huangyhg

floor beam design
hey! im designing a two span floor beams supported by girders at both ends for the two spans.using the aci moments coeffecients is the fastest way to determine the design moments. but the conditions did not allow me to use th said coeffecients. instead, i used the three moment equation in determining the fixed end moments. the fixed end moments are much higher then the midspan moments... i have noticed in many plans done by advanced structural engineers that beams supported by girders at midspan have many rebars than at the support. meaning the moments at midspan is higher than that of the support...what should be the design approach for floor beams supported by girders? thanks in advance..merry xmas guys..
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you need to explain your framing better. so there are two span floor beams, thus 3 supports for each beam. are all these supports girders, or only at each end? what is the centre support? how are the beams loaded and what are the spans?
the moments in the two span beams will be controlled by consistent deflections of the girders and beams.
the column to column distance is 6m(5 bays) and the other is 7m(2 bays) with perimter beams to support the slab. i introduce floor beams at midspan along the long direction to make by slab one-way..
the steel designers manual (uk) will give the appropriate co-efficients for a 2.span continuous slab (equal spans.
w = udl on 1 span and l = l for 1 span
for a 2 span continuous slab with equal loads on each slab.
moment = co-efficient x w x l
reaction = coefficient x w
mid-span sagging moment co-eff = 0.07
support hogging moment co-eff = -0.125 (centre support)
outer support reaction co-eff = 0.375
centre support reaction co-eff = 1.25
there are also variations of this for different numbers on spans and load arrangements.
just do an elastic analysis and use a simple moment redistribution (say 10% - 15%). aci permits this (you can't exceed 20%), just don't try to use (or adapt) the coefficients and redistribute moments.
if your model has fixity at the support, then the moments will be greater at the support; if the simple span case is modeled, then the greatest moment will be mid span. the details of reinforcement must match your model.
the floor beams and girder will be poured monolithically. so its a fixed connection??
it will be a fixed connection, but you need to be careful with how much moment you take from the last beam into the girder. this is a compatibility torsion situatioin, so even if you design that spandreal beam to take the entire end moment of the beam in torsion, it will likely not provide that support becuase of its twist. the twist will allow some end rotation of the beam and reduce the negative moment at the support and increase the positive moment at midspan.
i would design the end of the beam at the spandrel for the cracking torque of the spandrel girder (be sure to design for this torsion in the spandrel) and increase the moment at midspan accordingly.

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