## ETABS 18 underestimates/overestimates steel columns/beams compressive strength (AISC 360-10)?

## ETABS 18 underestimates/overestimates steel columns/beams compressive strength (AISC 360-10)?

(OP)

Hi, everyone.

Title says it all. I checked the axial compressive strength of a compact W-shape column/beam in ETABS 18.1.1 and compared it to manual calculations based on AISC 360-10 Section E3 and E4. It seems that ETABS calculates the compressive strength differently than AISC 360-10 even though the design manual clearly has the same formulae as AISC 360-10.

I've checked the flexural, shear, and tensile capacities reported in ETABS and they are equal to those I've calculated manually. Only the compressive strength results given in ETABS are different.

I've followed the calculations exactly and changed the input parameters accordingly (K-factor, unbraced length, etc.) and the results are always different no matter what dimensions I use. Is anyone facing this issue as well or am I missing something?

Title says it all. I checked the axial compressive strength of a compact W-shape column/beam in ETABS 18.1.1 and compared it to manual calculations based on AISC 360-10 Section E3 and E4. It seems that ETABS calculates the compressive strength differently than AISC 360-10 even though the design manual clearly has the same formulae as AISC 360-10.

I've checked the flexural, shear, and tensile capacities reported in ETABS and they are equal to those I've calculated manually. Only the compressive strength results given in ETABS are different.

I've followed the calculations exactly and changed the input parameters accordingly (K-factor, unbraced length, etc.) and the results are always different no matter what dimensions I use. Is anyone facing this issue as well or am I missing something?

## RE: ETABS 18 underestimates/overestimates steel columns/beams compressive strength (AISC 360-10)?

## RE: ETABS 18 underestimates/overestimates steel columns/beams compressive strength (AISC 360-10)?

## RE: ETABS 18 underestimates/overestimates steel columns/beams compressive strength (AISC 360-10)?

So, I only checked for Flexural Buckling (Section E3) since the torsional unbraced length is equal to the lateral unbraced length and therefore flexural-torsional buckling (Section E4) is not the controlling limit state.

Based on my manual calculations, the axial compressive reduced strength is 10869 kN while ETABS reports the compressive strength to be 9937 kN, quite a difference. This report is for a beam, so all the K-factors are taken as one.

I've attached the manual calculations and ETABS report below.

Edit: Had to remove the license information from the report for reasons.

## RE: ETABS 18 underestimates/overestimates steel columns/beams compressive strength (AISC 360-10)?

So are you sure your calculations are considering the same effective lengths for compression buckling?

If the L factor is the ratio of full length to buckling length, then 0.936 x 20000 does not equal 18000?

## RE: ETABS 18 underestimates/overestimates steel columns/beams compressive strength (AISC 360-10)?

Yes, the beam span is 20000mm in both ETABS and my hand calculations, and since there were columns at each end, the unbraced length in the major axis is taken as 18800 mm which is equal to 0.939 x 20000 in ETABS. Maybe I should've used a more accurate value of 18780mm for the major axis unbraced length, but the difference between 18800 and 18780 is quite small to have any significance in my opinion.

So yeah, I've used the same unbraced lengths in ETABS and my manual calculations for both the major and minor axes.

## RE: ETABS 18 underestimates/overestimates steel columns/beams compressive strength (AISC 360-10)?

If you do the hand check at 20000mm do you get closer to the Etabs value, maybe it is neglecting the reduction in length for some reason?

## RE: ETABS 18 underestimates/overestimates steel columns/beams compressive strength (AISC 360-10)?

Yup, I tried that as well, but the reduced compressive strength in my hand calculations is still about 10759 kN, a little bit closer but still far from the value of 9937 kN reported by ETABS.

I'm beginning to wonder whether there are other limit states I should consider. I'm using an I-shaped welded beam which is doubly symmetrical, so I believe only section E3 and E4 applies.

## RE: ETABS 18 underestimates/overestimates steel columns/beams compressive strength (AISC 360-10)?

## RE: ETABS 18 underestimates/overestimates steel columns/beams compressive strength (AISC 360-10)?

One thing Etabs might be doing is a buckling check, which is coming up with a different effective length that allows for the stiffness of interconnecting members making the K factor effectively higher.

Similarly it may be using the direct analysis method (notional loads), your output suggests this with the 0.8 factor noted on stiffness? Therefore could be working back from the combined actions check to the maximum axial load that can be carried by the system.