Existing Building Analysis Concrete Flat Plate

[craigory28]

Hello:

I'm currently analyzing an eight story building that they want to expand one floor.

The existing structure was built in 1960 in Wisconsin. It is a two way flat slab with drop panels and column capitols. We were checking to see if it was ok for seismic loads. (We have site class C and it turns out the loads are less than wind.)

However I can't seem to get the building to work for gravity loads and especially wind loads. The building has several expansion joints and the area I'm looking at has no stair or elevator towers. Therefore I'm using frames with the columns and flat plates.

The problem is that the flat plates do not give a lot of rigidity. I'm getting very high K values which makes my Pc exceed the existing column load. (Without the weight of the addition!!!) I'm using the slab length from face of capitol to face of capitol for my EI/l calculation. I'm using the width of my drop panel for calculating the I of the slab. I'm using an average depth of the slab as well.

For the most part I'm coming up with memebers that are 20% overstressed. I know I can have the concrete checked and it probably has more capacity. However I do that it will increase my E for the columns and thus increase my k. (Original plans show 3000psi for floor. 5000 psi for columsn. I assumed rebar is 40000psi for the time.)

Here is my questions:
1. Has anyone done something like this and can give advice?
2. Is it possible that lateral load was never considered?
3. Is it just silly to try to get a flat plate to work in this application?

Thanks for any help you can give me.

Craig

 

[Joecandraw]

Craig call the original design team if they are still around and ask them for there design cals.

 

[craigory28]

The building was designed 45 years ago. They are long gone.

 

[VoyageofDiscovery]

Are your frames infilled?

Are you checking it with the old wind loads?

VOD

 

[JAE]

You mention that your building has a number of expansion joints. In this case, are you applying BOTH windward and leeward wind to the "portion" of the building you are checking? For buildings with expansion joints, many times the portion of building being designed does not see both....just a thought that might reduce the wind on your model.

In 1960 it could well be that the wind loads used were smaller than what is now applied. However, you must use CURRENTLY applicable wind loads from the governing building code. You cannot use old code wind.

You may just have to notify the owner that the current building frame does not meet code and add infill shearwalls or some other sort of bracing. Perhaps the bracing is only required in the lower floors.

Another thought - perhaps tying the building together across the expansion joints could be considered. Most of the concrete shrinkage has already occurred and it could be that there are too many joints than truly necessary.

 

[J1D]

Eight story building to have one story expansion doesn't seem to be a big problem. I once involved in an expansion of a flat-plate parkade, three story existing, to add three stories!

But eight story flat-plate without shear core (stair well, elevator core, etc), shear wall, or bracing in seismic zone appears a bit odd to me. In this case, the bending moment in lower columns is definitely a critical factor, the beams (flat-plate) will have significant bending as well. The 20% overstress may be a hint that the structure is not modeled or calculated the same way as the original designer did.

The three story parkade was successfully expanded with steel bracing all the way up in a bay (to reduce the column load from lateral load) and using steel and metal deck for the new portion (to reduce the gravity load).