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RE: Bar Joists parallel to masonry walls

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Jim,

You present an age-old problem.   Now allow me to complicate it.

You also have the issue of bracing the wall that is parallel to the joist. The deck is what braces the wall unless you require a subframe in the plane of the deck. The deck is then a also compression element. If you move the joist too far back (like a full joist space), the compression on the deck L/r goes bad. And as you stated, if you move the joist directly adjacent to the wall, you have a deflection compatibility issue. This is often overlooked.

I generally split the difference and place the first joist back about 2'-6" from the masonry wall. I provide WT's cast into head joints of the masonry, and weld a continuous angle to the flange of the WT's. The stem has DBA's welded to the stem that project into the bond beam. This provides me with an 8" tall weld plate in the masonry wall. It makes welding a continuous angle to the embed plates easy. The angle serves as a diaphragm chord and transmits gravity load from the deck into the masonry wall. This is a whole lot easier than trying to cast in anchor bolts or use post installed anchors. The angles can parallel the joist in elevation, but most sharp iron workers will not match the camber elevation exactly. They know that they will loose a lot of the camber when all of the dead load is placed on the joist. So if your joist has a camber of 1 1/2", the iron workers may camber the angle about 3/4" as they weld it to the embedded WT's.

I can still use the deck to dump the load from the parallel masonry wall into the diaphragm, and I have enough distance that the deck will deflect allowing for a rigid wall and a relatively flexible joist. It is also enough distance that an iron worker will be able to stand on the deck and force it down to make the puddle welds.

Once the deck is in place, fatigue is not an issue. The snow, roof live loads, and dead loads are not fatigue loads.

By the way, you should still check the deck for the out of plane compression loads induced from the parallel masonry wall.

Regards,
Harold Sprague


From: Jim Wilson <wilsonengineers(--nospam--at)yahoo.com>
Reply-To: <seaint(--nospam--at)seaint.org>
To: seaint(--nospam--at)seaint.org
Subject: Bar Joists parallel to masonry walls
Date: Fri, 13 Aug 2004 07:54:37 -0700 (PDT)

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.
wilsonengineers(--nospam--at)yahoo.com
Stroudsburg, PA

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