Want quick responses to our most asked precast questions? Our blog series, “FAQ Fridays,” is designed to answer your most popular questions, organized by topic and product category. In Part 12 of this series, we discuss Transportation Structures & Bridge Design.
Transportation Structures and Bridge design are crucial to infrastructure requirements. Precast concrete is an ideal material for a variety of reasons. Below are the most common questions we get asked.
- PCI Online, On-demand Transportation Systems Courses
- BIM
- Design – General
- Design – Fabrication
- Means & Methods
- Materials
- UHPC for Bridge Elements
- Precast Foundations
- NEXT Beam
- Piles
- Camber
- Girder Repair / Fabrication Issues
PCI Online, On-demand Transportation Systems Courses
Does T520 (lateral stability) cover in-place girders or shipping?
The PCI eLearning course T520 covers in detail the topics of handling, shipping, and bracing using methods presented in the PCI Recommended Practice. An instruction manual with a free spreadsheet is available from PCI that implements these procedures.
Please click our Transportation Catalog that features PCI publications and a comprehensive list of training courses in our eLearning Center.
BIM
What experience have you had with using PG Super in design?
PGSuper is a well-known design program that was originally developed by the Washington State DOT (WSDOT). It is now being used for some other DOTs as well. The person at WSDOT who is responsible for maintaining the program is closely involved with the development of changes for the LRFD specifications, so has first-hand information on how to implement any changes to the programs when changes in the specifications are made. We recommend you speak with a PCI-certified precast producer regarding their experience with PGSuper and other design software.
What percentage of projects are using BIM for layout of all rebar? What percentage of the bridge structure needs to be designed as precast before you start seeing a net gain to make a BIM model?
Many PCI Mid-Atlantic fabricators layout all rebar in all their models. The time investment into 3D modeling is extensive and should not be underestimated but the positive output is far greater. Many use 3D BIM on all the products they produce, even individual beams; and have experienced net gains in plant constructability, information, drawing quality and consistency.
Design – General
You’ve shared in design seminars that the top steel calc is As= T/fs, where fs is limited to 1/2 yield. Can you clarify if the top prestressing strand can be used in this "As" and that 30 ksi is still the allowable steel stress?
The LRFD specifications do not provide direct guidance regarding this question. We recommend that top flange strands can be included in the area of reinforcement, and that the same working stress be applied to them as to reinforcement, that is, 30 ksi.
Are draped strands utilized in post-tensioning applications? If so, it would seem there would be significantly lesser safety considerations.
Draped or harped strands are typically utilized in concrete segmental construction by means of a deviator segment. For spliced bulb tees or tub girders with internal post-tensioning tendons, the profile is usually parabolic.
Do hold down location vary based on length of beam? We use hold down at 0.4L and 0.6L.
Hold down locations used to be closer to 3rd points but in recent years fabricators have seen typical hold down locations at 0.4 and 0.6. We prefer 0.4 and 0.6 since it flattens the angle of the strand out more, reduces the uplift force at the hold down, and may allow us to place the strand hold down at one location vs. breaking it up into more locations which requires having to add hardware.
While a ratio of the length can certainly be used in design, it is recommended that a fixed value be used for a set of similar girders so that the hold down location is not different for each girder or group of girders. This simplifies detailing of plans as well as fabrication. Fabricators may submit changes in hold down locations in their shop drawings to fit their available hold down locations. When reviewing shop drawings from fabricators, designers should allow ample tolerance for shifting a hold down location to avoid supports of the formwork or other spacing considerations for the hold down mechanism in the bed layout of the fabricator.
Design – Fabrication
Regarding types of debonding (snap sleeves, solid tubes, split sleeves, etc.) it would seem that the fluid pressure of wet concrete would cause deformation of the sleeve and would cause significant friction between the sleeve and the strand. Has this effect been investigated? Perhaps the strand-sleeve friction may be high, but there is minimal friction between sleeve and concrete because sleeve is smooth?
