Shrinkage & temperature reinforcement in thick members

[TpaRAF]

I need some insight on the AASHTO LRFD Bridge Design specs.

Section 5.10.8.2 states shrinkage & temperature reinforcement should be 0.18% of the gross sectional area for members less than 48" thick. The AASHTO LFD code required 0.125in² per ft of surface.

I am designing a bent cap that is 42" square. The LRFD code requires 3.18in² temp steel, while the LFD code requires only 0.44in² - this is a BIG difference.

The AASHTO LRFD commentary cites use of ACI 318. In that code, the discussion is limited to slabs only (§7.12).

My inclination is to use the smaller value, but I cannot justify this decision within the limits of the LRFD code.

Thanks in advance for your help!

 

[TTK]

I am not familar with the LRFD provision, but I think you need to multiply the 0.44 in2 number by 4 to get the required steel for all sides of the cap, not just one side.

This still only equates to 1.75 in2 of T&S steel, and without the LRFD code in front of me I can't think of where the difference might be....

Anyone know the logic behind this LRFD provision?

 

[TpaRAF]

TTK -- you're right, the LFD requirement would be 1.75in² for four faces. Conversely, 0.80in² per foot length is required by the LRFD criteria.

This would be #4 ties at 3", or #5 ties at 4½" -- much more than needed for the shear requirements.

Thanks, RAF

 

[bridgebuster]

I went through the same thing four years ago; my client agreed to use the LFD provisions for temperature reinforcement. At the time, we were designing four integral abutment bridges and found that the temperature steel in the substructures far exceeded what was needed for flexure. I spoke to one person who worked on the concrete portion of the code, he intimated that there were some bugs with respect to substructure design.

 

[TpaRAF]

BridgeBuster :


I concur the LRFD results seem excessive. The "break even" point occurs around 24" thickness.A square member eventually requires double the reinforcement at the 48" limit. Below the break pt, the LRFD requires slightly less. (let Ag = s²; plot 4s vs. 0.0018Ag)

Any idea if anything has been published or posted regarding this individual's comments?

Thanks for your help! RAF