Amount of load to design new steel beams for, when cutting floor opening
Amount of load to design new steel beams for, when cutting floor opening
(OP)
How should the beam loading be determined when checking steel beam framing that was placed some years ago around an area to provide supplementary support where a new stair opening was cut in a two-way slab floor. I don't know that modelling it in SAFE will necessarily give the answer, since it is not apparent what load should be applied to simulate the cutting of the opening. The steel beams were placed and grouted tight to the slab before the opening was cut. The opening removed 1/2 the column strip (bottom bars) in one direction, and all the middle strip (top bars) in the perpendicular direction. The steel beams were grouted tight to the underside of the slab with 2½" thickness of grout.






RE: Amount of load to design new steel beams for, when cutting floor opening
Please draft the plan, elevation and section views so we know what issues you are facing.
Given some geometry of the floor ans original beams, plus the new ? beams when the hole was cut, the floor loading is determined not by the structure underneath the floor, but by the weight and movement and friction above the floor. (And the floors above the current one).
RE: Amount of load to design new steel beams for, when cutting floor opening
RE: Amount of load to design new steel beams for, when cutting floor opening
RE: Amount of load to design new steel beams for, when cutting floor opening
There may be two separate questions to consider here:
1) How much flexural capacity do the beams need to resist the load that the beams could potentially attract were they rigid and to remain so. That's the question that I've tried to answer.
2) How much flexural capacity do the beams need to add to the waffle slab floor system in order to replace the capacity lost when the opening was cut.
The answer to #2 could be a fair bit less than the answer to #1.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Amount of load to design new steel beams for, when cutting floor opening
Because there is a schedule crunch, I issued beam strengthening drawings based on a loading pattern similar to what you showed, but I am still interested in how to determine the load the beam actually takes. Probably not an easy thing to do, because the waffle was already taking the load before the beam was installed.
RE: Amount of load to design new steel beams for, when cutting floor opening
Right. The difficulty in accurately assessing the loads on the beams is probably why this thread hasn't garnered too many responses so far. Coming up with something that's simple, realistic, and reliably conservative is tricky. Some thoughts:
1) You could use SAP to do a staged analysis. Too much work for me.
2) You did exactly what I would have done. My reasoning would be that most of the cost in this will be labor, not material. As such, if the beams needed reinforcing anyhow, reinforcing them with larger tees probably isn't a real big deal.
3) If I were the original designer, I would have detailed the W14x26's to not be in contact with the waffle slab for just this reason.
4) Wherever a beam has been installed, I would think it reasonable to assume that beam shares load with a few of the neighboring waffle ribs based on stiffness. If I remember correctly, however, your ribs are pretty shallow.
5) One approach would be to allow the beams to fail in bending moment. As long as the resisting moment that they do supply, taken in conjunction with the flexural capacity of the adjacent waffle slab, is adequate for the applied loads, all should be well. You'd want to conservatively design your beam support connections of course. And deflection analysis would be a bear.
It's considerably more aggressive, but I might be able to get behind #5 with the load distribution shown below. I'd argue that the load to the right of the beam can probably still be supported by the 1/2 middle strip that remains and that the waffle slab to the left of the beam is still pretty much going to act like a two way plate.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Amount of load to design new steel beams for, when cutting floor opening
1/2 column strip?
RE: Amount of load to design new steel beams for, when cutting floor opening
Yup. My bad.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Amount of load to design new steel beams for, when cutting floor opening
I too felt that the waffle slab half column strip could still carry its share of the load after the opening was cut. I was thinking that only the half column strip load capacity was lost. So since the positive moment taken by half the column strip is half of 60%, or 30% of the total panel moment, plus a percentage of the weight of the new masonry walls of the stair enclosure, that might be all that the beam has to take. The moment in the beam came to the following in each of the cases we calculated:
415 kN.m for Case 1: all of the ribs grouted to the steel beam
310 kN.m for Case 2: only some of the ribs grouted tight
226 kN.m for Case 3: 30% of the panel load
202 kN.m for Mr of W16x26 (fy=303 MPa) if laterally supported.
In each case the new beam would also take some of load of the new block wall; we took 100% of the weight of the new block wall onto the steel beam in our check.
RE: Amount of load to design new steel beams for, when cutting floor opening
If there's a physical gap between the beam and the waffle slab ribs, then no load would go to the beams. That's the point that I was trying to make and what I would have done for the W14x26's at least. My money says that those beam exist only to hold up the W8. I doubt the original designer ever intended them to mess with the general slab-ish behavior of all the stuff to the left of the new shaft.
Putting 100% of the masonry on the beam makes sense to me.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Amount of load to design new steel beams for, when cutting floor opening