Concentrated Load on continuous one way slab to supporting beams 4

[wrxsti]

hello i am trying to analyze concentrated load on decking to beam force transfer
in particular concentrated load directly over beam

in a previous post someone referred to image below

for distribution of concentrated loads for deck slab design


Is it plausible to use the same for force distribution on the supporting beams?

Instead of image below

Perhaps this could be used (image below)

Reference for reaction calcs image below

Also could you refer to further calculation to incorporate the stiffness of the slab into spreading to adjacent supporting beams
in the scenario of concentrated load directly on top of the beam?

 

[r13]

My question is, you have created and ran a FEM analysis, why the need to justify the distribution by hand?

 

[jayrod12]

why the need to justify the distribution by hand?

Because without validating the model, what good is it?

Garbage in, garbage out. If you can't somewhat verify that your model is acting appropriately, you could very easily have a model that is giving you the answers you want, but is horribly incorrect.

 

[wrxsti]

@BARetired

Yes, the more slab incorporated, the more distribution for sure

The intention was to mimic this procedure used to reduce the design bending moment for concentrated loads for slabs

considering 4inch support and 4inch thick slab
totally 12 inches

 

[r13]

The FEM results can be easily verified by turning off all other loads, and run the questionable load only. Hand calculation for an involved system can produce misleading results (due to over simplification), and further confuse the issue.

 

[r13]

if before this load was introduced another point load was existing other than center causing an deflection before the
center point load.
what procedure could be taken to modify the stiffness in the model to represent that?

If the same beam had another concentrate load (P1) at any location on the beam, that has caused large deflection prior to placing present concentrate load (P2), then the stiffness does not change, but, now you have a situation of - the slab strip is supported on uneven supports, with the loaded beam has settled the amount of deflection due to P1. Let's say, P1 is located on L/4 and produced a x" deflection on the beam at the location of P2 (not at L/4), thus, in your model, the elevation of supports is 0.0 x 0.0 before loading P2. The slab strip will have built-in stress/force due to P1, that has to be superimposed on the results of P2, when checking the effect of P1+P2 on the slab strip.

As you can see, this method is simple due to the simplification of the real structure with simple loading condition. But, as the situation gets complicate, the inaccuracy due to simplification will accumulate. So, for more involved structures (geometry, loadings..etc), I would rather verify the results using computer method, by judging the results one case at a time. I think you know how to verify the computer outputs by both ways, otherwise you should learn how.

 

[r13]

BA,

The narrow band is to conservatively capture the stress distribution suggested on the vendor's literature/drawing.

 

[MIStructE_IRE]

Things like this remind me of a quote I saw years ago ‘No Engineer should be more clever than is necessary...’ Now go and weld a plate onto that beam!

 

[r13]

To me, approach/method to structural design/modification is rarely one way, though there is preferable solution, but it could not be feasible for reasons. The time took to explore other approaches is not wasted, its a learning, which may not work now, but beneficial for later. Admittedly, to strengthen the beam is the most clean/preferable approach, but if the slab is capable of reliving the load demand on the beam, with its capacity can be verified, then why not leave the beam alone? If the slab is inadequate, and strengthen the beam is not feasible, then it is good to know the slab requires strengthening. My point is - don't jump into conclusion too quick before we understand the OP's needs/situation. Ask pertinent questions, if you are willing to offer your knowledge.

 

[MIStructE_IRE]

I’ve been through it and have offered my two cents with regard to the spring supports and analysis approach. It still doesn’t sit entirely right with me. If this is a way to not have to admit a mistake, and we’ve all been there, I find its best to just put the hand up and get the beam strengthened.

 

[r13]

For the sake of discussion, let's assume the FEM analysis indicates that the beam is adequate but slab is over stressed. What will you do?

 

[jayrod12]

The beam is adequate for the entire load without reinforcement?

If that were in fact the case, I wouldn't of even checked the slab.

If you mean that using the FEM analysis and determining that you need load sharing in order for the beam to work, but the load sharing causes overstress in the slab, then I would be reinforcing the beam and the problem would be solved. There isn't exactly an easy way to reinforce the slab. But there are multitudes of options available for reinforcing the beam.

 

[MIStructE_IRE]

If the beam is adequate - leave it be.

If the slab is failing - supplement it with additional beams or strengthen it with carbon fibre strips.

I believe a clear load path is best.

I suppose though we’re going off topic - OP, after all this, does your system now work or not?

 

[r13]

Sorry, I didn't frame my question correctly. Let me change the scenario to as below:

The load was provided after the floor design was done, and I knew well the beam will fall short with 100% of the load placed on it, and the call for strengthening/up sizing is undesirable for its trouble and cost.

So, I took out my pen and paper, performed a simple evaluation as described. The results indicated the beam will survive after consider load sharing.

FEM analysis was then carried out to confirm the finding. The beam found to be fine, but the slab was overstressed. Now the decision time has came - strengthen/up size the beam, or add one inch or two to the slab. I think the decision will depend on ...

 

[jayrod12]

Wouldn't adding an inch or two to the slab cause more issues?

Generally speaking, in my experience, strengthening a slab is far more onerous and costly than reinforcing a beam.

Maybe it would be easier to install a cross beam that enforces the load sharing to two adjacent beams. But that's situation dependent just like everything else.

 

[r13]

Wouldn't adding an inch or two to the slab cause more issues?

Good catch, agreed.

 

[wrxsti]

based on the fem model which i redid for the slab to cover entire bay
where in the load is distributed

The majority of slab is not overstressed

only the corners the stress values go up really high

this is NOT normal though right ?

 

[r13]

Stress concentration, the corners are highly restrained.

 

[jsmith234]

@retired13 how should it be accomodated?

 

[r13]

Similar to the corners of floor openings, additional reinforcement/diagonal bars will alleviate the stress, and prevent diagonal cracks.

 

[jsmith234]

i mean as shown that area is very small less than 1ftx1ft