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!

 

[UcfSE]

Is that per ft^2 of surface? (42/12)^2*.125=1.53 in2
Even with that the answers are pretty different.

 

[TpaRAF]

The sectional area is in square inches.
LRFD: 0.0018*(42in)^2 = 3.172in²
LFD : 0.125in²/ft * 3.5ft = 0.438in²

 

[yack]

Standard Spec's requires that the steel required in flexure be at least 1.2X that of the uncracked section. This makes more since than the ACI Code, and the section you mentioned above is often misinterpreted by plan checkers. I usually cater to their comments, no matter how misguided.

So far as your bent cap goes, put in the extra steel, it's improbable that you will make a cost savings that is worth fussing over. Can't really say since you didn't provide any particulars.

 

[TpaRAF]

The longitudinal reinforcing bars meet the 1.2Mcr strength req'mt as well as serviceability. They more than meet the s&t reinf requirements in that direction.

Additional s&t reinf is needed perpendicular to the primary bars. Stirrups or ties are used for both the shear and the s&t requirements.

This section of the code calls for 3.18in² per foot of length! This is equally distributed to four faces, so #4 ties at 3" or #5 ties at 4½" spacing are needed.

 

[yack]

The minimum is not for the transverse direction.

You would get better responses from someone in the Bridge Engineering Forum. I have not had the privelage of designing bent caps of any substantial size and have not used AASHTO LRFD.

Think: 1) shrinkage cracks occur over large areas due to shrinkage (volume change incompatibility) there is release at corners/edges, 2) For the reinforcing to control cracking it has to have the necessary development length.

Based on the above, as far as T&S in the transverse direction of your beam I would say #4@18" minimum. You may want to throw in some extra, so it looks extra nice, with very little cracking.

Cheers, it's Friday.

 

[TpaRAF]

Greetings, Yack !

Thanks for your advice. The AASHTO LRFD code is calling for the s&t reinforcing in each direction, distributed equally.

The #4 at 18" would provide only 0.133in² (LFD) or 0.533in² (LRFD) -- well under the required values of 0.44 or 3.18 in². Yet we have used #4 at 12" for years, as standard practice.

And that's why I'm seeking help with the design code!!

I wasn't aware there was a bridge forum. Thanks for pointing me there.

Regards, RAF