Plate bending stress

[tebodm]

Hi folks,

I am trying to design a connection for an equipment support frame to be located above an existing roof. The frame is designed to penetrate the roof at the existing column locations. Existing roof framing is bar joists bearing on continuous W16x31 that span across the tops of the columns. I've checked reinforcing the bar joists to support the equipment but the work will be too labor intensive so in talking with the contractor this is the preferred method. Their is a joist located directly above the column on the beam top flange where we propose to install a new stub column to go up through the roof. I've attached the connection I'm working on to bridge the joist bearing shoe and connect to the beam top flange. I'm stuck on how to design the plate (shown as 1") that is below the HSS6x4 that bridges the joist bearing. There is a column load of 24 kips and a small moment of 13 k-ft in the HSS6x4. I looked at it as a simple beam with concentrated point load and moment at midspan but that results in extremely large bending stresses. I also looked at it as a "cap plate" similar to Ch 4 of AISC Design Guide 24 for HSS connections, which told me the plate was more than thick enough. My gut tells me a 1" plate should do the trick but the numbers aren't making sense. Thanks in advance.

 

[Ron]

The HSS6x4 prevents bending in the plate near the center, so it bends only in the short span between the edge of the HSS and the bolt, so I would check shear across here and neglect bending.

 

[hokie66]

Why don't you just move the equipment supports slightly so they miss the joists? Is the W16 beam really not adequate when the load is slightly offset from the column below? Your solution should work, but it is expensive, and difficult to flash for watertightness. Don't forget to provide support for the roof deck.

 

[tebodm]

Thanks for the responses. I hadn't considered shifting the posts by 8" to avoid everything, sometimes it seems like you get so focused on a solution you forget their might be other options. We know its going to be difficult to flash, which is why we are using HSS which is easier than a W shape.

 

[dhengr]

Be judicious about which side of the main column you put your stub column, as a function of the W16 framing below. Get your bolts and base plate well below the roof deck. We used to put something like this in a pitch pocket above the roof surface. Although, I suspect there are some nice rubber boots like you see on plumbing vents, etc. Maybe that would be a reason for you to change your stub column to a round pipe.