SCBT Virtual Team Question and Answers from Ask the Expert

Updated July 26, 2005

Question 1: Can you please send me some information on Segmental Concrete Bridge design and construction? I would also like some information on their advantages and disadvantages?

Please visit the Library Area of the Segmental Concrete Bridge Technology V-Team Website, www.fhwa.dot.gov/bridge/segmental, for a list of references on the design and construction of segmental bridges. Please also visit the American Segmental Bridge Institute's (ASBI) website for publications on the design and construction of segmental concrete bridges. ASBI also conducts seminars on "Recommended Practice for Design and Construction of Segmental Concrete Bridges". Please contact ASBI for more detailed information on preparing seminars on segmental bridge design and construction.

Ask a Question

Question 2: I'm building a free-lanced model of a concrete railroad bridge that may also be called a viaduct. I'm building this on a curve and I would like to know if a concrete railroad bridge of this type would have been built as a "curved" structure, or would would it be a series of straight sections pieced together around the curve?

Answer: It sounds like you have a fun project building a model of a curved concrete railroad bridge. The curvature of the superstructure may be built of curved structural elements or of a series of straight members forming chords of the curve. For segmental concrete bridge construction, the curvature of the bridge is achieved by building curved segments. To simplify geometric control, a constant rate of curvature (constant radius) is used for the curved bridge alignment.

For your model of the viaduct, a curved structure will be very attractive!

Question 3: Please recommend a good text on the design of segmental bridges

Answer: Here are a few references on the design and construction of segmental concrete bridges:

1. Podolny, Walter and Muller, Jean, "Construction and Design of Prestressed Concrete Segmental Bridges", John Wiley & Sons, New York, 1982.
2. Chen, Wai-Fah and Duan, Lian, Editors, "Bridge Engineering Handbook - Chapter 11 - Segmental Concrete Bridges", CRC Press, New York, 1999.
3. AASHTO, Guide Specifications for Design and Construction of Segmental Concrete Bridge, Second Edition, 1999.
4. ASBI, Recommended Practice for Design and Construction of Segmental Concrete Bridges. (Visit ASBI website for further details: www.asbi-assoc.org.)
   
   

These references should give you a very good introductory to the design and construction of segmental concrete bridges. If you need further information, please let me know.

Question 4: On a superelevated section of post tensioned concrete box girder bridge, where will be the right location of intermediate pier and what will be the position of webs (Vertical or normal to the deck slab?) Will you consider the shift due to superelevation for positioning the pier?

Answer: The location of an intermediate pier for highway bridges is seldom dictated by the superelevation of the structure. It is generally determined based on factors, such as, site conditions, clearances to existing facilities, aesthetics, etc. For box girder with vertical webs, the webs are kept vertical while the top flange (deck slab) and bottom flange are parallel to the superelevation. For trapezoidal box girder, a constant slope is maintained for the exterior webs, the interior webs are kept vertical, while the top and bottom flanges are parallel to the superelevation. The key is to keep the geometry as simple as feasible to facilitate construction.

Question 5: What is the tolerance of the location of internal tendon duct for the precast post-tensioning segmental bridge? I have faced the problem to cast the segment with +-5mm of the tendon duct.Is it possible to increase to +-10mm? Any effects to the structure if the location of duct is out more than 5mm?

Answer: The AASHTO Guide Specifications for Design and Construction of Segmental Concrete Bridges, Second Edition, 1999 gives the tolerance of the location of internal tendon duct (tendon hole location) as + - 1/8" (3.2 mm). It is tighter than the + - 5 mm you are providing. Excessive misalignment (> 5 mm) may cause problems in erection, installation of the prestressing strands, jacking force, grouting of tendons, corrosion and other long term performance of the structure. In the fabrication of precast segments, a high degree of geometry control (including tendon duct location) is recommended during match casting of the segments to assure accuracy.

Tendon duct size and area are an important part of segment design and detailing. The maximum size of internal ducts is normally 0.4 x web thickness. The duct area is usually 2 to 2 ½ times the prestressing steel area. This is the basic for the tendon hole tolerance.

Good luck to you in maintaining tight tolerances in construction.

Question 6: How does an extrados post-tensioned bridge differ from a cable stayed bridge? What is FHWA policy on extrados post-tensioned bridges?

Answer: The cables of a conventional cable-stayed bridge support the dead load of the superstructure and the live load; while the cables of an extradosed bridge support only the live load. As a result, a conventional cable-stayed bridge has relatively shallow and flexible superstructure and deck system; while an extradosed bridge has heavier and stiffer superstructure to carry its own dead load, and the cables are then added to support the live load only, resulting in much shorter towers for an extradosed bridge.

FHWA has no special policy on extradosed post-tensioned bridges.

Question 7: I would like to ask for the provisions for seismic behavior in precast segmental bridges?

Answer: The provisions for seismic design, analysis and construction of segmental concrete bridges may be found in the following AASHTO Specifications:

1. AASHTO LRFD Bridge Design Specifications, Second Edition, 1998 and Interims through 2002.
2. AASHTO Standard Specifications for Highway Bridges, Sixteenth Edition, 1996 and Interims through 2002.
3. AASHTO Guide Specifications for Design and Construction of Segmental Concrete Bridges, Second Edition, 1999.

AASHTO may be contacted for further information on these publications:
Mail: AASHTO, P.O. Box 96716, Washington, D.C. 20090-6716.
Phone: 1-800-231-3475
Fax: 1-800-525-5562
International Fax: 304-728-2171
Web Site: www.transportation.org

Question 8: Does your organization have any information how much (pounds) material is used in bridge and road expansion joints per year? If you do not have how much volume is used, would you know how many miles of expansion material is laid per year?

Answer: The information on the quantity or miles of expansion joint materials used in bridge and road construction can best be obtained from the State Departments of Transportation. The planning, design and construction of highway projects are under the direct control of these organizations.

Question 9: What is the maximum span length can be used for span-by-span construction method.

Answer: I presume that your question pertains to segmental concrete bridges. The common span lengths for segmental concrete bridges are as follows:

1. Precast Segmental

   a. Span By Span	80 to 150 feet.
   b. Progressive Placement	100 to 300 feet.
   c. Balanced Cantilever	150 to 400 feet.

   
   
2. Cast-in-place Segmental

   a. On Falsework	100 to 300 feet.
   b. Balanced Cantilever	200 to 850 feet.

Other span lengths may be used with due consideration to design, analysis and construction.

Question 10: Have segmental, post-tensioned precast conc. been used for two level bridges.If so, where? Was it used on suspension type bridge?

Answer: We are not aware of any two level segmental concrete bridges or used in suspension type bridges.

Question 11: How to apply epoxy resin on surface of segment with many shear keys and thickness of resin.

