Temporary Foundation Design
Temporary Foundation Design
(OP)
I'm designing a temporary bridge foundation for 100 ft clear span bridge structure. Contractor would like to install on a log sill, as timber up to 1m in diameter is abundant on site. Looking for a reference that might deal with this situation. The foundation soils will permit 200 KPa allowable bearing capacity. I would like to determine minimum diameter (width B) for a 10 m log length. I've used a traditional design approach assuming a width B equal to the diameter, am I overestimating the contact area. Any thoughts?





RE: Temporary Foundation Design
Mike McCann
MMC Engineering
RE: Temporary Foundation Design
RE: Temporary Foundation Design
Mike McCann
MMC Engineering
RE: Temporary Foundation Design
Keep in mind that duration of loading should be a consideration. I doubt that you will be loading a log "mudsill" for a duration long enough for shrinakage of the log to be a serious consideration. Wood shrinks more parallel with the growth than it does perpendicular to the rings. Bearing on the log perpendicular to the grain should be checked to ensure you do not exceed a reasonable value for the specie. Using the diameter of log as the width of bearing should be acceptable assuming that you can live with the resulting settlement until the soil underneath reaches an equilibrium state.
Ralph
Structures Consulting
Northeast USA
RE: Temporary Foundation Design
I would expect a month or two of the log in place an any typical construction situation. Without leveling jacks to maintain the proper heights in the mix here, limit the moisture content and the stress.
Mike McCann
MMC Engineering
RE: Temporary Foundation Design
One of the difficulties with posts on this forum is that no one seems to remember to post the project's general geographic location.
For some challenges, what works well in Florida may be totally ridiculus in Montana or Maine. Local climate often has an influence on the solution.
Just venting a tiny bit.
Ralph
Structures Consulting
Northeast USA
RE: Temporary Foundation Design
As a data point, think of a laterally loaded pile - I wouldn't imagine that much more than half the soil on the compression side would be mobilized...
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RE: Temporary Foundation Design
RE: Temporary Foundation Design
Mike McCann
MMC Engineering
RE: Temporary Foundation Design
If you draw a radius at an angle of Phi from vertically downward, the radial and tangential stresses at that point have an obliquity of Phi. Beyond that point, the soil will tend to shear or slide on the steep side of the log and not support the applied load well. The width within these limits is 2 R Sin(Phi), or about 50 to 60% of the diameter. If the soil has cohesion, it will do somewhat better.
RE: Temporary Foundation Design
If the logs are tied together into one mat with through-rods (so the logs cannot spread apart), won't the soil (between the logs) simply deform and "extrude" upward and be trapped between the logs, eventually compressing to provide an effective bearing area almost equal to the diamter of the logs? Certainly using an effective width of 60% of the diameter of the log is reasonably conservation.
With all that has been said here, why not split the logs lengthwise (say with a portable bandsaw mill) and use them with the flat side down? Then add a layer on top running perpendicular to distribut the load from whatever falsework is constructed for the the bridge. By utilizing a mill to split the logs, one could also shave off a bit on the top to make them all of equal height, thus making the lay-up of the mat and cribbing easier.
Ralph
Structures Consulting
Northeast USA
RE: Temporary Foundation Design
But then shear in the wood member becomes more of a problem.
Any saddle bracket would have to be twice as long and thicker.
Mike McCann
MMC Engineering
RE: Temporary Foundation Design
Thanks for all the advice