Debonding as a method to control stresses at the ends of pretensioned members has been used for many years. The materials and methods have developed over the years and have performed as intended when properly installed. In our experience, there is no reason to doubt that debonding is effective and allows the strand to slip within the sleeve, thus preventing any transfer of force to the concrete.
From a fabricator perspective, what are some common RFI/mistakes that should/could have been addressed during the design phase?
Typically, the RFI process occurs on more complex projects or ABC (Accelerated Bridge Construction) type bridges. The more items that are not clear on the contract drawings, or left to be determined by the fabricator, the longer a fabricator will need to complete the shop drawings. Early ABC bridges did not contain enough time in the project schedule to perform the engineering tasks before production was required to start.
Regarding early stage RFIs: can producers speak to their willingness and experiences to be in contact with designers prior to releasing final plans and specifications?
Our fabricators receive frequent calls about questions from designers prior to issuing plans. These calls typically come from a bridge that might not be able to follow the standard and they want to be sure what is being proposed is feasible. Fabricators try to answer for the industry and not just what is best for an individual company. If the project is located closer to another fabricator, they will request the engineer follow up with that fabricator as well.
Have producers found an effective way to consistently avoid hairline (<0.1mm width) web cracks in the ends of girders caused by the prestressing? Is this a point of contention in the U.S.?
Pretensioned girders can develop web splitting cracks at the ends due to the concentrated loads being applied to the concrete as the force is transferred from the strands to the concrete. The cracks may form immediately at transfer, or they may appear with time as a girder is in storage. Generally, these cracks are narrow and will close when the girder is erected, and a composite deck applied. The ends of many girders are also encased in diaphragms, so for these girders, the cracks are not exposed. The width of such cracks is typically controlled by the web splitting reinforcement provided as required by the LRFD Specs. Some owners do not take any action regarding such cracks, recognizing that they are narrow and will typically close in the field. However, some owners are more cautious and may require that cracks be sealed or even injected. Repairs may not appear successful because cracks tend to continue to widen as the girders are in storage, often reopening after being repaired. Over the years, damage to girders along any such cracks has not been observed, so it would seem that they do not pose a significant risk of deterioration.
In terms of establishing continuity earlier than 90-days, the positive moment on the support shall be checked and detailed. Do you agree?
The 90-day limit mentioned in the specifications was intended as a simplification that will reduce or eliminate positive moment for most cases. An analysis can be performed to determine the effect, if any, of establishing continuity at an earlier age. Several factors affect the potential positive moment that may be developed if continuity is established at an earlier age, so there is no simple answer to the question. However, the 90-day limit is a complication for fabricators and contractors and can be eliminated or reduced in many cases.
Working in Pennsylvania and given all the different types of prestressed beams, is there criteria on which ones to use?
For PennDOT, it would be recommended to use PABT over any AASHTO /PennDOT I-beams. All of the box beam and NEXT beam sections listed on PennDOT’s BD standard are readily available. For the best solution for a specific bridge, submit a TS&L Quote request to the local fabricator.
For Lateral Stability, what is the maximum on site wind to be considered to delay the lifting of precast beams to assure safety and remain stable during installation?
We are not aware of any specified wind speed limits for erecting prestressed concrete girders. Owners or contractors may have such limits, which may depend on several factors. The effect of wind on the stability of a girder can be evaluated using the spreadsheet available from PCI.
Do you use corrosion inhibitor with bridge elements, and is it effective?
Corrosion inhibiting admixtures have been used for bridge elements. It appears that the material is effective, although properly constructed prestressed concrete girders are very durable, even without corrosion inhibitors.
Means & Methods
Bottom flange ties only at ends of girders?
We’re providing two responses to this question given interpretation of the intent. The more likely meaning is related to the confinement reinforcement in the bottom flange. While some owners require confinement bars for the full length of a girder, the LRFD Specifications only require confinement bars at the end regions of the girders.
The second meaning is that side forms require under-ties that pass beneath the soffit forms hold the two side forms in place at the bottom. Top ties are also provided to keep the top of the forms in place. These ties are required for the full length of the girder.