Answer: Proper joining of segments is a very critical part in precast concrete segmental bridge construction. It is necessary to follow good practices. The AASHTO LRFD Bridge Construction Specifications cover the fabrication and erection of precast concrete segments. I recommend that you refer to Article 8.13 of the LRFD Construction Specifications for developing the proper construction practices for your segmental project. You questions are adequately covered in the LRFD Construction Specifications.

Your office may have a copy of the LRFD Construction Specifications. If not you may obtain one from your local library or contacting AASHTO at www.transportation.org. or telephone: 1-800-231-3475.

Question 12: I am doing a research on the application of fiber reinforced concrete in the end zones of precast prestressed bridge girders. I am looking for cost analysis of a bridge girder (box-girder, I-girder, bulb-T). Our main focus is on the spiral and skin reinforcement in the end zones of a girder. If we eliminate the spiral and the skin reinforcement, how is it going to affect the cost (considering the cost of the material and the labor to place those reinforcement) of the girder?

Answer: Your concept of using fiber reinforced concrete in the end zones of precast prestressed bridge girders affect the cost incurred by the fabricators. It is best to contact the fabricators to share your ideas and to get an assessment on the feasibility and the savings in material and labor. The Executive Director for the Precast Prestressed Concrete Institute is John Dick. He will be able to give you the names of some fabricators to contact. Mr. Dick's phone number is: 312-786-0300 and e-mail: jdick@pcinst.com

Question 13: Can you supply names of corrosion inhibitors for PT strands for MnDOT as they are concerned that they may have unprotected strands during winter construction for up to three months?

Contributors: George Poirier, Daniel Wood, Jerry Potter, Jay Rohleder Jr., Tim Cooper, Tom Dehaven

Answer: Two commonly used corrosion inhibitors are VPI powder and water-soluble oils. Both can be problematic as the VPI powder is reported to be a carcinogen, and the water soluble oils can reduce the bond between the grout and the tendons. Many owners have stopped using corrosion inhibitors due to the necessity of flushing with water to remove the residual prior to grouting. It is nearly impossible to remove all of the water prior to grouting. It is recommended to blow dry using warm air to remove water prior to grouting. We understand that there is one corrosion product used overseas that does not require flushing, but is not currently marketed in the US.

The consensus of the group is to grout as soon as possible!

Specifications provide varying maximum time of ungrouted tendons. They vary by environment, location, and code. AASHTO LRFD Construction Specifications allows 7 to 20 days depending on environment. PTI allows 7 to 40 days for similar environments. Some State specifications allow 30 days. These specification references do not address or provide for cold weather construction.

It is important to understand the potential risks of delaying grouting operations in cold weather. These risks extend to both the anchorage block- out areas and the ducts. On previous cold weather construction projects were grouting was delayed, water had found its way into anchorages and ducts, and froze producing spalls. See attached images of spalls caused by the freezing of trapped water in PT anchorage block outs on a PC balanced cantilever constructed box.

Proper detailing of the block outs for PT anchorages can help to minimize this problem. If the block out for an anchorage is open to the deck surface, it should be designed so that the contractor can clean out any water that is trapped prior to the pour back. The contractor's attention to cleaning these areas from water and debris is critical.

Heating of a structure or tendon so grouting can be accomplished is difficult, especially over an extended period. Handling and curing grout in freezing weather requires careful planning and monitoring. Depending on the temperature range, heating of the ducts and or anchorage can be very expensive, and impracticable.

Question 14: We are at the concept stage of designing quite a no. of segmental bridges with external prestressing. Just want to seek your expert opinion with regard to the required width of walkway left between the external tendons inside the box (running longitudinally from end to end). We are not aware there is any requirements in AASHTO, but what is the common practice in the States? Would a 600mm width be too narrow and insufficient for future inspection and maintenance purpose?

Answer (by Larry O'Donnell): For standard details please see the AASHTO-PCI-ASBI Segmental Box Girder Standards for Span-by-Span and Balanced Cantilever Construction that was jointly developed by the American Association of State Highway and Transportation Officials (AASHTO), Precast/Prestressed Concrete Institute (PCI) and the American Segmental Bridge Institute (ASBI). These standards contain 22 sheets of standard segment drawings, design and construction information and standard rectangular box section drawings for substructures. These standards are available and can be ordered from ASBI or PCI at the following websites:

The 600 mm horizontal width between external tendons seems narrow for a walkway through the box and along the center line of the box. Although narrow, it likely would not prohibit future inspection and maintenance, but would make it more difficult. Most inspectors wish there was more space in typical box sections. See further discussion below. However, the 600 mm width seems more than adequate, if the walkway width refers to a width (spacing) between adjacent external tendons on the same side (half) of the box section.

In some cases, the minimum access dimensions have been influenced by local emergency response personnel such as fire department personnel. In such a case, the minimum dimension established was 30 inches (762 mm). This was determined to be adequate to allow access by firemen in full protective gear with a back board for extraction of an incapacitated person (inspector) from within the box girder. Coordination with local emergency response personnel may be needed. In addition, this 30 inch (762 mm) width allowed for the access of equipment to perform magnetic flux leakage tests and high-energy x-ray field inspections.

The AASHTO-PCI-ASBI standard drawings, referred to above, have been developed for typical box sections with the following depths and associated width of bottom slab: 1800 mm, 4500 mm; 1800 mm, 5700 mm; 2100 mm, 4260 mm; 2100 mm, 5460 mm; 2400 mm, 4020 mm; 2400 mm, 5220 mm; 2700 mm, 3780 mm; 2700 mm, 4980 mm; 3000 mm, 3540 mm; 3000 mm, 4740 mm. There is a standard Interior Pier Segment Diaphragm Dimensions drawing for span-by-span and balanced cantilever and an Expansion Joint Segment Dimensions drawing. On these drawings are dimensions for access through the diaphragms. Therefore the minimum horizontal walkway through the box should not be less than the opening through the diaphragm. The least width dimension of the opening through the typical box sections diaphragm is 520 mm for the 3000 mm depth, 3540 mm width bottom slab section.

A typical horizontal dimension from the centerline of the box section to the centerline of the tendon at a deviation block would be 995 mm to 1781 mm depending on the segment size. This would result in a much greater walkway space than 600 mm.

Question 15: What is a cost per square meter, or per cubic meter, of a segmentally constructed bridge superstructure using precast segments?

Our bridge is a 5 span (49m-70m-98m-70m-49m) curved soffit single box over a large river in Western Australia.

Answer (by Myint Lwin): The average square foot cost (time adjusted to 2002) of segmental bridges constructed over water varies from US$100 - 130 per square foot of deck area. The American Segmental Bridge Institute (ASBI) keeps a database of the construction costs of segmental concrete bridges. I suggest that you refer to the ASBI Cost Data for more detail of the bridge types and related costs. The ASBI Website address is: http://www.asbi-assoc.org

Question 16: Would it be possible to build a bridge ac cross the Straight of Hormuz with todays technology?

