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RE: Bar Joists parallel to masonry walls

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It seems to me that the best way to approach this is to put the first joist a full or nearly full space from the wall.  If you are going to attach the deck to the wall, the angle needs to be attached to the wall when you place the deck anyway.  I don’t feel comfortable trying to figure out if the deck ends up carrying the joist when the joist is only 6” away from the wall or what happens to the connection between the wall and the angle when the second joist from the wall is carrying twice as much load as the first joist.  The second joist is going to deflect more and the deck is going to act like a lever over the first joist and try to lift up on the wall.  On a short span, I wouldn’t worry about it.  On a 100’ span, you better worry about it.

 

I talked to someone in the fabricating shop of a major joist supplier about camber once.  He told me that it takes too much effort to change their tables to adjust the camber in a joist.  They do not pay any attention to special requirements for camber no matter what it says on the shop drawings.  It may be true or he may have been trying to lead me away from asking for one or two joists with a different camber than the bulk of the order.  In any event he convinced me to try avoiding requests for special camber if possible.

 

Another thing to think about with long span joists is that where you have cross-bridging, you can create a significant load against the masonry wall where the bridging ties to the wall at the elevation of the bottom chord.  Most of the joist manufacturers are willing to allow you to use horizontal bridging in that last space because of that problem.  I had an argument with an engineer from one joist manufacturing company that would not allow that because the Steel Joist Institute Standard Specifications made now allowance for that condition.  We compromised and he agreed to put horizontal and diagonal bridging in that space.  I agreed to not tell him that the diagonal bridging would be removed after the deck had been installed.

 

Roger C. Davis

Architect

SDS Architects, Inc.

205 N. Dewey Street

Eau Claire, WI 54703

(715) 832-1605

 

-----Original Message-----
From: Jim Wilson [mailto:wilsonengineers(--nospam--at)yahoo.com]
Sent: Friday, August 13, 2004 9:55 AM
To: seaint(--nospam--at)seaint.org
Subject: Bar Joists parallel to masonry walls

 

I am debating two opposing details for placing a bar joist adjacent a masonry wall.  Its quite common to see bar joists parallel to masonry walls set off from the wall by 6" or so.  But it seems to me, however that this will cause installation problems with the metal decking as well as performance issues under loading.

 

If a 60' span joist has 1 1/2" camber for example, how can the metal decking be successfully attached to the top of the masonry wall without either bending the deck or tapering the top of the wall.  This connection is imperative to transfer diaphragm shear to the shear wall.

 

Secondly, how can the joist deflect under snow/live loads without fatiguing the deck welds and ultimately causing some type of failure at that location?

 

Am I being too picky or am I just seeing some bad details?  It seems to me that the joists should be set off from the wall the same distance as the typical joist spacing to keep the necessary flexibility.

 

Regards.

 

Jim Wilson, P.E.

Stroudsburg, PA

 


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