Slab fondation on ground FEM 2

[Simlac450]

Hi guys,
I designed a slab on ground with uniform loads + linear loads on it on SAP2000 and I found the results intriging.

uniform load: self load, SDL=1 kPa, LL=1.9kPa.
line load on the perimeter (factored): 42kN/m
line load on the red zone (check picture)(factored):26kN/m
soil subgrade modulus(k): 9000kN/m^3
250mm thick.
I locked the corners in the xy direction

Im confused with the negative moment under the line load, do you know what happen here, I would have expected a positive moment everywhere

The results are in Knm/m

 

[centondollar]

Why would you expect positive moment everywhere? It is not a simply supported beam. The problem you describe is similar to beam on elastic springs with point loads, for which bending moment can be both positive and negative.

 

[Simlac450]

Well under a line load I would have expect a positive moment.. I asked for help not for hate you can keep you bad attitude for yourself

 

[Just Some Nerd]

Sure that positive and negative aren't flipped somehow? If there's nothing where the blue is and a line load where the red is, then it seems as much.

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Why yes, I do in fact have no idea what I'm talking about

 

[HTURKAK]

My points are,

- Uniform loads ( self load, SDL=1 kPa, LL=1.9kPa.) are negligible for moment effects ..

- Perimeter line load and line load on the red zone are effective ..

- hogging moment should develop under the line load on the red zone ..

- IMO, probably sign convention is not the default sign convention or mistake at loading direction ..

- Pls post the loading ,local axis diagram , deformed shape of the slab and global axis to get better responds..







If you put garbage in a computer nothing comes out but garbage. But this garbage, having passed through a very expensive machine, is somehow ennobled and none dare criticize it. ( ANONYMOUS )

 

[ThomasH]

A few thoughts on this,

The total uniform load is 2.9 kPa, is that a "dead" load or are you using different load patterns? Ultimately, the uniform load should not be the issue in this.

Line load on the perimeter, does that mean the two vertical sides or all four sides? Line load on the red zone, is there a line load in the blue zone or what causes that moment? You have no (or very small) moments around the perimeter, all four sides, does that mean that there are no vertical boundary conditions except the stiffness of the subgrade? I see that you have locked the corners in the xy-direction, why? Have you checked that there are no membrane forces in the slab that you boundary conditions have created?

You have given what I assume is the thickness of the slab, 250 mms. You also need to include the E-module for the slab, otherwise the solution won't be correct.

I would start by checking that the deformation of the slab seems ok. Because the deformation is the base for the curvature that that ultimately gives you the moment.

 

[Simlac450]

I locked xy direction because otherwise it gave me a stability error. The convention sign is positive below zero, negative above it. No boundary on the perimeter except the stiffness of subgrade (its a foundation slab on ground), E is the modulus of reinforced concrete (given value in SAP2000), I join the deflection diagram

 

[ThomasH]

You don’t need to lock all four corners for stability, if that is what you have done.
Do you think the deflection looks reasonable?
Have you checked the support reaktions? I suspect that you have springs with tension.

 

[HTURKAK]

- If sign is positive below zero and upward is negative, the mid portion is deformed upward .. (min -2 E -3 ).. Is it possible if all the loads at gravity ( downward positive ) direction? Apparently the springs at this region is at tension ..

- Check your gravity multiplier and the material unit wt ( if self wt defined as UDL , the unit wt of conc. shall be set to zero..)

- check your loads ( self weight 250 mm =6.25 kN/m2 ⇒ 6.25 kPa)

- You may prefer self locking or assign horizontal springs ( say 10 % of vertical ..) or assign horizontal supprts at some nodes at center zone rather than locking the corners in the xy direction..







If you put garbage in a computer nothing comes out but garbage. But this garbage, having passed through a very expensive machine, is somehow ennobled and none dare criticize it. ( ANONYMOUS )