Answer (by Myint Lwin): There are modern construction materials and bridge types that we can use to span across straits and ravines. The Strait of Hormuz seems to be an appropriate site to connect the banks of the Persian Gulf and Arabian Gulf. It is the narrowest part of the gulfs! Personally I feel that the Strait can be bridged with innovative design and modern technologies. For example, the Rion-Antiron Bridge connects Greece and Italy beautifully! The first step will be to do a feasibility study to determine the site conditions, and then select the types of structures and the best location for the bridge. The feasibility will answer many of the questions that you may have.

Question 17: Could you suggest me where to get good literature (text books, internet web pages, journals, papers, etc.) on how to DESIGN an "Extradosed" prestressed concrete bridge? I want to know in which conditions are they used and what are the design steps.

Answer (by Myint Lwin): Connecticut State is designing and building the first extradosed bridge in the U.S. (Japan has built the most extradosed bridges to date.) The name of the bridge is "Pearl Harbor Memorial Bridge". The Connecticut State DOT led a group of engineers to tour Japan to study the Extradosed Bridge Technology in Japan. A copy of the study report may be obtained by contacting the Connecticut State DOT. This report will have the information you are looking for. If you have difficulty in contacting the DOT, please let me know. Good luck and have fun with the design of extradosed bridges.

Question 18: Several questions here: I am a freelance commercial diver living in florida I support primarily DoD and other government agencies However after recent experience I am interested in starting a bridge inspection company. I have taken Bridge inspection courses from ADC (Association of Diving Contractors). Several of the jobs I have done locally on bridges I have noted serious spalling and scouring issues. What Florida based contractors are addressing these issues? How often should bridges be inspected? And finally what is the life expectancy of a typical segmental bridge?

Answer (by Myint Lwin):

1. What Florida based contractors are addressing these issues?

   I am not very clear on the question. Generally, the Bridge Owner, such as Florida State, plans and organizes the bridge inspection. The Bridge Owner may use in-house inspectors or contract out the work. Depending on the results of the inspection, the Bridge Owner prepares action plan for more in-depth inspection, repair defects or rehabilitate/replace the structures.

2. How often should bridges be inspected?

   Generally, once every two years.

3. What is the life expectancy of a typical segmental bridge?

   Generally bridges are designed for a 75 years service life. For major bridges, the Bridge Owner may specify 100 to 150 years design life.

Question 19: Could you please advise me if precast beam construction is a form of precast segmental construction; and what is the actual "definition" of precast segmental construction?

Answer (by Myint Lwin): Segmental construction may be defined as a method of construction in which the primary load-carrying members are composed of individual precast or cast-in-place segments post-tensioned longitudinally and/or transversely together to form simple or continuous-span bridges. Superstructures of segmental construction are generally of single or multiple box sections or a combination thereof, but precast beam-type sections may also be used. In recent years, precast beam sections are joined by splicing (post-tensioning) to expand their range of applicability.

If the precast beams are spliced to form longer spans and/or post-tensioned to form continuous spans, the construction method may come under the definition of "Segmental Construction". In this case, the segments are long with respect to their width.

For further reading and explanation of segmental construction, please see the references given in the Library Area of the Segmental Bridge Website: www.fhwa.dot.gov/bridge/segmental.

Follow Up (by Dean Van Landuyt): I would not consider construction involving precast girders as segmental construction regardless if they were spliced or not.

Question 20: 1-How long is Durability of epoxy resin in connection between 2 segments of bridge? 2-Dry joint between 2 segments of bridge is better or not in comparison with a epoxy joint.

Answer (by Jerry Potter): In the US, epoxy joints between segmental units have been used for over 25 years and have served extremely well when properly constructed. A few instances have been reported where the epoxy did not perform as intended, but these instances were generally traced to poor construction practices in mixing, handling or applying the epoxy. Excellent success has been achieved with epoxies meeting specification requirements for segmental construction when used in accordance with the manufacturer's requirements. There has been no concern expressed relative to the long term performance of epoxies for this application.

A few projects have been constructed with joints referred to as dry joints where the surfaces are in contact without any sealant or epoxy. The attached file contains a brief history of joint types and current practice in the US and should answer your question relative to dry joints

Question 21: Could the team recommend some examples of guidance for pre-qualifying segmental bridge contractors? In addition, what "credentials" would be recommended for consultant inspection of segmental bridge construction?.

Answer:

State-of-practice for Contractor and Construction Engineering and Inspection Personnel Qualifications for Segmental Concrete Bridges

Contractor Personnel:

The approach used for assuring the contractor has the capability to successfully complete segmental concrete bridges has been handled by several methods. Some of these are:

• Pre-qualification of contractors before bidding
• Specifying minimum experience requirements for critical contractor personnel
• Relying on the state's standard specifications for controlling and executing the work.

Pre-qualification of contractors varies with each state and is usually covered in specific administrative procedures and is generally related to general capabilities rather than specific structure type. However, for some segmental projects, contractors have been required to be pre-qualified for specific capability to construct segmental bridges.

With the above methods, specifying the minimum requirements for project critical personnel seems to be the most used. The typical positions included are:

• General Superintendent
• Superstructure Superintendent
• Contractor's (Construction) Engineer
• Casting Superintendent
• Erection Superintendent
• Geometry Control Technician (Foreman)
• Grouting Supervisor (Foreman)
• Stressing Foreman

Generally a minimum of experience (time and structure type) for each of the critical personnel is specified in the contract documents and verification of experience is required to be submitted with the bid documents or after bid and before award. Others have requirements to verify experience before the related phase of work begins. Also, most states require review and approval of replacement personnel.

A typical requirement for a Superstructure Superintendent may read as follows;

The Superstructure Superintendent (SS) shall be on-site and in responsible charge of day-to-day construction of the concrete box girder bridge superstructures. The SS shall have supervisory and construction experience and substantial specific knowledge related to the construction of post-tensioned concrete box girder superstructures constructed using form travelers with the balanced cantilever (span-by-span, i.e., project specific relative to design being constructed) method. The SS shall possess a minimum of 5 years of cumulative relevant experience accumulated over the past 12 years.

The specific experience will vary with the position level, criticality to the project and type and complexity of project. Generally 3 years is the specified minimum experience for any position although some states have specified less time for registered engineers.

Construction Engineering and Inspection Personnel:

The requirements for Construction Engineering and Inspection (CE&I) personnel has not been well defined, except this work for many projects has been provided by consultant services and the qualifications of the personnel has been included and evaluated in the consultant selection process and controlled by consultant management staff. Recent indicators by some states, is that specific minimum qualifications for critical construction administration and inspection personnel positions will be specified and enforced. For example, one state is developing criteria for the following positions:

• Senior Project Engineer
• Project Engineer/Manager
• Senior Inspector
• Casting Yard Engineer/Manager

Requirements for these positions are similar to the contractor's personnel, i.e., length of experience and knowledge in the specific segmental related activities. 1 year with a PE or 3 years for non-PE is the minimum for the Casting Yard Engineer.

