AASHTO LRFD Prestressed Concrete Allowable Tensile Stresses

[Brandon]

Question about the following provision of the AASHTO LRFD Bridge Design Specifications:

Section 5.9.4.2.2 - Tension Stresses

Table 5.9.4.2.2.-1 - Tensile Stress Limits in Prestressed Concrete at Service Limit State after Losses, Fully Prestressed Components​

Other Than Segmentally Constructed Bridges--> Tension in the Precompressed Tensile Zone Bridges, Assuming Uncracked Sections​

Referring to the bolded portions above, this seems to say that this check only need be made at the bottom of the beam location (i.e. the Precompressed Tensile Zone for typical prestressed beams). However, it is conceivable that tension could occur in the top of the beam at the ends at service load, but this is not the "Precompressed Tensile Zone", and therefore the tensile stress check would not apply to the ends of the beam.

Is this generally how others have interpreted this provision (i.e. only check service load tension at the bottom of the beam)?

 

[BridgeEI]

To me, the precompressed tensile zone refers to the concrete beam itself. If you have a PCB that is made continuous, the top of the beam will be in tension with the Service III load combination (dependent upon span arrangements). You would want to check the tension stress there.

My interpretation anyways.

 

[Brandon]

BridgeEI, except that there are "zones" of the beam that are in compression. Why would they specifically call out a certain zone in a very specific manner if they intended the check to apply to the entire beam?

 

[BridgeEI]

So you have a PCB that you're designing. You apply a prestressed force to it. The entire beam is now in axial compression. When the beam is set, deck is placed, and traffic is on it, somewhere along the beam it will be under tension. Whether that area is at the top or bottom will depend on the continuity. (Just trying to make it clear to myself..)

What I think AASHTO is trying to say is that in prestressed members subjected to tension, the allowable limit is a certain value. They don't want someone to apply this provision to a regular reinforced concrete section that doesn't have a prestressed component.

I don't have the code in front of me, but I think they probably say something similar about the compression limiting stress.

 

[1ManWolfpack]

Hey guys, also a bridge EI, first time poster...

Brandon: I also interpret it the way you do, based on AASHTO's definition of the Precompressed Tensile Zone: Any region of a prestressed component in which prestressing causes compressive stresses and service load effects cause tensile stresses. So in most cases, yes, just the bottom of beam.

BridgeEI: Don't forget about the eccentricity of your prestressing strands. Generally, they are located at the bottom of the beams, causing tension at the top. Also note, the table 5.9.4.2.2-1 is for prestressed concrete components, so hopefully these provisions aren't being applied to a regularly reinforced section!