ldavisfl
02-08-05, 09:22 AM
I am a mechanical engineer but do not specialize in framing. I am designing a steel beam for a sagging floor myself. I intend to have my design checked by a structural engineer before fabrication but I would like to design the beam myself for educational purposes.
I have a sagging floor in one room (12X14.5ft). The joists in my 1930 house are in good shape (no rot, etc). Although the virgin forest lumber is quite stong, the joists were simply too small withstand gravity and time. They've sagged about 1.5 inches in the middle of the room.... the room floor is essentially a big shallow bowl. I'd like to design a beam (and end supports for them) to straighten out the floor. The beam will span the room width so I do not have to interrupt my full-height basement with a support column.
I will run this steel beam normal to the existing joists. The beam would be supported at each end by suitable supports. The beam will be 14.5 ft long. I expect the beam will have to be pushed up (and the floor straightened out) gradually over time, which is fine.
Now to my questions... I understand that typical live and dead loads are 40psf and 10 psf, respectively. My question pertains to the application of this design load.
(1) For the purposes of designing the beam, I'd like to keep center deflection under 1/4 inch, and withstand the resulting bending stresses with a FOS of 1.25. Is it typical to apply the design load continuously to the beam or as a point load in the center of the beam. Obviously the point load will yield a stronger beam design, but which load application is the standard method?
(2) Applying the entire design load (live + dead) to the beam implies that the beam carries the load of the entire room. But, this will not necessarily be the case because there is already a pre-existing network of interconnected joists that are presently supporting the floor just fine. Is it customary to apply only a portion of the design load to the beam for design purposes, since the beam is not truly holding up the room all by itself? Or, is it typically assumed that the beam should be designed to be able to support the room because the precise floor geometry could possibly route a great majority of the load to the new steel support beam and a conservatively designed beam is best? How is this apportionment of design load approached?
(3) Finally, are W-type beams more common now than S-type beams? My Machinery's handbook has tables for both, but the choices of W-type beams are more numerous. Does anyone know a little of the history behind S vs. W? :coffee:
I have a sagging floor in one room (12X14.5ft). The joists in my 1930 house are in good shape (no rot, etc). Although the virgin forest lumber is quite stong, the joists were simply too small withstand gravity and time. They've sagged about 1.5 inches in the middle of the room.... the room floor is essentially a big shallow bowl. I'd like to design a beam (and end supports for them) to straighten out the floor. The beam will span the room width so I do not have to interrupt my full-height basement with a support column.
I will run this steel beam normal to the existing joists. The beam would be supported at each end by suitable supports. The beam will be 14.5 ft long. I expect the beam will have to be pushed up (and the floor straightened out) gradually over time, which is fine.
Now to my questions... I understand that typical live and dead loads are 40psf and 10 psf, respectively. My question pertains to the application of this design load.
(1) For the purposes of designing the beam, I'd like to keep center deflection under 1/4 inch, and withstand the resulting bending stresses with a FOS of 1.25. Is it typical to apply the design load continuously to the beam or as a point load in the center of the beam. Obviously the point load will yield a stronger beam design, but which load application is the standard method?
(2) Applying the entire design load (live + dead) to the beam implies that the beam carries the load of the entire room. But, this will not necessarily be the case because there is already a pre-existing network of interconnected joists that are presently supporting the floor just fine. Is it customary to apply only a portion of the design load to the beam for design purposes, since the beam is not truly holding up the room all by itself? Or, is it typically assumed that the beam should be designed to be able to support the room because the precise floor geometry could possibly route a great majority of the load to the new steel support beam and a conservatively designed beam is best? How is this apportionment of design load approached?
(3) Finally, are W-type beams more common now than S-type beams? My Machinery's handbook has tables for both, but the choices of W-type beams are more numerous. Does anyone know a little of the history behind S vs. W? :coffee: