load applied to lower flange of beam

huangyhg

load applied to lower flange of beam
i am a new entrant to the forum.
i have been puzzling over the following:
why are loads always applied to the top flange of beams?
is the capacity lowered if the loads are applied internally to the lower flange?
loads are not always applied to the top flange.  in fact, for todays symmetrical wide flange shapes what you percieve as the top flange may just as well be the bottom flange.
over the years as much flooring was designed as concrete slab on steel beam and the connections from the beams to the columns were generally easier to make with the beam external to the slab it just became normal practice.  
there is some help for the steel beam from the concrete floor above in that the floor slab helps brace the compression flange (usually the top flange in simple beam connections) of the beam.  this helps prevent a premature failure due to buckling rather than allow the beam to develop its full strength.
as for loading the bottom flange - its is done as industry requires.  for example overhead cranes for use in industrial warehouses run along the "bottom" flange of a beam.  there are also many applications where a load must be permanently affixed to the bottom of a beam.
and furthermore, for simply supported double tee beams, symmetrically applying the loads to the web or bottom flange counteracts ltb, so there are cases where ltb would be controlling if applied without lateral bracing atop that to apply the loads at lower points is beneficial to the lateral stability of the beam and then the strength.
this is ascertained through proper use of cb and equivalent procedures. you can read about this on galambos v edition.
you can apply the load to the bottom flange of beams just as easily as the top but be careful because if you apply a load to the bottom flange from one side only you may exert torsional stresses on the beam if the load was applied to the top flange it can be positioned easier over the shear centre of the beam
i tend to apply loads to edge beams through the top flange and through the bottom flange on internal beams to limit the downstands
kieran coyle
beng(hons),ceng, m.i.struct.e, m.i.e.i.
thanks to all
your thoughts have been very helpful
i shall be back
jeg
and further-furthermore, many loads are applied to the web of the beam, as is the case with simple header connections of secondary framing members into primary.  
it should be noted also that there are some specific loading conditions in which the lower flange is directly loaded, such as the bolting of monorails beams to the underside of support girders/beams.  in these cases, the beam capacity can be diminished if:
1. the percentage reduction of the flange cross section at the bolt holes is greater than that allowed by the applicable steel code.
2. the local flange bending should also be looked at, and gussets added if necessary.
hope this helps,