I have a structure with 200x1000mm ductile walls to resist lateral forces. To deign a footing for this wall is it correct to define a very stiff beam at the base of the wall (say 200x200 with moment of inertia increased by 10^6), and placing a column with the same dimension as the ductile wall (200x1000) under the beam as in the attached screen shots. I have also attached the reduced forces at the base of the wall as well as the base reaction of the column.
Stroxy,
You may also consider the following approach:
1. Define a linear support under a wall
2. Use the linear support results as load for separately designed foundation
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Definitely this is not a standard approach and I would compare it with the other solutions first. What I would think of is what kind of foundation are you going to have under this wall:
- spread footing
- continuous footing
- raft foundation
This decision implies which of the design module to use and what would be the best way of applying loads from the watt to the foundation.
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Exactly my considerations. Firstly I would like a spread footing under the wall, seeing that it is only 1000mm long. I am assuming that the reduced forces in the cut at the base of the wall is what would normally go into the foundation design, hence I am using that as a comparison. Am I correct in my assumption?
Having said that, should I want a Grid foundation of connected strips then I believe I should be able to model a grid of elastic bedded beams under the walls as you suggest and get a design for the beams. However is allowable bearing capacity checked in such a model i.e Elastic bedded beams?
Can it be assumed that for such short length the vertical load distribution at the base of the wall is approximately uniform?
Well yes and no I guess. Looking at the length the assumption of uniform loading should be valid. However as a ductile wall with boundary elements, the assumption is that the tension and compression forces are concentrated in these boundary elements at the wall ends.
Why do you ask?
My approach would be to use the RC Spread Footing module in the Continuous Footing mode and depending on load distribution I would either define N as the average or maximal value of reactions under the wall.
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I was reading this topic and wanted to share my humble opinion :
- ductile walls : this means you are designing a project in a seismic area ? if you do so, it is recommended to have stiff lower level system : this means raft, spread or continuous footing but tied up together by ground beams in both direction.
- uniform loadings for the design of the footing : it is a shear wall -> it means you will get bending moment in the reaction then the load cannot be uniform, it is trapezoidal loadings.
depends if your structure has bracing only provided by ductile walls or if you have long shear walls (then maybe this wall can be ignored from the primary structure)
- beams on elastic supports : this means your model will requier non linear analysis, right arthur? much more complicated and time consuming... i would separate the models for the buildings and for the foundations (you can get some api in this forum that take the reaction of a model in order to convert it as loads for another).
Have nice day
- beams on elastic supports : this means your model will requier non linear analysis, right arthur?
Only when you expect having uplift which I think is not what you want under the wall
@ Sautier
ductile walls : this means you are designing a project in a seismic area ? if you do so, it is recommended to have stiff lower level system : this means raft, spread or continuous footing but tied up together by ground beams in both direction
Yes, it is in a seismic area and the spread footings shall be tied together with ground beams in both directions
uniform loadings for the design of the footing : it is a shear wall -> it means you will get bending moment in the reaction then the load cannot be uniform, it is trapezoidal loadings
Yes it is a shear wall and the reaction will be trapezoidal, but the short length of the wall "could" mean that an average uniform load could be assumed i.e load signs would not change so as to negate each other (stand to be corrected). The structure is braced primarily by these ductile walls. There is an off-centre elevator shaft which actually introduces torsion into the response, but that could be the subject of another discussion.
i would separate the models for the buildings and for the foundations (you can get some api in this forum that take the reaction of a model in order to convert it as loads for another).
I would be very much interested in this if it exists!!
My approach would be to use the RC Spread Footing module in the Continuous Footing mode and depending on load distribution I would either define N as the average or maximal value of reactions under the wall.
Artur, with a 1000x200mm wall won't this approach give me a very wide (so as to satisfy allowable bearing capacities) 1000mm long spread footing?
"Yes it is a shear wall and the reaction will be trapezoidal, but the short length of the wall "could" mean that an average uniform load could be assumed i.e load signs would not change so as to negate each other (stand to be corrected). The structure is braced primarily by these ductile walls. There is an off-centre elevator shaft which actually introduces torsion into the response, but that could be the subject of another discussion."
To design your footing you shall take into account the excentricity due to the moment. For exempla use the Meyerhof formula :
Q = N / ((L - 2 x e) * B) where e is M/N. you will apply something uniform to to a reduced area.
Otherwise you could exceed the soil bearing capacity.
"I would be very much interested in this if it exists!!"
Follow these links :
A fabulous topic where rafal and arthur have put the links to all the api developped in this forum in order to make our daylives less miserable or much more easier
http://forums.autodesk.com/t5/Autodesk-Robot-Structural/useful-addins-for-Robot-API/td-p/3899448
Speficic link for the api we have talked about :
TS
@stroxy wrote:
My approach would be to use the RC Spread Footing module in the Continuous Footing mode and depending on load distribution I would either define N as the average or maximal value of reactions under the wall.
Artur, with a 1000x200mm wall won't this approach give me a very wide (so as to satisfy allowable bearing capacities) 1000mm long spread footing?
I think you may want to compare the results you have for the above approach and for a spread footing with the 1000x200mm column and then decide which one to choose.
So to do the comparison with the spread footing under a 200x1000mm column, I am going to save the substructure comprising the columns and nodal supports. Then export reduced forces from base of wall panels into excel and apply them as loads to the saved substructure, and design pad as footings. Theoretically should work.