Wood Floor Joists Reinforcement 1

[jimzpe]

Been reviewing an existing building second floor structural system to accommodate new (increased) occupancy loading. Present floor framing is 2 bye wood joists, which are, by the numbers, incredibly overstressed for the current occupancy. Thinking of "Sistering" each joist with cold form metal joists designed to handle the total imposed loading (anticipated + existing dead load plus new occupancy live loading) to simplify things. Anyone have any thoughts or words of wisdom?
My rational is to: 1)Keep connections simple between the wood & steel (only that required to transfer shear and provide stability), 2) Design the new metal joists one size smaller in depth than the existing wood joist - to not have to worry about floor squeeking and facilitate installation, 3) Not clutter up the first floor by installing new footings, columns and girders and, 4) Give a warm fuzzy feeling knowing the system will work no matter what.

 

[SperlingPE]

If the existing bearing is adequate, can you install your new joists between the existing joists? You are going to all the trouble fo designing the cold formed sections for the load, why go to the trouble of connecting them to the wood joists? Can you double up the wood framing members (i.e. cut the spacing in half)?

 

[jimzpe]

Have adaquate bearing at each joist support. Connecting to existing wood joists to 1) provide lateral (buckling) stability to the new cold form section, 2) use the existing wood joist only to transfer imposed loads to the new joist from the floor deck above & suspended ceiling below. The existing joists are so far off even doubling them (adding new identical wood joist to the existing one) does not meet required capacity.

 

[SperlingPE]

The new cold formed sections (cfs) would be tight to the underside of your existing deck. The cfs would be approximately 1" less in depth than the existing wood joists. So the connection between the two members would have to be designed so that both members work together because the load at the bearing location will be transferred by the exisitng member. Verify existing bearing condition is adequate (i.e. enough bearing length).
If new cfs are not tight to the underside of deck, you have kind of a flitch plate situation.

If no circumstances preventing you from using cfs between existing joists, why not?

Consider reviewing install procedure with contractor.

 

[valleyboy]

How much additional capacity do you need to achieve on the floor?

You can design the floor as a 'stressed skin panel', assuming that the decking material to the top surface of the floor acts compositely with the timber joists. Is this possible for your application?

Another approach would be to reduce the span of the existing joists by introcucing a beam support towards mid span.

VB

 

[RareBugTX]

Flitch Beams would be another option (i.e. 2 wood joists will be the bread, and the "ham" a steel plate in between, with a thickness say 3/8",1/2" or whatever your particular requirements.) these plates can be fabricated with the holes at the shop and then your lag-bolts or thru-bolts would come at the end.

RareBug

 

[pba]

I assume you can't take out the existing floor and replace?

I like your idea and see no problem with it how it works.

If you can get bearing at each end of new cold formed sections then the wood joists are nominally redundant and only provide restraint. All is well - simply fix the cold formed to the timber at reasonable centres such that the load transfer can occur through the fixings rather than by bearing. Make sure the bearings are well shimmed to transfer the loads onto the walls. The existing floor will probably have also deflected so you may need your reduced section just to get the new (straight) members into the space.

If you can't get proper bearing at each end then the forces can usually be transferred back into the timber by multiple fixings at the ends of the steels. This is the alternative to the flitched beam described elsewhere - here there are two steel plates with the timber in the middle. - It's still a flitched beam and the design process is the same.

 

[dicksewerrat]

First you have to get the deflection from 'overstressed' members. Then add to take all of the new load. The existing members may be 5 lbs from catstophic failure. Have you talked to any regulatory officials. This may be a case of losing th certificate of occupancy. and now that you know, what is your responsiblity to the users of this space?

 

[pba]

dicksewerrat,

I think the idea is that the new cold formed joists take the full load. The timber joists are thus not required as joists in the final solution but are simply packing.