Earthquake Loads applied to Moment Frames 1

[Calif]

Lets say you have moment frame that has 4 bays with different spacing and is 2 stories high. Earthquake loading is applied using the static equavalent loads to each story of the frame. Considering that there is unequal of the column spacing, are you to only apply the force along the same plane of resistance in one direction as a single load case or should the resistance be applied in both direction as two load cases. To illustrate what I mean, if your moment frame is along the x-axis, do you apply the earthquake forces in the positive x-axis only or in the positive and negative axis to determine the critical moment at each corner of the moment frame. The loads in both direction are not applied at the same time but as different load cases and considering the moment frame has unequal column spacing.

Thanks

Calif

The resisant virtues of the structure that we seek depend on their form; it is through their form that they are stable, not because of an awkward accumulation of material. There is nothing more noble and elegant from an intellectual viewpoint than this: to resist through form. Eladio Dieste

 

[msquared48]

If the scenario is symmetrical, one direction is sufficient. Not so if unsymmetrical.

Mike McCann
McCann Engineering

 

[Calif]

Do you how to get ETABS to do this? I am not sure if ETABS does this for irregular spacing. It applies the earthquake loads in the X and Y direction but I do not know if it applies in the opposite direction of the axis.

Thanks

Calif

The resisant virtues of the structure that we seek depend on their form; it is through their form that they are stable, not because of an awkward accumulation of material. There is nothing more noble and elegant from an intellectual viewpoint than this: to resist through form. Eladio Dieste

 

[Lion06]

I would do it in both directions. Even uneven floor loading (maybe 50 psf LL on one side and 150psf LL on the other side) can cause larger moments at one end.
I don't know anything about ETABS.

 

[Calif]

What software do you use at work StructuralEIT?

The resisant virtues of the structure that we seek depend on their form; it is through their form that they are stable, not because of an awkward accumulation of material. There is nothing more noble and elegant from an intellectual viewpoint than this: to resist through form. Eladio Dieste

 

[Lion06]

We use RAM Frame for lateral analysis. It generates somewhere in the neighborhood of 180 Load Combinations. That is not a typo, there are actually around 180 Load Combinations with all of the Wind cases and + or - for each of the lateral cases.
I don't know how anyone does this by hand or even with a computer having to manually generate all of the possible load combinations. It really is kind of crazy!