Question 22: I have been working in the segmental design industry for quite some time, however, a question in regards to how to interpret Article 7.2.2.1 of the "Guide Specifications for Design and Construction of Segmental Concrete Bridges - Second Edition 1999" has come up between a coworker and myself.

I have always assumed that the 0.5 load factor for temperature gradient is applied only when the service load combination includes full live load plus impact as well as temperature loading - AASHTO Load group IV and VI from Table 3.22.1A. Mainly, I have always treated temperature gradient as part of the temperature loading based on the T =(TRF+TG) equation.

My coworker, on the other hand, has interpreted Article 7.2.2.1 differently. He feels that the 0.50 load factor for temperature gradient needs to be applied to all service load combination where live load plus impact is included, mainly load groups I, III, IV and VI from Table 3.22.1A.

Which is the correct way to apply the temperature gradient?

The interpretation is consistent with the majority of designers using the Guide Spec / Standard Spec. The Guide Spec introduced the Temperature Gradient (TG) within the term "T" of the Standard Spec - this is commonly interpreted to mean that TG need not be considered unless T is included in a particular combination group. The fact that T is selected only in Group IV, V and VI meant that TG is to be considered only in those groups; Groups IV and VI would have 0.5 TG while Group V would have 1.0 TG. To include TG for all service load combinations involving live load plus impact (LL+I) may be considered conservative and inconsistent with the overstress provisions in Column 14 of Table 3.22.1A of the AASHTO Standard Spec. The different percentage overstresses in Column 14 reflect considerations such as the frequency and duration of concurrent application of the loading types included in a particular load combination. Fundamentally, full liveload plus impact and design temperature (uniform plus gradient) are both transient effects which makes their concurrent application at any point on a structure infrequent and of short duration. This is the reason Group VI (principally DL + LL+I + TRF + TG)is measured against an allowable stresses of 140%. On the other hand, Group I is measured against an allowable stresses of 100% (i.e. with no overstress allowance) because the set of circumstances (principally DL + LL+I) occurs with much greater frequency and at longer duration.

While the interpretation of the Guide Spec / Standard Spec represents the majority, her co-worker's interpretation deserves "equal time" because it reflects the evolving nature of the design codes, and the interim nature of the TG provisions in particular. The complex nature of TG, insufficient data over its effects on past and current designs, and historically unequal treatment of this phenomenon between steel and concrete designs, have contributed to the varying views over the severity of the TG effects. Early designs have been based on vastly different criteria ranging from no TG, to linear TG, to a non-linear TG more severe than the one in our current codes. The majority of the designers who chose to interpret as she does often refer to the lack of reported distress in existing structures attributable to TG. Others maintain that it is all too sensible to associate TG with LL+I, observing that TG is a phenomenon that recurs regularly - literally as often as the sun would rise - and perhaps in the order of frequency as design level LL+I. This association, of TG and LL+I, is in fact recognized in the LRFD Spec (Table 3.4.1-1) where the term "TG" is broken out of the term "T" and included in Service-I and Service-III in combination with LL+I. But here again, consideration of statistical significance (frequency and durations), not unlike the increase of allowable stresses in Table 3.22.1A of the Standard Spec, takes place in the LRFD Spec under Service-III, where the LL+I load factor is reduced to 0.8 for checking tension in prestressed structures.

In conclusion, we would endorse the interpretation as meeting the minimum requirements of the Guide Spec / Standard Spec. We believe that there are objective basis for wanting to add TG to all load cases involving LL+I, but while this would be consistent with the LRFD approach, it would invariably exceed the minimum requirements of the Guide Spec / Standard Spec. We hope this provides the requested clarification.

Question 23: The questions are repeated below.

The answers below are general in nature and applicable to any bridge widening project. Specifics of the bridge in question were not provided, but widening a CIP superstructure, if post-tensioned, depending on the amount of widening, methodology, etc. will likely present special design difficulties (differential deformations, stresses, creep, etc.) and should be evaluated and analyzed most carefully. With more specific information, more detailed responses could be provided if needed.

Question 23a:. Is it advisable to effect condition assessment of a 30-year old concrete bridge (CIP) over brackish waters, before proceeding with the design of its expansion one side?

Answer: Yes, however the extent and degree of assessment should be proportional to associated risk and cost. That is, there are varying levels or degrees of assessment that might be undertaken, and likewise there may be many other issues to consider in the process of decision-making. For example, if the existing bridge is exposed to significant impact from marine vessels, the risk of collapse from such impact should be established, and this weighed against any additional cost to prevent collapse in the widened bridge. A life cycle cost analysis is also recommended to evaluate the options of bridge widening and total replacement.

Question 23b:. The brackish water condition exposes environmental issues and would you advise the use of epoxy-coated sheet piles in the construction?

Answer: The term "brackish water" alone is not very descriptive in that the corrosive qualities of such water can vary greatly, as can its affect on reinforced concrete and structural steel over time. Also, the intended purpose of the sheet piling is not described. Therefore, although epoxy coated sheet piling has been utilized, at least to some minor extent in the United States for corrosion protection, no specific recommendation on its use can be offered. Regardless, a cost comparison with other corrosion protection methods is suggested before a decision is reached on this issue. A critical factor in such analysis will be the desired service life of the sheeting.

Question 23c:. Would you advise replicating existing timber-pile foundation of the bridge in the proposed expansion? (We do not have any information on the condition of existing piles, bridge abutments and wing walls.)

Answer: The condition/performance of existing piling should be a major factor in deciding whether a replacement or widening project is undertaken. Referring back your first question, a widening project should not be selected without first assessing the bridge's foundation performance and expected remaining service life. If not practical to physically examine the existing piling on this bridge, the history and performance of similar piling in similar environments and under similar conditions of use should be helpful in establishing a confidence level for the existing bridge piling. If the existing piling are determined sound and suitable for providing the desired service life of a widened structured, the decision on pile type to use in the widened foundation would then be driven by cost and other performance criteria, such as capacity, settlement, stiffness, etc.

Question 24:. I'm doing research on identifying defects on precast segmental bridge. Is there any research that has been carried out so far and please suggest reference or websites for me to refer to.

Answer: Please see Article 16.1 Textbooks and Other Publications in the Library Area of the Segmental Bridge Technology website for references and other websites in your area of interest. Please also check the Q&A Area for answers to questions similar to yours.

Question 25: What is the policy for "virtual team"?

Answer: The Office of Bridge Technology, Federal Highway Administration (HIBT-FHWA) recognizes that there is much knowledge within the segmental concrete bridge engineering community. The recent retirement of technical and experienced engineers in government prompted the HIBT-FHWA to quickly nurture and develop the next generations of technical expertise from within the ranks to meet the challenging transportation needs of the 21st Century. The virtual team for segmental concrete bridge technology is formed with the following key points (ACES):

• Accelerate the development of technical expertise and leadership within the government
• Create innovative and efficient ways of doing business through the e-government information technology
• Establish a community of practice with limited and manageable membership
• Sharing knowledge and information with all stakeholders to improve and advance the segmental concrete bridge technology

Question 26: Please discuss the long-term durability of post-tensioning tendons in Segmental Bridges.

