Reinforcement of existing steel joist.

[Miner99er]

My issue is that I have an open web joist (LH series) that is over stressed about 10%. I believe I have two options at this point.

A. I can "cover plate" the existing joist.

B. try to spread the RTU weight over additional joists by "x-bridging" between additional joist. I think this would be the preferred option but I'm a little unsure how much "help" I can get from the additional joists.

If anyone has any insight to this issue I would greatly appreciated it.

 

[JAE]

SJI has a book out that outlines various ways to strengthen existing joists.

SJI Technical Digest No. 12

 

[MiketheEngineer]

10% is not that much.

Get creative with your new support beams - you might be able to tie back to existing I-beam or girder joist to pick up to as much as 50% of the load?? Spreading out the load over 4 joists will cut down your overload to like 2.5%!!! - IF done right.

Think outside the box a bit - you might be surprised.

 

[DST148]

I had worked on projects for pharmaceutical industry where every now and then some thing would go up on the roof. But strengthening of OWSJ was rarely an option since the production could not be stopped, the area could be accessed only on the holidays, and maintaining cleanliness was another factor.
We had to either relocate the equipment if acceptable to other parties, spread out the load, or come up with a dunnage to support the new equipment. Budget was never an issue.
While designing we always kept a provision of additional point load of 2 kips anywhere along the span of the joists, but accessibility for strengthening purposes was always an issue.

 

[UcfSE]

Try this catalog from Vulcraft. There's a chapter on reinforcing joists. One of the authors also has an ePub on the AISC website that discusses joist reinforcing.

Designing with Vulcraft Steel Joists, Joist Girders and Steel Deck

 

[H57]

When using a spreader beam to transfer loads to additional joists there are equations to determine how much load gets transfered to each of the joists.

For instance if you were to hang a load from below the joist, using a beam between two joists, the load would normally be distributed to each joist (based on distance from the load). If you were to use a larger spreader beam you could spread the load out to additional joists. It has to do with relative stiffness of the joists vs. the spreader beam.

They reviewed this at a seminar. I would assume this is in the SJI technical digest that JAE identified.

 

[idecharlotte]

Use Post-tensioned cables. I've used them before on bar joists, concrete and glu-lams to gain strength.

http://www.idecharlotte.com

 

[shobroco]

I don't know where you are, Miner99er, but here's a dumb question: if you have snow load, have you removed it where the rtu goes? Depending on your snow load and the size of the unit, you could actually reduce the roof load instead of increase it with rtus or condensers. I have yet to go on a roof & see any snow drifted on or significantly around medium & small mechanical equipment.

 

[JAE]

A non-wind snow event is possible. And snow does accumulate on top of RTU's. The code does not allow negating snow on top of RTU's that I can see.