Warehouse Renovation 1

[McQSE]

I am involved in a renovation on a warehouse that is about 60+ years old. The lateral system consists of x-bracing in one direction & a rigid truss system in the opposite direction. I attached a drawing showing the floor plan and side view showing the x-bracing locations. The drawing also shows a section. The client wants to remove the 2nd floor (dotted lines in the section view). The 2nd floor consists of heavy timber girders and joists supported by timber columns. The heavy timber girders are supported by the steel columns at the edge of the 2nd floor. I have been studying the effects of removing the timber 2nd floor and have come the conclusion that the building should be fine without it. I am still nervous about removing it though. While my analysis has shown that the building is stable without it, it has to add a lot of stiffness to the structure. Does anyone have any insight on this issue? I appreciate your input.

 

[JAE]

When you remove the second floor - you are eliminating a diaphragm across the building. This floor diaphragm may be helping the clerestory columns and also may be transferring lateral forces across the building. I'm not knowledgable to see what the floor actually does here but I'd closely review the stiffness that the floor diaphragm provides. Note that your lower roof connects to your interior columns in their mid-height. The bottom chord of the lower trusses connect into the removed floor so any lateral thrust in the lower roof diaphragms would be a lateral load on the upper columns.

 

[kslee1000]

I think it is do-able. Guess the 2nd floor could be a later add-on, if it is the case, you are free to go ahead. Otherwise, you will face significant challenge in strengthening the steel columns, and foundations. As questioned by JAE, keep an eye on the lean-to roof connections.

 

[kslee1000]

Also, this building looks like it once was for industry use, then sold and turned into a warehouse. Check out the history will help.

 

[msquared48]

Without the mezzanine, this reminds me of a steel fabricating plant layout.

Any indication of crane rail attachments to the columns or to the underside of the high trusses?

Mike McCann
MMC Engineering

 

[BAretired]

I would be a bit leery about removing the second floor diaphragm because it may contribute significantly to the stability of the building. If your analysis shows that the frames at 20'+/- centers are adequate to carry all of the lateral forces without reliance on a diaphragm spanning between endwalls, then I would agree that it is probably okay.

If not, you may want to consider adding a couple of horizontal trusses in the plane of the bottom chord of the lower trusses.

 

[csd72]

Looks like the slenderness of your columns will increase due to the lack of diaphragm, unless the lower roof is properly braced.

 

[dik]

There may be building classification issues resulting from a change in 'Use' as well as bringing it up to current code requirements due to a major change. The unsupported height of some columns may change and they may have to be reinforced. There are also changes in the code loadings possibly.

Dik

 

[McQSE]

Thanks for all of your input.

I was told that the building was originally a warehouse for a department store. There are no signs that it was ever used in an industrial application. However, I can’t totally rule out that possibility.

I believe the building was designed for the frames to resist all of the lateral force and not rely on the exterior walls except for the end bays (the end walls are brick – no signs of steel in the end walls). In this case the diaphragm would not be needed to transmit the lateral force.

I modeled the building and found that the drift is nearly the same with or without the diaphragm. I think csd72 brought up a good point about the unbraced length of the columns. This is my greatest concern. It seems that the lower-exterior roof trusses provide bracing for the columns. It appears that the lower-exterior trusses are providing a rigid connection (maybe not the right wording) with the upper & lower chord connecting to the interior column.

I tried to attach some pictures. Maybe they will work.

 

[McQSE]

That did not work. Try this

http://i377.photobucket.com/albums/oo219/jmmcqueen/Warehouse/IMG_2634.jpg

http://i377.photobucket.com/albums/oo219/jmmcqueen/Warehouse/IMG_2636.jpg

http://i377.photobucket.com/albums/oo219/jmmcqueen/Warehouse/IMG_2626.jpg

http://i377.photobucket.com/albums/oo219/jmmcqueen/Warehouse/IMG_2615.jpg

http://i377.photobucket.com/albums/oo219/jmmcqueen/Warehouse/IMG_2614.jpg

 

[kslee1000]

Your photos have convinced me that this building wasn't for heavy industrial use, but colud be light manufacturing, or just a warehouse as you been told.

I still think the 2nd floor was a adds-on though, because couldn't think a good reason for the mixed use of steel-wood materials, also the wood handrailing on 2nd floor. However, it is not clear from the photos, the columns at the 2nd floor seem to be smaller than that at the 1st floor. What is the main eave hight? How was the transition of column size handled (details)? Very interesting project.

 

[McQSE]

It's hard to see from the picutures, but the columns are continuous. The columns are 28.5' tall.

 

[BAretired]

Nice photographs. It doesn't appear to me that the unbraced column length will change by removing the second floor. Am I missing something?

Best regards,

BA

 

[kslee1000]

From photo 2625, I have gain some confidence in your favor. I would try to analyze the full length column without intermediate brace point in the direction of web. If it works, you should be free to proceed as proposed. Otherwise, try strengthen the columns rather than relying on the lower roof trusses.

 

[BAretired]

kslee1000,

That would be very conservative. I doubt that the structure would work that way.

Why not analyze the structure as it is, then make sure that the lower truss members and their connections are capable of carrying the forces required by the analysis?

Best regards,

BA

 

[csd72]

There is no bracing to the column at the lower chord of the lower truss but there is bracing at the upper chord of the lower truss. This will increase your effective length a little but not a massive amount, still needs to be checked though.

 

[kslee1000]

BA:

You are correct on modelling the building as is for frame anaysis.
What I meant was to use full column length to compute KL/r, to determine the allowable compressive stress, without counting on the lower roof truss as brace. Hope this is not an over kill.

 

[BAretired]

kslee,

Okay, I see what you mean. That may work okay.

csd72,

I don't think you can consider the column laterally braced by either top or bottom chord of the lower truss if the outside columns are hinged top and bottom. Except for the interior column, the lower truss is free to translate.

In the present situation, the floor diaphragm may help a bit, but it is 40' by about 240' (length six times width) so probably doesn't help a great deal.



Best regards,

BA

 

[kslee1000]

BA:

It's my gut feeling that the trusses are too flexible to be count on as valid braces, and the lower truss translate with the main column too. That's the reason I suggested to try to get around by using the lowerest code allowable compressive stress. For renovations without original design document, I prefer to stay at safe side.

 

[BAretired]

kslee,

I agree with you.

Best regards,

BA