What is the purpose of the bearing plate if the prestressed beam is being supported by a rubber pad? For example, for a prestressed beam with an integral abutment. Is the bearing plate necessary in this case?
A bearing plate is typically required if the bearing plate connects to a sole plate when anchor bolts are used for superelevation or to serve as a mechanism for lateral restraint. Bearing plates are often preferred by fabricators to protect the ends of the girders because spalling is a concern when girders camber and slide during detensioning. A bearing plate is a good practice in all cases with larger skews as it reduces the likelihood of spalls at the end of the beam during detensioning or when the beam begins to camber and bear on the ends alone. In a situation like an integral abutment, a bearing plate is not required, but would still be helpful to prevent spalling during detensioning.
When the concrete is poured in the forms how can we ensure there are no potential cold joints? Since you will be going back and forth to fill the full height of the girder? On site construction projects, OPSS indicate that a cold joint is considered after 20 minutes interruption in the concrete pour operation. Is this accurate or is this more for shallow structures like a bridge deck or overlay?
Typically, when concrete is placed into a form in a precast plant, it is placed continuously or as close to continuously as possible, so that no cold joints form. (Also, not normally a back and forth motion). More importantly, anytime new concrete is placed on existing concrete, care needs to be taken to watch the flow of the concrete and assure the new concrete is mixing with the existing concrete. And if it is not mixing well together, the simple solution is to use a vibrator or a rod to mix the two layers of concrete.
How do you avoid segregation of the aggregate when vibrating, especially with the congested reinforcing? It seems like you would have excessive aggregate at the bottom and center of the forms with little or no aggregate at the top and outside of the reinforcing cage.
A proper SCC (Self-Consolidating Concrete) concrete mix is designed to suspend the aggregate and provide a uniform aggregate distribution across the entire depth of the cross section. There are pouring techniques used at the ends of beams that help provide a uniform aggregate distribution, but overall, a good SCC mix has no segregation. Tests are performed during mix development to assess the ability of the mix to pass through rebar in a beam; another approach is to pour a trial section of girder then cut the end off and visually inspect the aggregate distribution to assure there is an equal distribution of aggregate per area of the cross section.
A conventional concrete mix design with a higher slump may also be used with appropriate means and methods.
Materials
What time duration is needed before raking top flange ¼” for composite behavior?
A lot of conditions affect this duration such as the set time of the mix, concrete temperature, and air temperature. On a typical pour, it is likely 15 to 30 minutes after the top of the beam is screeded off.
What is typical f’ci and can use 7,200 psi?
Typical release is approximately 80% of the f’c. We use 8 & 10 ksi for most of our bridge mixes so the release for these is 6.4 & 8 ksi. 80% is a good rule of thumb but f’ci should be driven by the design and it is good practice to specify what is required for the design rounded up to the nearest 100 psi.
Release strength does not need to be a function of 28-day strength. Designers should recognize that release strength is a very important, driving parameter for casting efficiency for the producer. In many cases the release strength for a 10 ksi, 28-day strength could be only 6.4 ksi with no impact to the final strength of the product. For this type of design, debonded top strands are often utilized to reduce stresses at the end of the girders to allow lifting and handling.
Do fabricators ever use corrosion inhibitor with concrete at any project specially roadways?
Corrosion inhibitor has been used on bridge products. We’re not aware of its use on roadways. Corrosion inhibitor has been used on bridge deck panels or NEXT beams which may be subjected to direct traffic without an overlay.
For P/S beams that have draped strands, are there any limitations for the force resisted by the hold down device/anchor? Also, are there any limitations for the total hold down force? For example, we have seen a requirement where the total hold down force must be equal to or less than 75% of the girder self-weight.
Most fabricators will have a hold down force limit. Typically, this is dealt with by breaking the one hold down location into two or more to stay under the fabricators limit (usually this is done by the fabricator and not by the designer).
Yes, there is a limit on the total hold down force. If the force is too high, meaning too many strands are deflected, you can suspend the beam from the strands and that is not a good situation (beam wants to raise up off the bed which is a major safety concern).
General standard practice is to design with straight strands and use debonding to control stresses at the ends. Using debonding tends to reduce end stress cracks more than draping. If a debonding pattern is unattainable based on the limits of AASHTO or the specifications, drape the minimum number of strands required to meet the criteria.
I design with PennDOT DM-4 criteria and for f'ci what range is considered when I use f'c for final of per DM-4 of 8,000 psi?
If f’c is 8 ksi, f’ci typically is approx. 80% of f’c or 6.4 ksi. However, if a lower initial strength will work, designers are encouraged to specify the lowest compressive strength that will satisfy the design requirements. This will allow the girders to be detensioned at the earliest possible time.
What are some lessons learned so far from the design and fabrication of CFRP strand (vs. steel strand) prestressed and post-tensioned members?
Two lessons come to mind. The first is that fabrication of girders using CFRP strands is generally more challenging because stressing of the material, which has a greater notch sensitivity than steel, affects how that type of strand can be gripped. The second issue is that the strand is elastic until failure, so there is no yield point and therefore no plasticity. While this differs from the behavior of conventional strand, the significantly greater ultimate strength of CFRP strand makes it possible to achieve safe designs using a different approach from designs using conventional strand. A guide specification is now available.
Several recent articles on this subject have been published in both ASPIRE, PCI and ACI journals and publications.
Do precasters have any objections to the use of fibers in precast decks and beams?
No objections at all! Depending on the type of fibers, it may get expensive. If the question is referring to fiber as a replacement for other forms of traditional reinforcement, no objection.
You shared in an ABC presentation that crushed stone backfill was used; is this standard?
On the ABC projects for which our fabricators have provided precast, it is typical for the contractor to use crushed stone backfill. We are not aware if this is special for a total precast substructure construction. We understand the backfill is the same whether it is accelerated or conventional construction.
UHPC for Bridge Elements
UHPC and shrinkage impacts?
The impacts of UHPC shrinkage can vary. The early age autogenous shrinkage of some UHPC’s can be very high (500-900 microstrain) therefore formwork or other restraint should be avoided during curing when possible. Drying shrinkage in service is much less than conventional concrete.
If UHPC has no shear reinforcement, will it be able to resist centrifugal forces and impact load due to cars, trucks, etc.?
While UHPC does not have mild reinforcement, it does have internal reinforcement in the form of the steel fibers. The current research being performed is showing that the steel fibers provide sufficient tensile capacity to resist shear. This same capacity would be available to resist impact loads as well.
For the hybrid structure, did you have to cast the traditional concrete first? Concern would be bonding between the two dissimilar materials if UHPC was cast first.
Yes, the conventional concrete was cast first in a trial girder that was manufactured for FDOT. The interface is the primary research item, and it was found that the bond across the joint between materials was very good and could not be made to fail. A summary report and final report are available on the testing (see https://www.fdot.gov/research/documents.shtm):
BDV31-977-101 Hybrid Prestressed Concrete Bridge Girders using Ultra-High Performance Concrete Summary [245KB] Final Report [15.6MB]
The bond of UHPC to a roughened and prewetted (SSD) existing concrete surface is well documented. The strand and/or mild reinforcement that crosses the interface also contributes to the behavior of the hybrid product.
For earthquake provisions, shear is needed for most of the concrete materials. Will UHPC be able to resist seismic lateral loads and load reversal during a seismic event, or earthquake?
There is ongoing research regarding the use of UHPC for seismic applications. Results show that the UHPC performs better than standard reinforced concrete with regard to flexure and shear. This shear resistance is provided by the significant amount of internal steel fibers that produce a sustained post cracking tensile strength in excess of 1000 PSI. At first glance this might seem like a low value, however, the area of the members is still significant, which produces significant tensile and shear resistance.
What is Chloride penetration of UHPC?
C1202 testing results for UHPC are typically < 300 Coulombs; many are < 100. This is magnitudes lower than conventional concretes, which is why UHPC is being studied for main members in addition to connections for ABC.
How are fabricators addressing the difference in mix costs for UHPC?
