Did You Know That… Srss Orthogonal Direction Combination Can Be Waived?

If you’re having trouble making the design or drift work because of the SRSS seismic load combination shown below, well good news is you can waive the said requirement.

SRSS = SQUAREROOT [SPECX^2 + SPECY^2]

What am I talking about? Let’s break it one by one.

When the code specifies seismic load E in a load combination, it seems that it is a very straight forward E where you just substitute a singular value and voila, an ultimate load.

Trouble is, it’s not that simple because E may mean either of the following:

Vertical component added/subtracted to the selfweight;

The seismic load for accidental torsion;

The seismic load for drift;

The seismic load for drift waiving period limitations;

For design in seismic zones D, E, and F;

Overstrength for design of discontinuous LFRS; and

For orthogonal seismic load consideration. Let’s focus on this one.

As per code provisions, a single direction of action be it X or Y is not enough hence it is imperative that a certain percentage of earthquake loads at a certain direction needs to be added to the earthquake load on its orthogonal counterpart. This increases torsion to the LFRS which it must safely resist.

Section 1633.1 of UBC 97 states that two methods may be used to account for this:

The 1+0.30 rule; or the

SRSS method

The first method would require you to have 32 combinations (remember that we have positive and negative eccentricities hence the 2 cases in one direction). Multiply these with the different number of dead and live load combinations which they will be added to.

Ortho Load Combo 2 by yours truly

For the second method however, we only need to define an SRSS load combination or load case. The effects of spectral seismic loads will be combined statistically.

SRSS in load combo Cropped by yours truly…

SRSS in load case … including this one

But why the need to waive the second method?

Compared with the 1+0.30, the SRSS method gives generally larger forces. Of course this may not always be true locally and globally so I’ll leave it to you to confirm this. I’ve had some projects before where there is a difference in the applied forces and the design using SRSS gives an O/S but the 1+0.30 still has some allowance left.

This is indispensable and probably the last option when you can no longer amend the current framing nor reduce the applied loads.

And what is my basis for saying that this can be waived? Read 12.5 Direction of Loading of ASCE 7-10. You will not find any provisions for SRSS. If this was disregarded in the newer code, then it might be because they found out that SRSS is way too conservative that it is very uneconomical compared to a probably more realistic 1+0.30.