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New Mechanical Units

New Mechanical Units

New Mechanical Units

(OP)
I have an existing building with OWSJ spanning between gerber beams. New mechanical units will be placed on platforms that span between the gerber beams. My calculations show that the existing 22 guage deck (8' span) does not possess the capacity for the new snow accumulation. I am also finding that the existing OWSJ (at the peak of the snow accumulation) will need reinforcing.

I am thinking of adding new wide flange beams or OWSJ to be placed inbetween the the existing joists. By doing this, I would eliminate two birds with one stone (deck span is halved and existing joist load is halved, no reinforcing required). I am wondering if this is a viable option? How easy is it to place new joists next to existing joists? Should I go with wide-flange beams instead? Are there other options?

Thank you.

RE: New Mechanical Units

WF beams will be easier to deal with when it comes to connections. You would somehow have to slide the joist bearing seat between the deck and gerber beam, and attach properly or remove the roof deck. With the steel beams you can attached extended shear tab and bring it straight up into place.

RE: New Mechanical Units

I'm a bit embarassed with my years in the field that I have never heard of a "gerber" beam... Gerber is a popular baby food in the US ponder

I looked this term up and it brought me to an old post on eng-tips with the same reaction to gerber.

Before you think about putting in new steel beams and joists, what kind of MEP obstructions may prevent this as a possibility?

RE: New Mechanical Units

You can get joists from joist manufacturers where they have a moment splice at midspan. The contractor can then fit in the joist seats over the supporting beams on either end and then bolt the joist together at midspan.

The challenge always is how to then attach the joist the existing metal deck to create adequate lateral bracing of the joist. This might be done by having the joist manufacturer add ear tab plates on the side of the top chord with pre-drilled holes to allow screw connections into the bottom of the deck flutes.

RE: New Mechanical Units

Can you reinforce the existing joists to work, then add small angles or something to support the deck under the new load? You would need a fairly clear ceiling space to install a long new beam or joist.

RE: New Mechanical Units

Your roof deck spans 8'-0"? That's a long way.

I agree with the post by JAE, a spliced joists is a good option if space is available. I disagree with using a WF makes the connections easier as fitting the beam in will be difficult. With regards to attaching the joist to the existing decking. I understand the concern with making an attachment. I have ofter attached the new joists to the existing joists on each side with X bridging at 3'-0" o.c. if puddle welding the deck from above is not allowed.

If you are spanning the new RTU between the existing gerber beams, and you are not adding any additional load to the deck, why does the deck have to comply the the current building code? Are you imposing snow drift on the existing deck, what is the size of the unit?

RE: New Mechanical Units

When I have roof members overstressed, I like to add a raised platform to support the new mech unit that spans from column to column, or wall to wall. That may be faster and easier than trying to do the work from below.

Just an thought.

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