Foundation design: pad footings with biaxial moments
Foundation design: pad footings with biaxial moments
(OP)
I am currently designing concrete pad footings for transmission towers with biaxial moments combined with horizontal forces. All the reference literature I have only consider the case with full compression under the full area of the base i.e no tension to underside of the base which would imply that my resultant base reaction has an eccentricity of less than or equal to Lx/6 or Ly/6, were Lx & Ly are the base length and breadth on plan. This approach is resulting in a concrete pad footing size which in my opinion is uneconomically large, take for example the following loading (unfactored);
Mx=1539kNm Fx = 150kN; My=1750kNm Fy = 130kN, V=122kN concrete density of 24kN/m3;top of base at ground level.
My base works out at 6.8m x 6.8 with a depth of 3.45m if I use the approach outlined above.
Does any one out there have a design approach/formulae to determine a base size in which only part of the base is under pressure.
Please help.
Mx=1539kNm Fx = 150kN; My=1750kNm Fy = 130kN, V=122kN concrete density of 24kN/m3;top of base at ground level.
My base works out at 6.8m x 6.8 with a depth of 3.45m if I use the approach outlined above.
Does any one out there have a design approach/formulae to determine a base size in which only part of the base is under pressure.
Please help.






RE: Foundation design: pad footings with biaxial moments
RE: Foundation design: pad footings with biaxial moments
Steve Braune
Tank Industry Consultants
www.tankindustry.com
RE: Foundation design: pad footings with biaxial moments
RE: Foundation design: pad footings with biaxial moments
I purchsed RISAFoot. It does the analysis for moments about either axis, as well as talking into account the moment caused by the horizontal (shear) force.
You can get mnore info about the program at WWW. RISATECH.com.
RE: Foundation design: pad footings with biaxial moments
RE: Foundation design: pad footings with biaxial moments
Trilinga
RE: Foundation design: pad footings with biaxial moments
Also, if you have a copy of AASHTO or AREA around you can use the table that they've employed for checking bearing pressures. This table takes the guess work out and is so easy to use.
RE: Foundation design: pad footings with biaxial moments
Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.
RE: Foundation design: pad footings with biaxial moments
I sent this message yesterday, but surprisingly it did not meet the forum.
Can you please post the complete web page address for Mathsoft Collaboratory site, Civil Engineering folder? Thank you for your help.
Regards
AM
RE: Foundation design: pad footings with biaxial moments
DaveAtkins
RE: Foundation design: pad footings with biaxial moments
Once inside the site there go to the Civil Engineering folder, and make a search for Footing
You will get (I have got today) 13 items, some of mine and some of others, and I think nor less than 3 serve somewaht to your intent.
RE: Foundation design: pad footings with biaxial moments
Thank you for this information. I will explore it. Another question - have you [ or any one in this forum] used PCA-Mats software [ prepared by PORTLAND CEMENT ASSOCIATION ]for designing foundation? What is your feeling about this programme, in case you have used it. I never used it before but having a look at it. Any feed back will be helpful.
Regards
AM
RE: Foundation design: pad footings with biaxial moments
RE: Foundation design: pad footings with biaxial moments
RE: Foundation design: pad footings with biaxial moments
Note also that other interesting foundation worksheets in the Mathcad Collaboratory may escape the Footing keyword so it is better browse a bit as well, yes.
RE: Foundation design: pad footings with biaxial moments
DaveAtkins
RE: Foundation design: pad footings with biaxial moments
Thank you all for your valuable suggestion. Yes, we are trying to get staad.foundations. It is nice to get a good feed back though. I tried PCAMATS any way but could not run it. Programme has a few limitations in my opinion. I modelled a raft foundation for a triangular tower. One leg has downward reaction, others have uplift. Program did not run - it gave floating point error. I do not have the knowledge to fix this, but will try again. Programme is not wrong - it is my understanding. Again, any input from any of you will be helpful.
ishvaaag, thanks for the tip with MathCad. I do not have MathCad - can you please advice how to open *.mcd files without having MathCad? It appears they had a free download at some stage, but does not seem to work now! There are a lot of valuable information in this site. I would like to explore it.
Best regards
AM
RE: Foundation design: pad footings with biaxial moments
AASHTO 16th Edition (pages 51-53) contains very clear, simple and accurate method to solve your task.
Maximum soil pressure under rectangular footing under biaxial bending:
qmax = K*R/BL
K - coefficient from the chart, function of eccentricities in both direction.
R - vertical load
B and L are footing dimensions.
this method assumes triangular pressure distribution diagram under the footing. You can find the chart at www.yakpol.net/ftg_graph.jpg
Hope it helps!