Removing Masonry prtitons

[ttoma]

The architect requested our opinion in removing a number of masonry partitions from the second floor of a 2-storey steel framed building built in the sixties. I reviewed the existing drawings and found no lateral resisting system. The beam/column connections were shear connections, no bracings, no shear walls except for the masonry ones.
The base plates for all columns have 2 bolts.
Some of the partitions to be removed are only in the second floor.
I assumed all the columns are fixed. Checked whether they can resist the lateral loads if they act as cantilevers. Analyzed the building and found that the building would conform to the current code. The base plates can resist all of the forces.

Question: can you assume columns to act as cantilevers to resist lateral loads?

 

[JAE]

You are assuming that columns are fixed which have only two bolts? Not sure I buy that one.

 

[hokie66]

As it is a two storey building, I take it the base plates with two bolts are at the ground floor or footing, and the columns are two storeys high, going up to support the roof structure. If my assumption is correct, the columns are propped cantilevers with pin supports at both floors. Whether or not the columns are adequate to resist the lateral loading is for you to analyze. Deflections will be much greater for propped cantilevers than for fixed bases.

Then again, I have assumed that there is a lateral load resisting system at the ground floor level. Is that correct?

 

[archeng59]

before assuming the column bases are fixed, check the anchor bolt embedment and verify the foundation can resist the moments induced by the fixed column bases. The baseplates might be ok, but the anchors might not have adequate embedment and the footings/piers might not be reinforced or sized to resist the moments.

my initial impression would be to keep the masonry walls. Determine if all of the walls are needed and if not, how much of the walls can safely be removed?

 

[JLNJ]

It seems unlikely that the building would ever sway enough for the columns' cantilever action to be developed. It sounds like you have masonry shear walls whether you analyze it as such or not.

 

[csd72]

I agree with JLNJ. You have rigid ,masonry walls that will take the majority of the shear up until the point that they fail.

 

[ttoma]

Thanks all for the help.
JAE:
I assumed a 2-bolt base plate has a moment capacity equals to the force in anchor bolt/compression block times the distance between center of bolt to compression block.
Hokie66:
Your assumption is correct. We are not touching the structure in the first floor.
JLNJ and csd72:
What I am worried about is the effect of removing the masonry walls in the second floor, which changes the configuration of the lateral resisting system. When you say that enough sway will not happen to prompt a cantilever action, is it correct even when all the masonry walls in the second floor are removed?

 

[csd72]

ttoma,

I you remove ALL the masonry walls on the second floor then what you say should work.

I have designed buildings similar myself.

But keep in mind that the cantilever action will amplify the reaction at second floor. You will therefore need to check the connections,diaphragm and the lower walls for the additional loads.

csd

 

[JAE]

ttoma,

I just can't believe that a whole building can be laterally braced with only two bolt connections at the base of some steel columns with no beam-column moment connections.

Yes, you can certainly use the method you describe to get some sort of max. moment capacity, but remember that you probably don't know how deep the anchor bolts are embedded, the bolts are probably A307/A36 type bolts (not high strength) and the stiffness of these columns has to be pretty low - thus allowing very high lateral deflections in the building.

 

[csd72]

JAE,

From the post, I am assuming that they are double floor height columns and the two bolts are at the ground floor. The column is then a continuous cantilever supported at the second floor level.

The two bolts at ground floor then act as a pinned connection.

csd

 

[JAE]

csd72 - Ya, I thought of that too, but the original post says there was "no lateral resisting system" in the entire building...thus I assumed (maybe incorrectly) that that meant there were no walls below the second floor.

 

[hokie66]

csd72 makes a good point about the amplification of the loads due to the columns being cantilevered above the second floor. Could result in twice as much load delivered at that level compared to system with shear walls above.

 

[msquared48]

I agree with JAE too here. Definitely pinned base.

In addition though, if the CMU walls do prove to be shear walls, then would you not be creating a "soft story" laterally by removing them?

Mike McCann
McCann Engineering

 

[hokie66]

Yes, but the soft storey is the topmost storey. Nothing wrong with that arrangement. Problems arise with seismic forces when the soft storey is an intermediate or bottom floor.

 

[msquared48]

OK. I misread the post and thought this was more than two stories.

Mike McCann
McCann Engineering

 

[haynewp]

I would expect the column bases will be pretty soft with 2 bolts and a footing not meant for moment resistance. Have you tried to figure the amount of deflection at the roof level using the fixed base column system? The column base will rotate a decent amount to engage the 2 bolts (which is a PR connection) plus the footing to soil interface will rotate. I try to stay away from fixed base columns. You might actually get more lateral resistance out of the beam to column "shear" connections (which you don't want) than the 2 bolt base connections. The existing beam shear connections will see some stress due to the lateral while waiting for the soft base connection to stop the building from deflecting.

If you actually do have shear walls still at the first floor and do a propped cantilever column system, then you should be focusing on the horizontal reactions at the first and second floor level and not justifying a fixed base condition.

 

[DaveAtkins]

If you can trace all the forces to the soil, you can feel comfortable with what you are doing. A two bolt base plate CAN take some moment (but the footing needs to take the moment as well). But even if the column bases are assumed to be pinned, they can cantilever above the Second Floor (if each column can take the combined axial force and moment).

But how is the Second Floor framing attached to the First Floor CMU walls? The lateral loads cannot magically get into these shear walls.

DaveAtkins

 

[jike]

The answer is as Dave stated. You must be able to trace the loads to the foundations. The connections of how the structure will transfer the loads to the diaphragms and then to the walls and then to the foundations are the key. Do the walls and their connections have the capacity to resist these forces?

 

[haynewp]

You have to also look at the deflections. It is unclear to me if there are shear walls at the first floor or not.