Answer: The response is a statement prepared by Clifford L. Freyermuth, Manager, American Segmental Bridge Institute, William N. Nickas, State Structures Design Engineer, Florida Department of Transportation and Andrea Schokker, Henderson Professor of Civil Engineering, The Pennsylvania State University, in response to the article "Enduring Strength" published in the September 2003 issue of Civil Engineering.

The article "Enduring Strength" in the September 2003 issue of Civil Engineering raises questions about the long-term durability of post-tensioning tendons in segmental concrete bridges. The article discusses corrosion discovered in external tendons of two span-by-span segmental bridges in Florida that required replacement of 12 of the total of approximately 4500 external tendons in this type of bridge in Florida (0.26 percent). Although the extent of the observed corrosion problems was limited, these incidents served as a catalyst for many agencies to look for constructive responses to the question, "How can these sinews be better protected?" In this context, concerted investigations directed towards improving corrosion protection of post-tensioning tendons have been made in recent years by the Florida Department of Transportation, the Federal Highway Administration, post-tensioning material suppliers in the U.S., the American Segmental Bridge Institute (ASBI), and the Post-Tensioning Institute, as well as many other State Departments of Transportation, research agencies, and consulting engineers involved in design and construction of segmental concrete bridges. These investigations have identified improvements in post-tensioning materials, design details, and construction procedures necessary to avoid recurrence of the problems discussed in the referenced article. Comprehensive inspection of segmental and other post-tensioned bridges in other States made at the request of the Federal Highway Administration did not disclose any significant corrosion problems, although deficiencies in grouting (voids) were observed.

Some specific responses to items in the referenced article are as follows:

• An NCHRP Research Report 20-7/Task 92, "Durability of Precast Segmental Bridges", June 1998, by R.W. Poston (one of the authors of "Enduring Strength") and J.P. Wouters discusses (among other issues) the Ynys-y-Gwas Bridge in the United Kingdom, and the Interstate 195 Bridge over the Seekonk River in Rhode Island which are also discussed in the "Enduring Strength" article. In regard to the Ynys-y-Gwas Bridge, and other segmental bridges in the U.K., this NCHRP report states that "the problems with segmental bridges in the U.K. have been specifically with thin mortar joints and poor design and construction practices". In reference to the Interstate 195 Bridge in Rhode Island, the NCHRP report states, "Despite the high incidence of voided tendons, no significant corrosion of the prestressing steel had occurred in some 30 years". This report concludes, "The durability performance of precast segmental bridges has been excellent in the United States to date".
• There has been no restriction on construction of segmental bridges with external tendons in the United Kingdom since the U.K. Department of Transportation ban was lifted in 1996. The global investigation of post-tensioned bridges in the U.K. found very limited corrosion or durability problems.
• While the durability performance of segmental bridges to date in the U.S. has generally been excellent, durability of post-tensioning tendons in segmental bridges now being built does not "rely on the same corrosion protection system used on bridges where problems have been encountered". The following are only a few of the specific changes implemented by the Florida Department of Transportation and other State and Federal Agencies:

  1. Prepackaged, pre-qualified, anti-bleed thixotropic grouts are used which are mixed with high-energy mixers. The anti-bleed characteristics of these grouts virtually eliminates problems related to voids in grouts due to bleed water.

  2. All post-tensioning anchorages are inspected for bleed water voids 24-48 hours after grouting. Post-tensioning anchorage details have been modified to facilitate inspections of grouting at anchorages. Inspection of tendons grouted with anti-bleed thixotropic grouts have disclosed only a limited number of small voids. Any voids encountered are repaired by vacuum grouting.

  3. The problems with duct splitting were resolved by introduction of new American State Highway and Transportation Officials (AASHTO) specifications for high density polyethylene duct in the late 1990's.

  4. Corrosion protection details of post-tensioned anchorages have been revised to ensure that deck runoff or wind-blown salt water will not gain access to the anchorage.

  5. The ASBI initiated a "Grouting Certification Training Program" in 2001 to train and certify engineers and construction personnel in proper techniques for mixing and placing grout, as well as protection of post-tensioning anchorages. To date, 604 personnel have participated in the first five training events, including 120 representatives of State Departments of Transportation and other transportation agencies. Eighteen States and Agencies currently require this training (or equivalent training) in bid documents, and ten additional States are considering this requirement. The next ASBI training event will be April 5-6, 2004.

  6. The Florida Department of Transportation has concluded numerous comprehensive Post-Tensioning Durability Workshops. These workshops deployed a five-part strategy to improve the durability of Florida's post-tensioned bridges. Bridge practitioners involved with design, construction, and maintenance came together to study specific enhancements to materials, workmanship and operations for these important structures. The most recent event in Orlando, Florida on July 24 - 25, 2003 was attended by more that 300 engineers and construction personnel from around the country.

  7. AASHTO adopted comprehensive new grouting specifications in 2001.

Notwithstanding the corrosion problems noted in the referenced article, National Bridge Inventory data shows conclusively that the overall durability performance of segmental concrete bridges, and pre-stressed concrete bridges in general, is outstanding. The changes in materials, grouting technology, construction procedures, and anchorage protection details implemented by the Florida Department of Transportation, and most States and Agencies that use post-tensioned construction, are considered to provide a more durable and robust system that represents a new standard for corrosion protection of post-tensioning tendons. This new corrosion protection standard provides added assurance that segmental concrete bridges will provide the 100 year minimum service life endorsed by the Board of Directors of the American Segmental Bridge Institute in 1999.

Question 27: I'm looking for information on technical specifications and values in geometric control on segmental concrete bridge

Answer: Article 16.1 Textbooks and Other Publications posted in the "Library" Section of the website will provide you the information you are looking for.

Question 28: On referring to your library - Article 2.7.3, I note that this document claims that dry joints with external tendons are now not allowed by AASHTO LRFD.

I have a copy of the 2002 version and this does not appear to be the case. Can you pls advise if this is the case?

Answer: The 2003 Interims to the AASHTO LRFD Bridge Design Specifications Section 5.5.4.2.2 includes the change adopted at the 2002 AASHTO Bridge Subcommittee Meeting that eliminated the use of dry joints for precast segmental bridges.

Question 29: Is there an inspection manual available from FHWA or otherwise, specifically for segmental bridges built in 1980?

We are not aware of any manual developed specifically for segmental bridges built in 1980. There have been various Maintenance Manuals developed which include inspection requirements for specific segmental bridges. Most of these manuals are retained by the various owner DOT's.

