Beam on elastic subgrade

[Reng1]

I have been given a dessign project to design a concrete grade beam foundation based on ''beam on eleastic subgrade analysis''. I have never performed this analysis. I have been given a soils report with the subgrade modulus. I feel that I understand how to perform the analysis, but I have some questions. The building is one story and supported by masonry bearing walls and steel columns. I plan on using a program named Enercalc. The question I have is how do I analyze a concrete grade supporting a masonry wall and a column offset from the center of the wall approximately 18'' using the beam on elastic subgrade analysis? Any help or comments would be apprciated.

 

[minorchord2000]

My experience with this type of problem is that Enercalc will not help. I use STAAD or RISA 2D and use soil spring nodes at 1' on center let's say. I usually use a conservative value of 250 pci for the soil spring. If you have a geotech who can help you, ask him/her to convert the subgrade modulus into a soil spring value that you can use. Once your beam is properly noded, you can load the beam with the acutal loads and check your response at the soil and for the beam.

 

[BigH]

Why not look up Bowles ("Foundation Analysis and Design") - 5th edition. He has an entire chapter on "Special Ftgs and Beams on Elastic Foundations" (Chapter 9). While some have panned his book, Bowles was originally a structural engineer. I think you'll find the answer there - and why not do it the old fashioned way - rather than some canned programme. This way you can get a better handled on it.

 

[Ash2007]

I feel you have to be very carefull in choosing the elastic subgrade modulus, because that is the main key in your design which will depend on the soil stress and displacement criteria. and choosing the right coefficients is the true name of the game!

 

[cedent]

We have a local geotech that often gives us values in the range of 100 - 150 pci. I have often wondered what is involved (for the geotech) in determining these values. Could someone point me to a reference?

Thanks.

 

[BigH]

Most likely "seat of the pants." You can get an estimate of the subgrade modulus from Baker's "Highway Engineering" handbook - and also from the chart that has been placed on SlideRuleEra's web page.

 

[jdonville]

cedent,

Sounds like you have clayey (A-7-6 or maybe A-6) soils, based on the range given.

The 'actual' subgrade modulus will depend on the degree of compaction and moisture content (relative to 100 percent Standard Proctor) of the subgrade soils, as well as the size and shape of the footing/slab/whatever.

Jeff

 

[connect2]

There are alot ways to analyse raft or mat foundations, such as 1) on winkler springs and 2) elastic half spaces or continum models. Rarely does either give a definitive answer, as both represent the soil behaviour quite differently, and rarely as a structural engineer do you find that sufficient information is provided for either. In the end it becomes a parametric evaluation of each model to determine the maximum principle moments (Mxy), shears, settlements and contact pressures that the raft structure can be exposed to, and then cya for all when sizing and reinforcing the raft.
It always seems to be our problem to never get a mat that is a nice geometrical shape, symetrically loaded. I think one of the most interesting areas is combining the frame into the mat structure. Soil structure engineering what a bizarre field. Bigger is definetly better. and yet despite it all there are a pile, excuse the pun, of mats performing well based on winkler springs alone. 'Lets float the thing' so says the architect! ok one to many beers, gotta go.