reinforcing open web joists

huangyhg

reinforcing open web joists
this is probably too long but here goes.  some background...existing roof consisting of owsj joists spaced @5.5' spaning 42 ft.  arch needs to add 20psf dl to roof.  i came up with two scenarios..1.reinforce existing owsj which is very labour intensive but cheap materials and option 2. add beams between exiting owsj, effectively reducing ta to 2.75'...very costly for materials cheaper labour.  each option turns out to be too expensive, does anybody have any other possible solutions or suggestions.  there are a total of 35 beams/owsj to be considered.
thanks
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can you support the joists with a new beam line at mid span of joists?
no that's not possible, because it will interfere with the bay sizes.  
what is causing the plus 20 psf added weight?
i think the three options presented (reinforce, add beams, add column/beam line) are your primary options short of my first sentence - why add the load?
can the supporting beams hold the 20 psf additional load?  can the supporting columns and footings?
20 psf seems a bit much.
the additional weight is due to concrete pavers being placed on the roof to achieve the required fire separation.  this option was developed by the fire protection engineering consultant as being the most efficient.  the beams are being reinforced for the additional load and the columns/footings are adequate.
just discussed an option with a co-worker, he seems to think a plausible solution is to place a beam between every second joist.  i can't seem to rationalize how this could possibly work?
it could work because new beams at every other joist space will reduce the load at every joist (look at your roof framing plan).  but does the deck work?
daveatkins
have you taken a look to see if the existing joists could take the additional load without reinforcing?
perhaps a different fire protection method would work better with the structural budget, which appears to be nil.
20 psf is quite a bit to add to an existing roof.
it appears that possibly your fire protection engineer developed his 'most efficient' solution in a vacumn, without considering the impact of various options on other disciplines and parts of the building. and that this solution may not be the 'most efficient' when the total cost of the solution is calculated.
i would request that all fire protection options be explored in light of their total effect on the entire building (structure, mechanical, electrical, etc) and the various options (and associated total costs) be evaluated on that basis.
then the client and other disciplines will have an idea of the total impact (includings $$$'s) of each option and can make an informed decision on which way to procedd.
i see dave, looking at this too long. i have to recheck the joist..thanks.
"perhaps a different fire protection method would work better with the structural budget, which appears to be nil."
surely this is better than trying to reinforce the structure.  the current solution sounds like "let's find the heaviest way possible to achieve a fire separation and go from there."  lol, hyperbole that i learned from another esteemed eng-tips   
concrete pavers on a roof to achieve fire protection is a new one on me unless it has something to do with the roofing. can they use lightweight pavers to reduce the 20 psf? how about adding fire-rated gyp to the bottom of the joists instead?
lkjh345 stated it best! this may be the most "efficient" for the fire protection scheme but not for the overall project. the total cost of this must be taken into account for all systems! the architect needs to review other options.