Other reference documents that may provide guidance are as follows:

• FHWA published a report in 1988 that provided guidance for developing Inspections Manuals for Segmental Concrete Bridges. The Report is FHWA/IP-88/038, Guidelines for Developing Inspection Manuals for Segmental Concrete Bridges, September 1988.
• Another reference document recently published is FHWA NHI 03-002, Bridge Inspectors Reference Manual, October 2002. This Manual contains a section providing guidance on inspection of Segmental Concrete Bridges.

Question 30: I would like to ask a question regarding to the bearing type selection for segmental bridges. We are doing design of a bridge using span-by-span construction method. The span lengths varies from 140 ft to 160 ft. The design units are typically 6-span continuous. The DL reactions at bearing locations are around 1,400 Kips (two bearings at each pier). Currently we proposed to use steel-reinforced elastomeric bearings. Based on the force and length of unit, the elastomeric bearing could be as big as 4'x 4'x 1' thick. I would like to ask what is the recommended practice for bearing selection for segmental constructed bridges and if elastomeric bearing are used, what is the size of the bearing that can be achieved provide all the uncertainties in manufacturing and installation tolerances are satisfied.

The bearing type selection for segmental concrete bridges is similar to the bearing type selection for other concrete bridges. The bearings shall be designed to accommodate the translational and rotational movements of the bridge. The movements, with due consideration to the types and locations of the deck joints, are due to the effects of temperature changes, time-dependent causes, such as, creep and shrinkage, elastic shortening due to prestress, traffic loadings, construction loads and so on.

Steel-reinforced elastomeric bearings are suitable for longitudinal and transverse movement and rotation. However, they tend to be large when the movements are large as you are describing. For large movements and rotations, and loads, you may consider spherical bearings, disc bearings, pot bearings, etc. Guidelines for bearing selection and design may be found in the AASHTO LRFD Bridge Design Specifications. Manufacturers of bearings are very helpful in providing technical information for their bearing designs and dimensions.

You may also want to contact some State Departments of Transportation for their construction specifications for the various types of bearings. These specifications have provisions for acceptance criteria, installation and testing requirements, quality control and quality assurance to assure proper functioning of the installed bearings.

Question 31: I am designing a bridge that has to meet the following criteria:

• span of 160 metres
• max depth of bridge below deck is 1.5 metres

How do I decide upon the bridge type?

Answer: Please refer to Article 6.0 listed in the Segmental Bridge Library area of the segmental bridge website. This article discusses the type and size selection and other aspects of preliminary design that you are interested in.

Question 32: Please comment on the advantages and disadvantages of segmental concrete bridge decks using integral wearing surfaces versus a waterproof membrane and a replaceable non-structural latex modified concrete wearing surface.

Answer: (by Jerry Potter) Generally, there have been four types of overlays used for segmental concrete bridge decks. These consist of a waterproof membrane and asphalt wearing course, a non structural concrete overlay, a thin epoxy overlay and a monolithic concrete overlay. The non-structural overlays have consisted of regular concrete, high performance concrete, latex modified concrete or rapid setting latex modified concrete. Other concrete overlays have also been used such as reinforced thickened concrete overlays, fiber reinforced concrete overlays and special concrete formulations such as polyester concrete.

The monolithic concrete overlay has been primarily used for precast construction and has been easily included during casting the precast segments. The AASHTO Subcommittee on Bridges and Structures adopted a change to the AASHTO LRFD Bridge Design Specifications in 2002 as now included in Section 5.14.2.3.10e. This Section recommends that protection be provided by overlays instead of monolithic concrete. The Commentary for the Section provides additional information and reasoning for the changes.

A paper written and presented by Michael Sprinkle, P.E., Associate Director, Virginia Transportation Research Council at the ASBI Convention on November 4, 2003 titled "Deck Protection Systems for Post-tensioned Segmental Concrete Bridges" provides excellent guidance relative to issues to consider in selecting a particular protection system.

Emphasis is now being placed on longer service life and reduction on impact to traffic. Accordingly, deck protective systems that extend the service life of a bridge while minimizing the need to interrupt traffic with maintenance or overlay repair and/or replacement is recommended.

In general, the type of overlay selected depends on the objectives of the project, the project environment and the owner's preference. Each of the overlay systems have been used successfully when properly designed and constructed.

Question 33: I am a recent college grad and have been offered a position with a leader in the post-tensioning industry. I have thoroughly studied post-tensioning concept. The question I have is, in your opinion does the PT industry appear to have longevity? Is this something I can make a career out of? Are PT applications becoming a industry standard?

Answer: (by Myint Lwin) The prestressed concrete technology (pretensioned or post-tensioned, precast or cast-in-place) has advanced to a high degree of refinement. It has extensive application in bridge engineering for over half a century. The technology was originated by Eugene Freyssinet of France, introduced to the U.S. by Arthur Anderson of Washington State, made practical and popular by the teachings and writings of the late Professor T.Y. Lin of the University of California, and practiced by bridge engineers. In my opinion, the PT industry is a mature industry and will be here to stay as long as we design and build bridges and structures. Many engineers and entrepreneurs have had challenging and successful career in the PT industry. There are many opportunities for us to capture and challenges for us to meet in applying and advancing the prestressed concrete technology.

Question 34: How often should an owner perform an in-depth bridge inspection on a segmental bridge? Are there any guidelines from the ASBI or the FHWA?

Answer: (by Myint Lwin) In accordance with the Code of Federal Regulations, each bridge (including a segmental bridge) is to be inspected at regular intervals not to exceed 2 years in accordance with Section 2.3 of the AASHTO Manual for Maintenance Inspection of Bridges.

Question 35: Could you explain the Strut&Tie model to check structure capacity for shear (at end support & intermediate support) and calculation of local re-bar at anchorage zones (for segmental concrete bridge).

Answer: Article 5.6.3 of the AASHTO LRFD Bridge Design Specifications explains in detail the Strut-and-Tie Model for areas of interest to you. Further discussion of the Strut-and-Tie Method may be found in Chapter 9 of the Prestressed Concrete Structures Text Book by Michael P. Collins and Denis Mitchell.

Question 36: For design of segmental bridges that include an integral wearing course and additional thickness for milling, what are suggested design guidelines for thickness of wearing surface removed and superstructure design to keep distress at the time of concrete removal to acceptable levels?

Answer: The basic design approach for an integral wearing course and thickness for milling during construction is to design the structure by discounting the wearing course and extra concrete for milling in the design section. It is considered as dead load only. The design is based on a net section without the extra thickness. Accordingly, the stresses in the structure after removal of the concrete should be within allowable stresses with some minor differences for creep effects and possible redistribution using the total section. Of course the integral wearing surface and additional concrete not milled during construction will participate in the load carrying capacity of the structure and will be stressed similar to the rest of the top slab concrete. However because the actual section is larger than the design section, the stresses should be below the stresses determined during design for the reduced section. If the depth of removal is to be below the integral wearing surface, the section used for design should exclude the additional removal if known at time of design. If not, an analysis would have to be performed to determine the effect of the additional removal on the structure.

