analysis theory for retaining wall 1

[GerhardSA]

hi guys,

I designed a besement like structure a few weeks ago, I did the initial bending moment calculations assuming the walls as cantelevers( the roof can be removed), only taking into account the pressure of the soil.

I then checked it on Prokon2.2 as a retaining wall and got the same moment I did with my hand calculations as the maximum moment for the base. But the max moment for the wall part was a full 23KNm less. The structure has no toe, the wall and floor differ 50mm in depth.

I compared this to other engineers designs in our company with the same program, there is always a difference in BM.

Does anyone have any insight on this? Is there something I'm missing?

Someone told me the axial force in the wall reduces the BM, I read something like that but couldn't confirm, and not by that much.

 

[asixth]

I don't see how the retaining wall in that diagram will get axail load. If there was a slab sitting ontop the wall it may induce a bending moment due to P-eccentricity.

The difference in bending moment may be due to differing earth pressure co-efficients, different assumed unit densities (I take water as 10kN/m^3 and soil as 19kN/m^3...It is too late at night to convert these units to imperial), or moments are taken at different locations. Personally, I like to analyse a wireframe modelled to centreline goemetry, this will give a greater moment than it would if the moment was calculated at the top of the slab, opposed to centrelines.

 

[GerhardSA]

Yes, that's it. The moment are taken about different places on the structure (base).

As for the axial load, it is the weight of the wall. It is negligible in this case, but some of my colleagues says that axial force can reduce moment if the axial force is enough?

anybody maybe have any material or explanations for this?

I tried explaining it with a wireframe analysis, but only deduced that as far as negligible forces go the weight of the wall will act as a destabilizing force with slight deflection.

 

[miecz]

This has nothing to do with axial force. The bending moment is a function of the height, cubed. If the height of the stem is 3.4m and the wall 3.7m, then the wall moment will be (3.4/3.7)[³] times the moment used to calculate soil pressure. So there's your 23kN-m. However, for the design of the heel, I would use the same moment as for the design of the stem.

 

[youngstructural]

The axial force cannot reduce the bending moment... Both are simply a function of your newtonian laws and summation of forces. Also to be a bit more accurate, the difference in bending moments calculated is due to where you consider rotation to occur, ie: where the "hinge" of the system is, not where you "take" the moments. Is the program taking the correct location?

The axial force has a beneficial effect on the wall with regard to the bending moment capacity. Your wall can take more flexual load when there is a vertical stress, but again this vertical load has no effect on the bending moment demand, only increases the capacity.

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[dik]

Axial loads from an interaction vantage have mostly a negligible effect. From minor P-Delta they can slightly increase the moment. I normally neglect them; the system is generally stiff enough that this can be done, in particular, since the real soil pressure is so difficult to determine.

Dik

 

[escrowe]

I see the Couloumb method cited on your drawing.
Are your colleagues assuming soil/wall friction in their calculations?
A portion of that is treated as axial load.

 

[miecz]

Please ignore the last sentence of my earlier post. In this case, the heel should be designed for the base moment.

 

[dpenco]

I need to design fence post which would have Permahedge applied to mesh between posts.
Has any body figured the percentage of blockage due to Permahedge ?
Not much info available from manufacturer of this product.
ASCE 7-02 stipulates as solid sign method. Any method to figure area of blockage to total area of Permahedge.

 

[escrowe]

1) Huh?

2) Start a new thread.

3) Huh?

(to avoid blockage, do not consume Permahedge)