Steel Beam - Bending Safety Factor
Steel Beam - Bending Safety Factor
(OP)
I am designing an agricultural greenhouse type structure that will never be inhabited or be used for shelter by anyone (fenced off and presents minimal risk of danger to anyone). AISC defines the safety factor against yield in beams in bending as 1.67.
Since this structure has an estimated life of 20 years and really presents no problems, does anyone see an issue with reducing this factor to say 1.25? If the beam was over loaded & deformed, the owner would not care - possibly would just replace it.
What do the experts think!?!?
Thanks,
Paul
Since this structure has an estimated life of 20 years and really presents no problems, does anyone see an issue with reducing this factor to say 1.25? If the beam was over loaded & deformed, the owner would not care - possibly would just replace it.
What do the experts think!?!?
Thanks,
Paul






RE: Steel Beam - Bending Safety Factor
RE: Steel Beam - Bending Safety Factor
Also note that the failure isn't always yield. Even in allowable stress, the formulas are set up in such a way that you'll hopefully get your factor of safety over the governing failure mechanism. If your failure mechanism is plastic hinging where there's a redundant stable system you might get out okay even though it's still a failure. If your failure mechanism is something else like section yield, lateral torsional buckling, your connections, or even hinging without any redundancy then your whole thing breaks.
RE: Steel Beam - Bending Safety Factor
Your importance factors are figured into your wind and seismic values for the lateral resisting system. Never heard of one for the vertical except when combined with vertical seismic forces...
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Steel Beam - Bending Safety Factor
It is the standard factor of safety and I get why, but since this is not a typical building, but rather a small structure, I'd like to get more than 60% of the capacity of the beam.
If this is really a horrible idea, does anyone have any other suggestions on how I can be less conservative in my design?
Thanks!
RE: Steel Beam - Bending Safety Factor
Stick with the code, Hammer. Temporary buildings have a habit of becoming permanent buildings.
BA
RE: Steel Beam - Bending Safety Factor
If you look at labor and coating costs, then just tweaking structural weight slightly may have a pretty negligible effect on constructed cost.
One advantage to using a code is it gives you some confidence in the results. You know it SHOULD work, since all the other structures built with that code stood up okay. If you intentionally undercut the code, you lose that confidence.
You mention this would not normally be inhabited, but if it's a greenhouse of sorts, it should have workers in it periodically, and it needs to be safe for them to work in. You'd have an awfully tough job justifying that reduction in safety factor if someone got killed in it. Reminds me of the Dallas Cowboys practice structure that collapsed.
RE: Steel Beam - Bending Safety Factor
It is a fact, that in many locations agricultural buildings are not tightly controlled by local building officials or building codes. The logic is that a few dead cows, pigs or hay bales isn’t the same as a bunch of dead people. But, that won’t prevent you from being the goat if anything goes wrong, and the guy ends up with a bunch of dead tomatoes if the roof comes down. Don’t be less conservative in your design, be more thorough and thoughtful in your design. I wouldn’t argue with you about the difference between 1.67 and 1.5 in a few very carefully considered locations, but not at an early failure mechanism. And certainly, you must have some agreement with the owner that you will not be hung out to dry if something goes wrong in saving him a buck up front. Is the structure shaped such that it sheds snow, and will the owner clean up the sloughed off snow around the bldg., so that the bottom 6' of the side walls don’t become retaining walls, with an unbalanced loading. What can you do in your design to make this structure very easy to fabricate, erect and maintain? Remember, an hour saved there will buy you a hundred pounds of steel. Simple, repetitive details to fab and assemble and maintain may be worth more than a few pounds of steel saved, up front. How is the structure going to be enclosed? Clean and simple and easily repaired, I would hope. Remember the humid interior conditions and the toll that can take on steel, therefore a very durable paint system, maybe a material other than steel.
RE: Steel Beam - Bending Safety Factor
I actually work for a company that does little structures like this all the time and they are also pressuring to lighten the system as well (in this case the client is pushing harder than my company). We build thousands of these small structures so we are not looking at saving a few hundred pounds of steel, rather a few million pounds of steel. The design is about a cheap and quick as can be to install - all that is left to do is lighten the steel (aluminum is cost prohibitive these days). I can tweak hardware, but literally, this one beam makes up about 40% of the weight and cost of the structure and that is compounded in galvanizing costs, shipping costs, labor costs, etc.
I suppose the best way to reduce loads is to do wind tunnel testing.
Thanks all!
RE: Steel Beam - Bending Safety Factor
RE: Steel Beam - Bending Safety Factor
BA
RE: Steel Beam - Bending Safety Factor
RE: Steel Beam - Bending Safety Factor
JS - higher strength steel is a option.
Just to give an idea, each structure is about 30' long by 15' wide, monosloped open to the air. The beam in question is a 4" round tube that I would like to reduce the wall thickness of in order to reduce the weight. Each foundation costs about $350, so the idea is to reduce foundations first, then thin out the pipe. No chance I turn it into a truss economically. I am looking at going to a thin roll-formed shape.
Thanks all.
RE: Steel Beam - Bending Safety Factor
locally reinforce the critical section would be cheaper than high strength steel, no?
Quando Omni Flunkus Moritati
RE: Steel Beam - Bending Safety Factor
Dik
RE: Steel Beam - Bending Safety Factor
BA
RE: Steel Beam - Bending Safety Factor
RE: Steel Beam - Bending Safety Factor
Based on the info provided, it's possible an I-shaped member will increase your bending capacity, reduce deflection, and perhaps be easier and cheaper to connect.
RE: Steel Beam - Bending Safety Factor