Calculating Friction Factor
Calculating Friction Factor
(OP)
Has anyone ever used this before to calculate the friction factor for a spread footing? I have attached an excerpt from the Civil Engineer's Handbook (Merritt).
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http ://files.e ngineering .com/getfi le.aspx?fo lder=07aa0 1b9-55c1-4 6ea-bbb0-4 2c7f4f2fd4 9&file =Friction_ Angles.jpg
I am looking at placing about 2 feet of crushed stone under a spread footing to reach a factor of safety of 2 for sliding and overturning. I have seen other engineers use the tangent of ¾ of the friction angle. I am thinking of using 38 degrees for the crushed stone, which is at the lowest end for gravel. This would yield a friction factor of 0.78. It just seems much higher than I have used before. I have typically just used the friction factor as determined in the soils report which I have not seen higher than about 0.55 for compacted structural fill; I just do not have a report to use in this situation.
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I am looking at placing about 2 feet of crushed stone under a spread footing to reach a factor of safety of 2 for sliding and overturning. I have seen other engineers use the tangent of ¾ of the friction angle. I am thinking of using 38 degrees for the crushed stone, which is at the lowest end for gravel. This would yield a friction factor of 0.78. It just seems much higher than I have used before. I have typically just used the friction factor as determined in the soils report which I have not seen higher than about 0.55 for compacted structural fill; I just do not have a report to use in this situation.
"Structural engineering is the art of modeling materials we do not wholly understand into shapes we cannot precisely analyse so as to withstand forces we cannot properly assess in such a way that the public at large has no reason to suspect the extent of our ignorance." -Dr. A. R. Dykes





RE: Calculating Friction Factor
In any case, effectively codes have become truly conservative respect friction, for as you say .55 is common for gravels, and angles of inner friction sometimes are not allowed to be counted for calculus above 35º.
Again, as the average behaviour, the more high values really attained for friction and inner friction are available as average, and so it must seem that the conservatism in using lower values must be a way of getting characteristical values standing 95% of the times.
RE: Calculating Friction Factor
2/3Tan(38) = 0.52
RE: Calculating Friction Factor
RE: Calculating Friction Factor
"Structural engineering is the art of modeling materials we do not wholly understand into shapes we cannot precisely analyse so as to withstand forces we cannot properly assess in such a way that the public at large has no reason to suspect the extent of our ignorance." -Dr. A. R. Dykes
RE: Calculating Friction Factor
"Structural engineering is the art of modeling materials we do not wholly understand into shapes we cannot precisely analyse so as to withstand forces we cannot properly assess in such a way that the public at large has no reason to suspect the extent of our ignorance." -Dr. A. R. Dykes
RE: Calculating Friction Factor
Check an AASHTO Bridge Design Specifications Manual for a table of ultimate friction factors. For mass concrete poured on clean gravel, gravel-sand mixtures, and coarse sand, f = 0.55 to 0.60 and the friction angle is delta = 29 to 31 degrees. The table is based on U.S. Department of the navy (1982), probably NAVFAC.
Tan(29) = 0.55 and Tan(31) = 0.60.
Yes, you still need a safewty factor. The f values are ultimate values.
RE: Calculating Friction Factor
the same AASHTO code is. 29 deg for say cemented gravels against concrete? Inner friction is over 40º and the gravel is attached to the concrete. The 2/3 factor is customary but scarcely more than that.
RE: Calculating Friction Factor
"Structural engineering is the art of modeling materials we do not wholly understand into shapes we cannot precisely analyse so as to withstand forces we cannot properly assess in such a way that the public at large has no reason to suspect the extent of our ignorance." -Dr. A. R. Dykes
RE: Calculating Friction Factor
RE: Calculating Friction Factor
RE: Calculating Friction Factor
RE: Calculating Friction Factor
1) At some point you can include passive resistance.
2) My professor (JM Duncan) told us the biggest uncertainty in soil strength is friction angle. Apply your safety factor to friction angle and then see what you get. If you took a friction angle of 38 degrees and a safety factor of 1.5 you'd get tan(38/1.5)=0.47, which may be what is being accomplished when you use the "2/3" value.
3) Another dimension to this whole problem is to what extent you fully mobilize the interface friction between the two media (i.e., concrete and aggregate/soil). I'd think the wet concrete would pretty much knit to the aggregate and you'd get pretty much full moblization; however, some folks use 2/3rds to figure this out too.
Hope this helps.
f-d
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