Wind/Seismic Load Application and Torsional Analysis

[jsc43]

It seems that RISA-3D applies the Wind and Seismic Loads at the center of gravity of the deck.

Does it calculate the center of rigidity as well?
And if so, does it look at the torsional effects on the system if the the two points are not the same?

 

[JoshPlumSE]

RISA is a Finite Element based analysis. Therefore, the stiffness of the system is developed solely through the matrix based mathematics used to solve for your forces and displacments.

The "center of rigidity" is therefore inherently accounted for in the analysis. It's never directly calculated or reported. But, it is reflected in the final results.

The best way to understand this is to model it yourself. Set up a couple of simple example problems and you will easily see the additional torsional deflection that arises from this type of loading.

 

[Indianola]

Direct torsion is taken care of, but "accidental torsion" is not. Both wind and seismic have accidental torsion in them per the new codes. These have to be modeled as a separate load case.

In the case of wind, you just apply the torsional load cases. In the case of seismic, you have to hand calc the amount of torsion, and offset the applied loads to supply that much torsion.

 

[Stazz]

I thought RISA takes care of all those things. Doesn't the program generate nodes for a 5% offset for accidental torsion? Calculate the magnitude of these torsions? and generate the load combinations accordingly for these cases.

 

[JoshPlumSE]

Well, when you use the RISA seismic and wind load generators then these accidental torsion and partial wind loads are generated for you. But, that only works for "tiered" structures with rigid diaphragms at each floor level.

When you have industrial structures or such, then you might have to apply some of these code requirements by hand.