PCI producers are developing in-house batched UHPC mixes that are significantly less costly when compared to field cast proprietary UHPC mixtures. The costs are in the range of two to three times the cost of normal plant produced concrete and a fraction of field placed UHPC. With UHPC, we can eliminate almost 50% of the concrete and most of the web reinforcement (including the labor to install it). Based on this, UHPC beams may very well be cost effective based on first cost. Life cycle costs are even better, since UHPC is much more durable than conventional concretes
Precast Foundations
With precast foundations, how do you ensure full and or uniform bearing?
If the subgrade bearing surface is aggregate, contractors have different methods of ensuring optimum compaction and a high standard of care for the final grading. More often, the subgrade system is prepared low, and either with shim stacks or levelling devices, precast foundations are set to proper elevations and flowable material is pumped through openings in the foundation until it flows out all sides ensuring full and uniform bearing. Early ABC (Accelerated Bridge Construction) projects used non-shrink grout for this fill. This is not recommended as it is very expensive. Typical bearing pressures under footings are less than 1000 psi. Therefore, there is no need to use a high strength grout for this purpose. The more common material now in use is flowable fill, which flows easily, is much less expensive, and has adequate capacity.
NEXT Beam
Will there be preliminary charts added to Chapter 6 for the NEXT E-beam and voided slabs?
PCI Northeast has provided preliminary design charts for NEXT F, E, and D sections in the 2nd Edition Northeast Extreme Tee (NEXT) Beam Guide Details. These can be downloaded for free on www.pcine.org under Bridge Technical Resources.
Piles
Do you know when PCI Bridge Design Manual Chapter 20 - Piles will be published?
The Pile Committee is working on this currently. They recently completed work on the Recommended Practice and a Standard which encompasses all types of projects (buildings, marine, and bridges). The piling recommended practice may be found here.
Camber
What is the typical variation in camber for precast girders?
This varies based on the prestress force to design the element. The more prestress the potential for higher camber. PCI MNL-135 Tolerance Manual addresses Camber Variation from the Design Camber. You should also speak to the fabricator about camber and the anticipated variation. It is good to provide details that allow variation in camber to be addressed in construction.
Should deck slab shrinkage effects be considered in camber calculation?
Deck shrinkage does have a small effect on the camber of girders and could be considered when estimating cambers. However, compared to the potential variability in the other quantities that can affect the camber, this particular effect would be very small and is typically neglected.
Girder Repair / Fabrication Issues
As a bridge inspector I see a lot of prestressed beams voided slabs and also boxes that have the end corners cracked and spalled. I assume it is being caused by the stresses - how important is this? I know in some ways it may push the stress blocks out the beams.
Spalls on the bottom corners at ends of prestressed beams or boxes are usually caused by camber and sliding at detensioning. This is more likely to be a problem when beam ends are skewed. These small spalls can be easily repaired.
For additional information, please download MNL-137 Manual for the Evaluation and Repair of Precast, Prestressed Concrete Bridge Products. Note there is a charge for this publication.
You may also download for free PCI Northeast “Guidelines for Resolution of Non-Conformances in Precast Concrete Bridge Elements.”
TS#13 refers to missing projected reinforcing steel. Why are steel bars missing, isn't there a QC check prior to concreting?
- Generally, TS#13 is for bars, such as those used for continuity connections that may be cut off while trimming strands at the ends of girders.
- Or broken while bending after fabrication.
What is the estimated lifespan of prestressed girders in order to be repaired or undergo maintenance works for safety?
The repairs that are recommended in the PCI repair manual are considered to be of equal quality when compared to the base concrete. If similar repairs are used for maintenance, they should also be sufficient to last the remaining life of the girder.
Should cracks at the ends be sealed when they are found during maintenance inspection?
If cracks are found during routine inspections that are in excess of the crack widths noted in the repair manual, similar repairs could be done to in service girders with similar cracks.
In this FAQ video all about Accelerated Bridge Construction (ABC), learn how precast concrete systems can revolutionize your projects and discover benefits, key considerations, and more!
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