Foundation design: pad footings with biaxial moments 2

[jum]

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.

 

[JedClampett]

"Foundation Engineering" by Peck, Hanson and Thornburn has a method in section 24.4. It basically works out to trial and error.

 

[JoeTank]

Fintell"s "Concrete Engineering Handbook" is also a good reference for eccentrically loaded foundation with partial bearing conditions.

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com

 

[ishvaaag]

You can freely download worksheets for these things from Mathsoft Collaboratory site, Civil Engineering folder.

 

[CSEllc]

I don't know hao many or how often you have the similar condition, but I found that after going thru the trial and error process on a few footings I opted to buy a footing design program.

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.

 

[JAE]

I would second CSEIIc's recommendation.

 

[trilinga]

The same topic was discussed in a different thread earlier. Refer thread507-4647

Trilinga

 

[Qshake]

The Foundation Engineering text by Braja Das has examples for bi-axial moments. Very good for examples.

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.

 

[Focht3]

Those are huge footings - why are you using pad footings instead of drilled piers? What soil parameters are you using? Where is the site located? Based on the loads, it looks like this is a tangent structure for an electric transmission line. If not, what is it?



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[am]

ISHVAAG

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

 

[DaveAtkins]

I have always designed footings with biaxial moments using page 133 of Foundation Design by Teng. It is a chart, and it works even when the load is outside the Kern limit. Plus, it is not trial and error and it is very fast.

DaveAtkins

 

[ishvaaag]

http://collab.mathsoft.com/~mathcad2000

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.

 

[am]

ishvaaag

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

 

[fleisher]

You can try a new program from STAAD called STAAD.foundations. It is similar to SAFE and more powerful than PCA Mats. It is an overkill, however, if you are just doing single footings. Really good for pile caps and mats.

 

[JedClampett]

PCA Mats is very similar to other PCA generated programs such as PCA-COL. It works quite well, I suspect, if you are doing designs exactly as PCA expects you to. Anything else will aggravate you mightily. I'm not aware of any other software for this type work, but I'm sure the other forum contributors can help.

 

[ishvaaag]

I have not used PCA Mats so am not able to provide comments about it.

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.

 

[DaveAtkins]

I have decided that the 3D software I use (RISA-3D) is adequate for modeling mat footings (I use springs and a finite element analysis). I don't see the value in buying PCA Mats.

DaveAtkins

 

[am]

ishvaag, fleisher, jedClampett, DaveAtkins

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

 

[yakpol]

Jum,
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:
q_max = 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

http://www.yakpol.net/ftg_graph.jpg

Hope it helps!