load distribution to members with unequal moments of interia
load distribution to members with unequal moments of interia
(OP)
I am working on a design to transform an existing office building (60 psf) to an laboratory (120 psf by owner request). There are existing floor joists at 4 ft o.c. and the existing slab is good for about 300 psf @ 4' span. My solution was to come up between each joist with a new steel beam. It was more cost effective then stiffening the existing joists. I'm going to have them shim and weld the beam to the underside of the deck to ensure a tight fit.
In my design the joists have an I=171 in^4 and the new steel beams have an I=88.6 in^4. I distributed the floor load based on stiffness. 66% went to the joist and 34% went to the beams.
A fellow engineer disagreed with my approach. He said the slab was so stiff that essentially the entire load will be transfered to the joists and the beam would just be taken for a ride. His suggestion would be to make the beams moment of inertia as close to that of the joist. This would add approx. $30K to the job.
I feel the distibution of load is valid. For the beam to deflect it needs to take load, right. Any thoughts would be helpful.
In my design the joists have an I=171 in^4 and the new steel beams have an I=88.6 in^4. I distributed the floor load based on stiffness. 66% went to the joist and 34% went to the beams.
A fellow engineer disagreed with my approach. He said the slab was so stiff that essentially the entire load will be transfered to the joists and the beam would just be taken for a ride. His suggestion would be to make the beams moment of inertia as close to that of the joist. This would add approx. $30K to the job.
I feel the distibution of load is valid. For the beam to deflect it needs to take load, right. Any thoughts would be helpful.






RE: load distribution to members with unequal moments of interia
If you shore up the existing joists, your approach is valid.
DaveAtkins
RE: load distribution to members with unequal moments of interia
RE: load distribution to members with unequal moments of interia
Why not create a little computer model with spring constants for joist and beam and model the slab as a beam element? This will give you the distribution to each but it should be close to what Dave described.
RE: load distribution to members with unequal moments of interia
I would say (if you're not relieving the DL stress by jacking) that the joists take all the DL and the their trib width of the LL. I would assign the trib width LL to the steel beams. That is assuming the slab is simple between joists. Ah, now thinking about it, if it's simple between joists, it could be continuous over the intermittent beams. Maybe the steel beams get 0.625*4 of the LL, and the joists get 0.375*4 of the LL.
I also agree with Dave about jacking and the validity of your load distribution.
RE: load distribution to members with unequal moments of interia
RE: load distribution to members with unequal moments of interia
Listen to the guidance of your fellow engineer! I think that he is steering you in the right direction! If you add steel beams of insufficient stiffness, the joists will still be overstressed. The trick is to choose a beam that is not only strong enough but stiff enough to take away enough load from the joist so that both the beam and the joist are within allowables.
If you are doubling the live load and the joist is carrying the full dead load, the joist will still be overstressed if it is carrying 100% DL + 66% LL. Conclusion: you need a stiffer beam. In fact, the beam will need to be at least equal to or close in stiffness to the joist even if you jack up the joist to distribute the dead load.
RE: load distribution to members with unequal moments of interia
It isn't always easy to support a jack from the middle of an elevated floor. On one job that I heard of, they cambered the beam to get a reverse deflection into it and them jacket the ends up close to the girder and columns until it was at the right elevation. They couldn't use a normal camber, it would have been too great, they had it done by a heat straightening expert, he was able to put about the right amount of camber into the beams.
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: load distribution to members with unequal moments of interia
RE: load distribution to members with unequal moments of interia
Assuming that the beams can be placed, the deflection of a joist will be slightly greater than a beam of equal moment of inertia due to shear deflection which is greater in a trussed member than a beam with solid web.
BA
RE: load distribution to members with unequal moments of interia
RE: load distribution to members with unequal moments of interia
RE: load distribution to members with unequal moments of interia
For the beams, w/o further analysis, I would use 50% LL, and would design the beams for some of the DL as well, just to be safe.
RE: load distribution to members with unequal moments of interia
Yes you are correct....it's called Hooke's Law.
RE: load distribution to members with unequal moments of interia
You're looking for 120psf and what's your new spacing with joists and new beams?
RE: load distribution to members with unequal moments of interia
BA