4 Story Wood Over 1 Story Steel Podium

[LoneStarEngineer]

I am working on a Hotel project which has a 1 story steel podium (Non-composite slab) and 4 stories of wood above it located in California (Seismic Controlling).
We have masonry shear walls in the building which we want to use as our main LFRS. I have a few concerns about this structure:

1. If using the Two stage analysis approach, how do you impose the reactions of the flexible upper portion down to the rigid lower in a computer model. Base shear can be done but how would you impose the torsion and the overturning effects from the upper portion. What tools/software do you use to analyze the rigid lower and model the reactions?

2. I am having doubts on whether a 3 1/2" thick non composite slab podium would even work. Can someone please shed some light here if you have come across this type of setting.

3. Is using the masonry shear walls to resolve all of the lateral a good idea? Or would it be better to use the wood shear walls as well? Again I am more concerned about the connection to the non composite slab.

4. Is there a good example of these type of structures or any good references? I have the example from WoodWorks (5 Story wood frame over Podium) which I think is really good but I am looking for more clear explanation of the link between steel podium and wood and how the forces are resolved.

5. Any other important factors/suggestions that need to be taken into consideration?

I am new to seismic and this is my first wood over podium building so need some advice on how these type of structures are analyzed in other offices.

Thanks.

AM-EIT

 

[RFreund]

I'll do my best to give you some responses, but I'm not sure I can get a full scope here...

1. If using the Two stage analysis approach, how do you impose the reactions of the flexible upper portion down to the rigid lower in a computer model. Base shear can be done but how would you impose the torsion and the overturning effects from the upper portion. What tools/software do you use to analyze the rigid lower and model the reactions?

I'm assuming that you are using wood shear walls for the tower and masonry shear walls at the first floor for this situation, but I suppose it doesn't matter. You would perform you lateral analysis on the tower ignoring anything below. You get your reactions at the base of your shear walls - you have point loads up/down at your chord studs and a shear along the wall. Now factor your reactions per the 2-stage recommendations and apply those loads to a podium model. You will need beams directly below your shear walls. The overturning (up/down point loads from the chord studs) will be taken out "locally" by the beam and columns (check connections). The shear is transferred to the diaphragm. You may need to add collectors and/or treat the beam as a collector (axial load). Usually the beams are composite so there is a pretty solid load path from shearwall to beam to diaphragm.

2. I am having doubts on whether a 3 1/2" thick non composite slab podium would even work. Can someone please shed some light here if you have come across this type of setting.

I have come across it. I don't have any great references here and I do wish there were better checks. However, if you have composite beams and you think of it as a flexible diaphragm you could treat it the same was as a wood/flexible diaphragm. Design the beams as chords/collectors and check the allowable shear of the deck based on mfg's tables.

3. Is using the masonry shear walls to resolve all of the lateral a good idea? Or would it be better to use the wood shear walls as well? Again I am more concerned about the connection to the non composite slab.

If you can use the masonry shear walls only, it's probably best. Just avoids lots of details (hold downs, penetrations in shear walls, etc). However usually the layout does not support this as you can only cantilever a wood diaphragm (or any diaphragm) so far. If you use wood shear walls the connection from holddown to beam is typically a small HSS stub cantilevered up from the beam which has a plate flush with the top of the slab that the hold down is welded to. The bottom plate can be anchored to the slab with post installed anchors to transfer shear.

4. Is there a good example of these type of structures or any good references? I have the example from WoodWorks (5 Story wood frame over Podium) which I think is really good but I am looking for more clear explanation of the link between steel podium and wood and how the forces are resolved.

The one you have is the one that I'm aware of. I'd be interested to hear if there are others.

5. Any other important factors/suggestions that need to be taken into consideration?

MEP coordination - penetrations in the top/bottom plate, in the chord studs, pipes that cannot penetrate the steel beams below the walls.


EIT

http://www.HowToEngineer.com

 

[LoneStarEngineer]

Thanks Rfreund. That was some really good information. We do not really have a great layout of the shear walls here. So we are going to add some braced frames on the first floor.


Assuming a Flexible diaphragm is killing my Braced frame and foundation. I may just have to go with a rigid diaphragm and take most of the shear in to the shear walls.


While I can picture the connection here, I did not get where the "Post installed" anchors are fitting in the detail. Would you mind elaborating a little bit.

Thanks.