Why doesn't STEEL determine the maximum stresses in the calculation of load combinations?
In fact, it may occur for biaxial loading that the combined internal forces do not result in the maximum stresses.
Since this problem is known, the option "Calculate stresses from single load cases in RC and then combine stresses according to RC criteria" has been implemented in STEEL 8.xx.
You can set this option among the "Details" available in the add-on module STEEL.
Here is a short example:
Single-span beam I 200, 5 m long (restrained in X)
2 Load cases
LC1 --> direction Z (variable) line load 1 KN
LC2 --> direction Y (variable) line load 1 KN
1 Load combination
Now, the load cases are combined in the results display for CO1.
Thus, in case of I=2.50 m you get the maximum/minimum internal forces for M2 and M3.
MAX M2, L = 2.5 m, 3.13 KNm LC1
MIN M2, L = 0 m, 0.00 KNm
MAX M3, L = 0 m, 0.00 KNm
MIN M3, L = 2.5 m, -3.13 KNm LC2
So, the stress analysis for the calculation of stresses from the internal forces of the CO would have the following result:
Sigma total, 12.04 KN/cm2, (design internal force only from MaxM3 CO1)
Tau total, 0.22 KN/cm2
Sigma v, 12.04 KN/cm2
And for STEEL 8.xx, the stress analysis for the calculation of stresses from the individual load cases of the CO would have the following result:
Sigma total 13.5 KN/cm2 (internal forces from MaxM2 and MaxM3 CO1)
Tau total, 0.27 KN/cm2
Sigma v, 13.5 KN/cm2
We assume that STEEL calculates the stresses from the internal forces of the CO first and combines them subsequently.
In this example, the different internal forces of the CO come from different load cases, which can be seen in the numerical output of the results (MAXM2 for LC1 and MINM3 for LC2).
Hence, the calculated internal forces in the results listing are found in different rows because they can occur or not occur. Thus, they must not be combined as otherwise further problems would occur for the combination.
STEEL 8.xx, however, is able to perform the individual designs for M2 and M3 but cannot combine these stresses as they cannot occur for STEEL at the same time because they come from different rows of the numerical output.
Unfortunately, as far as the program is concerned, there is no other solution. Therefore, a check by the user is required. This is the reason why the alternative to be set in the detail settings has been implemented.