Floor Loading 1

[haggis]

Hi all, I had posted a question previously about floor loading and received a response from Rawand which I appreciate considering my info was vague.
I'll try to describe a little better as to what my question was.

I have a concrete pit which will eventually contain machinery and the floor decking will be subject to pedestrian traffic only. However,before the machinery is installed, the pit will have to be plated over temporarily. At this time, the decking will be subject to scissor lift traffic for overhead work.

The pit is 26'-6" long x 16'-6" wide. I had planned on placing a beam 26'-6" long on the centre line of the pit,welded at each end to the curb channel of the pit and having (2) 4" dia pipe supports each one being 8'-4" from each end. Perpendicular to this beam I would have beams 16'-6" long at 2'9" centres again each end welded to the curb channel and resting on top of the centre beam. The pit would then be plated over with 3/8" plate, each plate being cut to 2'-9" x 8'-3" so that they have bearing on all four sides on the steel.

The scissor lifts would be 3400 lbs including the men on board.

Designing for a load of 150 psf could I take a strip load on the centre beam of 8.25 x 26.5 x 150 = 32793.75 lbs then divide by 26.5 to give me a distributed load of 1237.5 lbs/ft or is this beam subject to several point loads ?.

For the perpendicular beams, I could do the same using 2.75 x 8.25 x 150 = 3403 lbs , divide by 8.25 for a load of 412.5 lbs/ft.

This seems overly simple,but I have used a piece of software called BeamBoy and the results I get tell me that I can us S6 x 12.5 sections for the support steel throughout.

On the perpendicular beams for example the results look favorable ef.of 0.027" Max M of about 1800 lb-ft and bending stress of about 2900 psi.

Using the same method on the centre beam the results are
Def. of 0.09" Max M of 5900 lbs-ft and stress of about 10000 psi.

This seems OK or am I missing something? The choice of s6 x 12.5 is because we have miles of it on hand and it is temporary.

Thanks for any help in advance.

 

[ishvaaag]

Well, I don't know what the software does.

Anyway the main thing I see lacking in your approach are two, one in the calculations, and other in the use.

Respect the calculation, you can't use the entire tributary width of the plate for the calculation but one to be evaluated effective width. Or alternatively you make a 3D model where at least the plate is separately modelled and its influence properly passed to the main and perpendicular beams. Even so since stresses look low you may have a satisfactory structure for your design.

You may find guide on effective stresses in texts of

Galambos

Merrit

or Gaylord, Gaylord and Stallmeyer.

This one for stiffened plates gives for effective width the lesser of the actual width or

beff = t·0.95*SQRT(4·E/stress)

where t is the thickness
SQRT the square root function
E is the modulus of Young of the steel in the plate
stress is the compression stress in the plate at the factored level


Respect use, once the calculation is properly made and if the design is still satisfactory, you may still get local plastifications (kinks). At your thickness and loads these should be small, but some investigation of this would be sound thing to do.

 

[Neilmo]

Haggis,

I would also check whether the scissor lift is putting any heavy point loads into your temporary floor. If it is, this case will also need checking out as it may be more critical than your 150psf.

Regards,
Neilmo

 

[haggis]

Thanks for the quick responses from Ishvaaag and Neilmo.
I should clear up that I am not trying to be a structural engineer although I am doing the work on this. It will be sent to a Registered Engineer for review and certification.
I just don't want to look like an ass.

Ishvaaag, I thought the tributary width was the sum of half of the spans on either side of the main member but have since learned that the fact that the secondary members are continuous,it is slightly different.

Neilmo, If the scissor lift sits on a plate section, isn't the load transmitted through the plate a distributed load on the supporting members? The only thought I had was that is if 2 of the wheels happen to sit on a plate seam, directly above a member that this might be considered to be a point load on that member. Would this be true?

Thanks for your patience, people.

 

[Neilmo]

Haggis,

If the scissor lift sits on a plate section then yes, it could be assumed to uniformly distribute the load into the supporting members. The distribution of this uniform load will of course depend on exactly where the wheel(s) are in relation to the supporting members. But if the scissor wheels sit directly on a supporting member then, yes again, this is a point load (or 2 point loads if 2 wheels) on that member.

So you are right on both counts.

I would always play safe and check out the worst case scenario, as I suspect the point load case on the supporting member is more severe than the UD case.

Glad to be of help,
Neilmo

 

[haggis]

Neilmo,

Thanks again for your help. Despite my limited structural knowledge, I now feel a lot better about this project.

Best Regards,
Haggis

 

[chicopee]

I was reading your description but after a while I got lost on how you were going the weld that structure together. While it seems that your analysis is whithin ballpark I would tell you to weld stiffners at points of concentrated loads on the curb channels and your W beam as to resist any crushing of the webbs.