Temporary Foundation Design 2

[foreng]

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?

 

[msquared48]

Shrinkage across the grain with long term loading if I read you right, as this shrinkage will affect the CIP height of the bridge assuming it is concrete. I would hope you will use well seasoned logs and nothing green.

Mike McCann
MMC Engineering

 

[foreng]

The bridge has a steel girders and timber deck resting on a sill log to permit temporary construction access. I've treat this foundation as a rectangular footing with width B equal to the diameter of the log. Not sure if this assumption is correct, I'm assuming the struture will settle until the area on the underside reaches an equilibrium.

 

[msquared48]

Depending on the timber material used, the perpendicular to grain bearing will control at somewhere between 300 to 600 psi bearing. More than this allowable will crush the log. You might want to employ a curved metal bearing seat the diameter of the log under the steel beam at the bearing point of the log to help spread out this load and reduce the bearing stress. The load should distribute to the soil structure from the bearing seat at a 1:1 slope along the length of the log.

Mike McCann
MMC Engineering

 

[RHTPE]

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

 

[msquared48]

True, but if this is used to establish a grade, any shrinkage will affect the elevations.

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

 

[RHTPE]

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

 

[swearingen]

I would not use the diameter of the log as B. I don't have a basis for what it should be, but I would imagine that you shouldn't use over 3/4 of the log's diameter, if not less.

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...


If you "heard" it on the internet, it's guilty until proven innocent. - DCS

http://www.eng-tips.com/supportus.cfm

 

[foreng]

The site location is North-West of Canada, low humidity, will use seasoned logs so shinkage will not be a factor. Compression perpendicular to grain work out. The structure will be in place for a couple of months. One of the engineers in the office said to use 1/3 of B to calculate bearing capacity, that seemed a bit low.

 

[msquared48]

1/2 to 2/3 is reasolnable, and if that is not enough, use transverse logs to increase the bearing area - more like cribbing.

Mike McCann
MMC Engineering

 

[aeoliantexan]

I assume you will place granular bedding to get full contact under the haunches of the log.

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.

 

[RHTPE]

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

 

[msquared48]

True.

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

 

[foreng]

The sill will be placed on a compacted granular layer 0.4 m thick, over the entire length of the log approx 10 m, with full contact under hauches. The purpose of the exercise was to determine the initial log size requirements and if not available, look at transverse logs or a log bundle. I'm going to try 60%o of the diameter, seems reasonble. a quick calc yield a diameter of around 0.9 m, a bit large. I'd like to avoid splitting the logs, but an option that has been used in the past is a 3 log system to get more surface area. Logs are lashed together with 3 wraps of a 3/4 in cable in 5 locations along the log.

Thanks for all the advice