Diaphragm and Shear in Existing Building (Metal deck and Bar Joist)

[NFExp]

I am looking for tips or ideas on how to approach a Retail project I am working on... The project involves modifying the existing 1 story building 520'x120' 17' tall, by raising the front and side parapets on average about 9' but in some cases 18'. The existing building is masonry perimeter walls with steel beam/columns on the interior (30' grid), with bar joist (30' span) and metal deck.

The biggest challenge that I see is that the existing building does not have enough shearwalls and the diaphragm attachment / deck gauge is unknown. I think the building was built in the 70's but we do not have original plans. I did a quick initial inspection for quoting purposes. I couldn't see much except for the facade framing and roof framing above the ceiling. Most of the units are rented out so you cant see much in terms of the walls.

I do not think there is enough shearwalls for the original building but now the parapet will add an additional 560plf of shear. The answer may be it to add some metal braces between columns (and retrofit footings) but I know that is not practical because of the existing businesses.

Am I over thinking this..At this point I am only quoting this project. The architect is a good client of mine and I dont want to give a super high fee or reject the project.

Thanks in advance.

 

[BAretired]

You don't have enough information to quote a fee. Tell the architect about your concerns and work on an hourly basis until more is known.

BA

 

[JAE]

From your statements it appears that you have a good, and correct, handle on the issues....the biggest issue being the actual deck-to-structure attachments and the resulting diaphragm shear capacity.

I have a 1982 Wheeling Diaphragm Design Manual that shows diaphragm shear capacities for various puddle (spot) welds and panel seam (sidelap) connections.
This document (attached) will only provide you with a general view of what was possibly done back in that era. It may be that the deck was attached with some other type of fastener (but probably welds) and some other frequency of connections.

You might perhaps get up close to the roof framing from below and "see" residual evidence of old spot welding and spacing.

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[NFExp]

Thanks JAE.

I spoke to the Architect and told him my concerns.. He is contacting the developer to explain the situation and asking for higher fees.. I will wait to hear back. It seems that this project is a go regardless so I guess I'll plan on a discovery inspection and braced frames every 90' depending on the diaphragm capacity.

 

[DaveAtkins]

Depending on the building code that was in effect in the 1970's in your area, MWFRS loads may very well be lower now than they were then. For example, in my State of Wisconsin, you simply used 20 psf on the entire projected area back then. The new ASCE 7 code is much more sophisticated, and typically results in lower wind loads.

DaveAtkins