The thickness of integral overlays or wearing surfaces and additional concrete for milling should be a project specific determination based on the environmental exposure of the bridge and the owner's maintenance program. Overlays usually are from 1.25 to 2.75 inches in thickness. The general objective is to remove and replace the overlay before chlorides or other corrosion agents reach the bottom of the overlay.

Question 37: I am looking for information on a bridge located in a bay area and the method of construction is span-by-span. At its lowest level, the bottom of the segmental box is only 7'-6" above MHW line and will be 6" below the 100 year flood line. I am wondering whether this type of layout is acceptable, and if it is Ok, what kind of protections shall be provided to enhance superstructure durability. I also like to know whether there are some developed criteria that designer could follow regarding to this type of issues.

The other related question is the post tensioning of substructure, recent Florida Structures Design Guideline (LRFD, Aug, 2002) states "All vertical post-tensioning must be 12 feet above MHW" for structures over water, this will essentially eliminate the precast post-tensioning type of substructural construction (consider all kind of advantages that precast could offer) for most of bridges. If possible I like to know some background evidence that supports this restriction.

Answer: You have to work with the bridge owners to decide what is best for the conditions under consideration. The answers would vary from State to State and from site to site. I am not aware of any general criteria for the types of issues you are encountering.

You will have to contact the Florida DOT for the background information or the reasons for the restrictions.

Question 38: Is jacking apart of balanced cantilevers before casting the closure joints a common practice, and what are the considerations for this?

Answer: The jacking of cantilevers generally occurs when integral connections between the substructure and superstructure are used. In this case, all of the forces generated from long term creep and shrinkage and shortening due to continuity and/or positive moment post-tensioning could cause large forces on the substructure depending on the stiffness of the substructure units. To offset the long term forces, the concept of jacking is introduced so the net forces on the pier will be balanced between the initial jacked force and long term forces with time, thereby reducing the maximum forces on the pier. The jacking creates a force on the pier in the opposite direction and is relieved by the shortening due to long term creep and shrinkage. Accordingly, the long term forces on the pier are minimized. Also, the geometry of the completed structure may change and structural demand in the superstructure may be controlled by the forces. The magnitude of the jacking force is specific to the project design geometry and details.

Alternatives to jacking is to use bearings and let the structure move between expansion joints or design the substructure to accommodate the forces.

Question 39: What will be the difference in price per sq. meter of deck area between cable stayed bridge having a span of 500 m and a balanced cantilever segmental bridge having the same maximum span?

Cost effective cable stay bridges using cantilever balancing segmental construction are those with main spans ranging 150 to 610 m. Recently there are several cable-stayed bridges beyond the 600 m in China, Japan and Europe. For post-tensioned concrete segmental bridges, 500-meter main span is out of the current feasible range. The cost difference between cable-stayed and segmental bridges varies significantly and depends on numerous factors such as location, terrain, deep or shallow water, accessibility, traffic control and management, cable testing requirements and materials availability. The choice in your case is not based on cost but on technical feasibility.

Question 40: How do I calculate the time dependent effects such as creep and shrinkage in segmental prestressed bridges?

The AASHTO LRFD Bridge Design Specifications, Second Edition, 1998 covers this subject well in Article 5.14.2.3.6. The specification "commentary" also provides additional references on this topic for study. AASHTO may be contacted for further information on publications at:

Mail:	AASHTO 444 North Capitol Street N.W., Suite 249 Washington, DC 20001
Phone:	202-624-5800
Fax:	202-624-5806
Email:	info@aashto.org
Web Site:	www.transportation.org

Question 41: We are looking at the design of a 5 span box girder bridge, max span 90 m, with the road slab on a vertical curve of constant radius, radius approx. 5000 m, using the cast insitu balanced cantilever method. Are there any particular construction issues associated with the vertical curvature we need to consider?

The vertical curvature of the bridge profile is considered in the geometry control for the structure and unless unusual, does not require special activities. The vertical curve mentioned should not present any special need other than to assure the vertical curve geometry is included in computing the casting and final geometry control curves. The capability of the formwork adjustments required for the casting curve should accommodate the adjustments required for the vertical curve.

Question 42: We are going for segmental construction by under slung launching Girder. Here segments shall be supported on under slungs which are finally supported over Launching Girder. Once the erection and launching of one span is complete, post-tensioning shall be applied for that span. In case no prestressing is done, under slung of each segment is subjected to the Wt. of that segment. I want to ask that when prestressing is started, Is weight of the middle segments transferred to the under slungs of the segments near to the support? In that case is it true that under slungs of end segments as well as there supported points shall be designed for much heavier loads as compared to the original wt. of end segments? Or the case is like this, that as prestressing is done, under slung of each segment is relieved from some load and whatever the load is relieved from launching girder, shall directly be transferred to the bearings. What precaution should be made to get the load transfer directly to bearings when prestressing operation is in progress.

Erection of segmental bridge components is usually based on a prescribed sequence of construction activities which require analyses of each stage or operation to assure the stresses in the structure or components and erection equipment are acceptable. This would be applicable with any method or process of segment erection. The effect of prestressing and its sequence of application would be one stage in the operation that would have to be analyzed and the effects considered in the system. Definitely, as prestressing is applied, the structure begins to react to the force applied and the changes have to be considered in the structure components and the erection equipment. If a series of segments are supported individually under a gantry and are pulled together in a unit by prestressing, the support for each segment would change depending on the effect of the prestressing force on the structure. Some of the supports may be increased significantly as the structure camber changes due to the prestressing unless the prestressing is concentric with the structure neutral axis. These changes have to be considered in the erection procedures and accommodated in the erection process. Also, the transfer of the structure load to the bearings would be another step or phase in the erection process that will require proper evaluation. There are several methods of transfer including support by the gantry and lower to the bearing, temporary bearings and then transfer, placing the load directly on the bearings as the structure is loaded, etc. Each method depends on several factors including the type and size of structure, the erection method, the erection equipments, etc.

Question 43: We are looking at the design of a balanced cantilever structure with monolithic piers. The piers will be set at 30 degree skew to the bridge deck. The concern with the skew pier is that every time when a segment is cast, a torsional rotation of the box would occur and accumulate until the completion of the cantilever. Would there be any particular design and construction issues apart from geometry control.

From your question, it is understood that this is a cast in place balanced cantilever bridge. You have correctly identified that there is a coupling of the bending of the skewed pier and the apparent "torsional" rotation of the superstructure as the various segments are cast. Similarly, there are lateral "apparent horizontal" deflections that will appear due to the pier's skew. In general, these are geometric control issues that do not introduce significant internal forces. However, dependent upon the static scheme when continuity of various cantilever tips are made and subsequent loadings, it is possible to "lock in" parasitic "torsion" and/or "lateral bending". This same basic issues occurs for "vertical bending" with "non-skewed piers", but your particular case involves more 3D geometric/force effects than the conventional 2D case.

