Seismic Load Effects

[Structures33]

Hey - I know this has been in several threads in the past and after reviewing those, I still have questions. I am too wrapped up in the details and I can't see the big picture anymore.

I understand that E is broken into horizontal and vertical for the load combinations and that we will have different E values for each level. (which means different load combinations for each level??)

A hypothetical (I'm wanting to know what is right/wrong in this scenario) -

So let's say we have a 1-story cmu square building with wood roof trusses. We will calculate base shear, V, and corresponding F1 value for the top of the wall/truss connection. Is this used for design of shear walls, diaphragms, connections (what do we use this to design vs. the load combinations)?

Then, we will need to calculate QE(axial) - the axial effect of V (or Fi) on the individual component, QE(shear) -the shear effect of V on the individual component, and QE(moment) - the moment caused by V on the component. (Is it possible that we won't have all of these effects?)

Then, for Qv, we need the D values - the vertical effects. So we use the dead load of the structure at the selected floor/level and we come up with D(axial), D(shear) and D(moment).

Then these are put into the load combinations - so each set of combinations will have 3 versions.

Example of combinations (strength taken from ASCE 7-05 12.4.2.3))-
(1.2+.2*Sds)*D(axial) + rho*QE(axial) + L + .2*S

(.9-.2*Sds)*D(axial) + rho*QE(axial) + 1.6*H
AND
(1.2+.2*Sds)*D(shear) + rho*QE(shear) + L + .2*S

(.9-.2*Sds)*D(shear) + rho*QE(shear) + 1.6*H
AND
(1.2+.2*Sds)*D(moment) + rho*QE(moment) + L + .2*S

(.9-.2*Sds)*D(moment) + rho*QE(moment) + 1.6*H


thanks for any help.
-dazed&confused...

 

[JAE]

I think you have the general idea correct - but dividing up the load combinations for shear, moment, axial is not really the way to visualize it.

The loads can be combined as you list them - such as the first two you list...but without the (axial) or whatever qualifier.

Just analyze the structure under each load combination. Each combination will produce all kinds of shear, axial, moment effects in members. Design for the worst effect.

I would agree that you take your F1 and apply it to the roof....follow the design through the roof diaphragm (but check for the code minimum diaphragm shear) and then into the collectors (using the overstrength factors on connections, collectors, etc.).

Then into the shear walls and to the footings.

 

[Structures33]

JAE - Thanks for the response. So it sounds like you're saying whole axial, shear, moment effect comes into play after the LC is applied - which is what we normally do for any kind of design - I was thinking this application was different. So the only difference is that we are dividing the E into horizontal and vertical to contribute to the LC. Then we take the individual results of the LC's and design according to the critical.

Here's why I'm confused about the axial, shear, moment issue -- IBC 2003 Commentary provides an example table showing what we need to compile for E values (section 1617.1.1.1). This table shows the combinations broken up into axial, shear and moment like I showed in the first post. How is this table used??

Thanks

 

[JAE]

I don't have that commentary - not sure what it is saying so can't help you there.

Just remember that E in the load combinations means:

"all the loads applied to your structure, whether vertical or horizontal".

So you were correct in including .2*Sds*(dead load) in a combination wherever E shows up in a combination. It is a vertical seismic effect, E, that is calculated using a portion of the vertical dead load.

In our math we lump it with the dead load, but it should be factored by 1.0 (since it is an E) and not factored by 1.2, 0.9, etc. (since it is not a D).

If you could post some of that table, perhaps all of us here could respond to it.

 

[Structures33]

Ok I've attached the page from the commentary. The table is at the bottom of the page.
Thanks

 

[JAE]

Yes, I think this is just an attempt to describe what I stated above. The seismic load effect E is not just a horizontal load but an effect on the structure that includes both vertical and horizontal components and both of these components affect axial, shear and moment in a frame's members.