Skew Correction Factors

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sabre
Skew Correction Factors

I have a beam slab bridge with a C-I-P Deck. PGSuper appears to be applying a skew correction factor to the LLDF for moment with a formula from AASHTO LRFD Table 4.6.2.2.2e-1 for a typical cross section style (f) using the formula 1.05-0.25tan(theta); even though it is greater than 1.0. It could be round-off.

Also, the skew correction factor for shear is applied (Table 4.6.2.2.3c-1) even though two of the values (b and d) are not in range of applicability; apparently contrary to the commentary for this section. In this case it's 13%.

Rick Brice
Please post your PGSuper

Please post your PGSuper project file. My guess is, based on your other recent question, is that your bridge has a spread box beam configuration.

This is a type b cross section. The skew correction factor for type b is the same as type f. The correct equation is the one you listed.

PGSuper detects when parameters are out of the range of applicability. The user is forced to resolve this situation. Most engineers tell the software to just ignore the range of applicability. Perhaps you did this.

sabre
file

Side-by-Side slabs (zero voids).

sabre
skew correction applied regardless

I tried the various alternatives in the LLDF input window (halt, ignore, etc). The skew factor is still applied. Looks like I'll have to input the LLDF's by hand if I believe the skew factor is not appropriate. It's similar to AASHTOWare BRR; which doesn't compute skew factors.

Rick Brice
From your PGSuper project

From your PGSuper project file, I see that you have chosen to override the calculation of the LLDF with the lever rule. The software is doing this.

The skew correction factor is still used. This is by design. If you used the LLDF from the LRFD without modification, for type a, b, c, h, i, and j sections the main LLDF is computed with the lever rule. The skew correction factors are still applied. Nothing in the LRFD says to omit them when the lever rule is used. When you chose to override the other LLDF equations with the lever rule, why would those cases be different and the skew correction factors be omitted?

I can't speak as to why BRR omits skew correction factors.

If you don't want to skew correct your LLDF then using the user defined option is the best solution.

sabre
AASHTO LLDF

Given the geometry of the girders and the configuration of the superstructure, the lever rule is the only option for design in accordance with AASHTO 4.6.2.2.

Commentary in 4.6.2.2.3c indicates that "verifiable correction factors are not available for cases not covered in Table 4.6.2.2.3c-1." One the one hand, it might be better to say that some correction is better than nothing. On the other, the formula may simply not be appropriate outside the range of applicability. Especially if two values are not in range.

I will likely input my own values based on rational analysis. But as programmers, you guys should consider flagging your output when correcting LLDF's with formulas that are not in range.

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