Post Stress in Steel?

I don't believe it is a common practice, but that does not mean it is an unacceptable practice. If you are relying on the checker plate to act as a top flange, field welding has to be adequate to transfer the horizontal shear between top flange and plate, not the usual tack welds.

The added rod must be attached at each end of the bottom flange. When stressed, it will add a compression and a constant moment to the beam. If overstressed, it will cause lateral buckling in the bottom flange.

I tend to believe that a more economical solution is to select beams capable of carrying the load on their own, without reliance on composite action at the top and tension rods at the bottom.

In over fifty years, I never heard of such a thing. In the first place, you can only use a small part of the checker plate, it is limited by width thickness ratio. How are you going to size these elements? It is easier, and lower labor cost to use an adequate beam.

Your scheme is probably illegal in the sense that it won't meet code.

The dead load deflection can be eliminated by pre-cambering the beam. If you wish to take out deflections due to live load (or perhaps 1/2 live load or a most-likely-live-load) then a precamber beyond the dead load deflection will give you a slightly positive camber under dead load alone.

Generally for codes, the deflections are guidelines such that the deflections do not present any serviceability problems i.e. during service, the deflections do not present a problem of usage.

If the beams alone can take the stress criteria (or ULS criteria) then you can envisage to use the 1/4" plate as a deflection limiter (recalculate the effective moment of inertia limiting the width of the plate for the calculation by consideration of any shear lag that may occur in the plate) but you do need to check the welds for longitudinal shear as BAEL mentioned earlier.

A second look at your OP. Something I missed. You should not be anywhere close to the yield point. Depending where you are, there are codes that govern allowable stresses. I suggest you find someone who knows his way round. Have you checked for permits? If you are in the USA, there are probably specific requirements for elevators as well as the general building code structure requirements..

bill,

There are many ways of beefing up a beam. I do not see anything wrong with your proposal from a theoretical point of view, but I question the economics of it. Wouldn't it be simpler (and cheaper) to use a beam which satisfies design requirements without relying on composite behavior or tension rod(s) at the bottom flange.

This system may work theoretically but it is not very efficient since the E of the rod and the beam are the same. The increase in stiffness from the rod could more easily be achieved by increasing the bottom flange size by the area of the rod. However, in either case, the effect will not be significant unless the rod area is comparable to the flange area.

Cambering a beam can be used to hide the dead load deflection but it does not increase the stiffness of a beam. Also, since the live load in your case will be many times larger than the dead load the dead load deflection should not be a significant factor in the design and camber should not be necessary.

If you wanted a stiffer structure you could create a king post truss by dropping a post from the midspan of the bottom flange and connecting the bottom of the post to the ends of the beam with two rods. The stiffness would then depend on the length of the post and the diameter of the rods. This method allows a shallow beam to carry a much larger load, however the overall structure is deeper. Depth is the most efficient way to increase stiffness and moment capacity.

I saw an interesting structure in Germany ( I have forgotten which town).

A long thin vertical tube was required to support a large mass. In order to ensure there was no buckling (I assume) three wires were placed equidistant around the tube and supported by sets of spacers alomg the tube. It worked very well and I was impressed by the combination of simplicity and "brilliance".

Your solution to your problem seems to combine relative simplicity with some "brilliance".

If any personnel ar to use the elevator at any time, or could be beneath or near it, make sure you have thoroughly checked for potential problems. The same applies if it could cause any damage to property etc. Good luck - it sounds good.

Thanks:

I had not considered either of these methods.

bill_michaels

There ya go! Weld sideplates on the beams.

This is the best answer I have seen... standard practice, not difficult to accomplish, adds a lot of stiffness to beam without a lot of weight. Good answer, and a GA vote.

Wouldn't it be easier to select a beam which didn't require sideplates?

Hello ba/ael.

Easier? Probably. In your #1 you already voiced this, but OP has not embraced the suggestion. I am not certain why not; it is a valid engineering solution. It sort of sounds like this platform is constructed and in service, and an attempt to "fix 'er up" is being made.

Our friend did not really tell us much. ASTM A992? LL/DL? Span? Spacing? Country? Applicable codes? Is this platform already constructed, and this is a fix?

There are many possible solutions... Deepen beam, add sideplates, use two junior channel welded back-to-back instead, add more beams... these are all just shots in the dark.

Besides, why construct something simply when it can be complicated and wonderful?

Hello doorman and everyone else who wonders why I don't simply consider a stiffer beam:

I certainly appreciate all the interest in my question. My second reply obviously didn't get through.

No, the platform is in the planning stage. I certainly wouldn't build anything until I was sure of the design. As to the using of a stiffer beam, we wish to not go over 8" inches in total thickness of platform.(preferably closer to 7") We already have an ample supply of standard 6x3-3/8 beams in both 17.25 and 12.5 lbs per ft, and of course would prefer to use them. As we already have them, cambering them is probably out of the question.

Of course, heavier beams add to the weight, which increases the load on the cables and hydraulic system. Are there stiffer beams than the 6 x 3 3/8 ones weighing 17.25 pounds per foot which are not over 6 inches deep?

Even the 12.5 ones will, according to my AISC book easily support the desired load, but I find the deflection undesirable. The 1/4" thick diamond plate is 8' x 20' and all one piece. In normal use, the platform will be supported by latches along the perimeter. When something needs lowered, the latches will retract and the platform will hang from 6 steel cables, three on a side to be lowered by a hydraulic cylinder.

Deflection is not a problem loaded, but as this plate forms part of a garage floor I fear deflection will be suffifient to "pond" water from melting ice and snow in it's normal "supported from the edges" state. Of course one can always place a basement jack or two under the middle of it, but I would much rather be able to tighten rods until the floor became level or slightly convex when it had a vehicle parked on it.

I'll have to think about all this again, and greatly appreciate everyone's input.

bill_michaels

bill,

The most efficient way to improve the stiffness of your 6" beams is to add a plate on the bottom flange and to weld the top flange to the diamond plate.

There are heavier 6" shapes. In Canada, we have W150 x 30 and W150 x 37 (W6 x 20 and W6 x 25 in the U.S.A.). The depth is 157mm and 162 mm (6.18" and 6.38") respectively. The flange width is 6" for each.

Thanks Doorman, bad week I needed a kind word.