Question 44: Can you please give me information how we can obtain drawing of segmental box girder concrete moulding

I am not sure I have properly interpreted your question. However, if your desire is to obtain specific details and information on precast concrete bridge segment casting cells or forms, I would recommend you contact a major form supplier such as EFCO Corp. If instead, you are interested in details of precast concrete bridge segments themselves, I recommend you view the Segmental Box Girder Standards for Span-by-Span and Balanced Cantilever Construction, available through the American Association of State Highway Transportation Officials (AASHTO), the Precast/Prestressed Concrete Institute (PCI) or the American Segmental Bridge Institute (ASBI). You can preview what the Standards include by reading an article by Cliff Freyermuth entitled AASHTO-PCI-ASBI Segmental Box Girder Standards: A New Product for Grade Separations and Interchange Bridges in the September-October 1997 issue of the PCI JOURNAL. Another interesting article relating to segmental bridge casting machines may be found at http://www.cif.org/Nom2001/Nom29_01.PDF (.pdf, 0.33 mb). I am hopeful this information will satisfy your needs. If we can be of further service, please let us know.

Question 45: Could you please tell me on average how long in duration would a segment take (superstructure activities) on a cable stayed bridge, such as the Sidney Lanier Bridge in Georgia, I have researched approximately 6 days, but this is only from word of mouth.

There are many factors that could impact the cycle time for placing cable-stayed concrete bridge segments. The most important of these is whether or not the superstructure segments are completely precast. If so, after completing a learning curve, segments might be cycled in approximately 4 to 7 days. Cast-in-place (cip) segments are typically cast in traveling formwork which must be launched for each cycle. After casting, each segment must then gain the required strength prior to repeating the erection cycle. For entirely cip segments, a complete cycle would probably be 7 to 10 days. A superstructure with structural steel edge beams might be cycled in a few days less time.

Question 46: Do you ever create physical models for your design work, or are they all created in the computer? I'm curious how one could create a physical model, using materials that would accurately model a real concrete bridge.

In the United States, vehicular bridge design criteria and guidance are provided by the American Association of State Highway & Transportation Officials in the AASHTO Bridge Design Specifications (see also the Library Area of the Segmental Concrete Bridge Technology V-Team Website, www.fhwa.dot.gov/bridge/segmental, for a list of references on the design and construction of segmental bridges). A second recommended website for publications on the design and construction of segmental concrete bridges is the American Segmental Bridge Institute (ASBI) at http://www.asbi-assoc.org/. To directly answer your question, physical modeling for production bridge design is almost never utilized, other than wind tunnel testing for determining the aerodynamic properties of specific cross-sections for major cable-stayed or suspension bridges. More definitive answers in regards to structural modeling might be obtained through personnel at the FHWA Aerodynamics Laboratory located at the Turner Fairbank Highway Research Center in McLean, Virginia. Their web site is http://www.tfhrc.gov/aerodynamics/index.htm. Good luck with your investigation.

Question 47: What is the advantages of external post tensioning system (apart from ease of access for inspection and future replacement of the cables)? Would you agree that if these two factors are ignored (inspection and replacement)then internal post tensioing system would be more economical to use?

You are correct that various factors impact decision-making in regards to the use of specific structural features, such as internal versus external post-tensioning (PT). For example, a bridge designer would be constrained in structure feature choices by specific bridge owner requirements (design code, aesthetic criteria, durability criteria, PT tendon inspection and/or replacement requirements, etc), and beyond that by his own evaluation of structural efficiency, and overall cost effectiveness (material, labor, risk, time and ease to construct, etc.). In the United States (US) where low bid contracting is the norm, specific structural design choices such as structure type and erection assumptions are driven primarily by the overall cost effectiveness for/at a particular bridge/site in question.

Specifically in regards to external versus internal post-tensioning utilized in the design/construction of segmental concrete post-tensioned box girder bridges in the US, this choice is driven primarily by span length(s) and erection method(s), which again relate directly to the cost of construction. For example, in long span or significantly curved bridges built by balanced cantilever methods, cantilever PT tendons are typically located in ducts formed within the girder top flange and webs. Alternatively, for long straight bridges consisting of a great many relatively short spans, span-by-span construction methods are commonly utilized with PT tendons located within the box girder, but external to the concrete. Therefore, except when an ability to replace PT tendons and/or visually inspect such tendons is specified in the contract, I would not agree that these capabilities alone drive the use of external tendons.

A more detailed discussion of Internal versus External post-tensioning tendons at our web site Library, Article 2.2 at www.fhwa.dot.gov/bridge/segmental/seglibrary.htm.

Question 48: We are beginning the preliminary design of a cast-in-place segmental bridge and have encountered some questions in regard to the construction. It is a 5-span unit with a 470-foot main span. Span 2 of the 5-span unit is in a circular curve with a 3500-foot radius. This curvature requires the roadway to be superelevated from the normal crown section to a constant 2.5% cross slope.

Our questions are as follows:

1. In regard to construction with a form traveler and placement of post-tensioning, is it better to chord the individual segments or chord the entire span?
2. As noted above, the top deck slab for this bridge must be superelevated in the curve from the normal crown section to a constant 2.5% slope. It seems that there are two possible solutions to achieve the superelevation. One is to keep a constant top slab depth and vary the interior core form, and the other is to hold the interior core constant and vary the top slab depth. In either case the web angle would be held constant. Which is the better choice to provide the superelevation in the section?
3. In regards to constructibility, is it better to provide the longitudinal continuity post-tensioning as draped tendons in the webs running the entire length of each span or as a series of bottom slab tendons centered about the midspan?

Regarding your specific questions, the following comments are offered by our team:

1. In regard to construction with a form traveler and placement of post-tensioning, is it better to chord the individual segments or chord the entire span?

   In our opinion it would look better to cord the individual segments as would be done in a precast bridge. We are assuming this bridge will be built in balanced cantilever so care should be taken to investigate the loads and stability as it relates to the non-symmetrical cantilevers to include loads in the piers.

2. As noted above, the top deck slab for this bridge must be superelevated in the curve from the normal crown section to a constant 2.5% slope. It seems that there are two possible solutions to achieve the superelevation. One is to keep a constant top slab depth and vary the interior core form, and the other is to hold the interior core constant and vary the top slab depth. In either case the web angle would be held constant. Which is the better choice to provide the super elevation in the section?

   We would recommend keeping the top slab constant. The travelers should have room for making adjustments to handle the superelevation.

3. In regards to constructibility, is it better to provide the longitudinal continuity post-tensioning as draped tendons in the webs running the entire length of each span or as a series of bottom slab tendons centered about the midspan?

   If the segments are corded, the continuity tendons would be better in the bottom slab. Draped tendons would also add to the risk of grout voids in the tendon ducts.

(To view PDF files, you need the Acrobat® Reader®)