sudgarde modulus

[elidavao]

i did designing the total grade beam in 3-storey hotel building by using staad pro.. based on geotechnical soil report spread footing foundations are recommended for the support of the proposed building... i used elastic mat as my foundation support. Is there a formula or any possible way to arrive the value for my subgrade modulus if the only given is soil bearing pressure? unlike my previous design, subgrade modulus is already given of 125pci...

 

[kslee1000]

If the subgrade modulus is required, the geotech will need to provide it based on the building geometry and the loads. However, if the spread footing system works, I don't see the need for such information, since the settlement will be kept within the allowance by default.

 

[elidavao]

thank you... kslee1000.. coz im neophyte in this world of design.. hope you could help me again to become a better one... it is ok that for me now i mostly depend on staad pro? than the result of my actual design? once again thank u...

 

[kslee1000]

You welcome.

I don't see any reason why for not using programs that are capable of carrying out the tedious analyses required for the mat foundation. However, I would encourage you to look into the analytical results, and provide your own thoughts on it using your understanding in structural engineering.

For simple footings, either spread or strip, I would encourage you to practice long hand calculations until you have solid grips on such topics. By then, computer method is fine.

 

[fattdad]

Subgrade modulus is used by the structural engineer to design concrete floor slabs with point loading. The modulus represents a spring that is positioned on a rigid surface with the underside of the slab on the top of the spring. Designing the slab under these circumstances has little to do with long term settlement or bearing capacity. There is actually an ACI (Americal Concrete Institute) committee looking at some of the pitfalls of just using the subgrade modulus for industrial floor slab design.

That said, don't get too caught up on trying to correlate subgrade modulus to bearing pressure - they're different animals. Case in point: I worked on a project where there was storage of paper. The subgrade modulus may have been 200 or 300 pci (considering the local stresses in the upper few feet of the subgrade). The areal load over thousands of square feet, replicated a big foundation-type load. This resulted in a "seat of settlement" that extended to depths of 20 ft or so. We had to use a preload to mitigate long-term settlement potential from such a heavy load over such a big area. Sometimes you just have to look at these things separately.

Good luck.

f-d

¡papá gordo ain’t no madre flaca!

 

[genomty]

elidavao

Ks is not a soil property as shear strenght or modulus of elasticity thus,it is not easy to establish a value, in fact, some memebers will tell you that, there is not such thing as Ks and that it is in fact a convenient relationship for structural design purposes and no test will give you a reliable parameter, which is certainly true. There have been a lot of threads regarding to this this issue and most of them can be summarized in the previous sentences.

However what can structural engineers do if softwares require this value and geotech guys do not provide this value for the mentioned reasons. Well, and this is just a suggestion; since Ks is a conceptual relationship between soil pressure and deflection and most getechnical engineers hardly provide a Ks value in the geothechnical reports because of it is difficult to perform a plate-load test except for very small plate dimensions, for structural analyses purposes you can review "Foundation Analysis and Design" by Joshep E. Bowles, fifth edition chapter 9-6, this section present some numerical estimations, perhaps the most usefull in your case could be the one suggested by the author, which use the bearing capacity and respective settlement, both should provided by your geotech consultant in the geotecnical report.

Regards

 

[kslee1000]

To my understanding, Ks is derived from the subgrade modulus Es, and Es is in a sense similar to the Young's modulus - correlation between stress and strain. It's a soil parameter that should be estimated by the Geotech based on the materials on site, deformation/settlement characteristics of such materials, type of foundation system, and type and magnitude of loadings. Although Es is not as exact as the measure of Young's Modulus, it is an indication of how stiff/flexible of the soil subjects to instantaneous external loadings.

For small or structures mainly subject to uniform loads, Ks serves no better than an elastic analysis (straight line approximation). However, for larger industrial type structures with irregular loading patterns, the Ks is highly desirable to be used in the structural calculations, since it will provide better picture on variations of internal stresses of foundation due to deformations, thus allow the structural guys to provide better reinforcing arrangement.

To me, Ks & Es shouldn't be just picked from a handbook without fully understood the soil properties/behaviors. I agree that these parameters are normally not provided, but to be requested by the owner (at extra cost?) or its representative.

 

[fattdad]

By way of example I offer the following: If you have a soil with a modulus of subgrade reaction "k" of 50 pci (a real low value) and you increase the subbase of the industiral floor slab from zero inches to 10-1/2 inches the "effective" subgrade modulus would be 100 pci for the design of the slab. Clearly, if you have a slab that's 200 ft by 300 ft with a design load of 500 psf, this doubling of the subgrade modulus is not even closely related to the potential "seat of settlement". It just helps the structural analyses deal with very localized shear forces.

Hope this helps illustrate some of the fundamental concerns with just focusing on the subgrade modulus. That said, to a structural engineer, an appropriate value for subgrade modulus is critical. I typically use correlation to CBR value and specify a minimum subbase thickness in my geotechnical engineering reports.

f-d

¡papá gordo ain’t no madre flaca!

 

[elidavao]

thank you to all.... it realy helps to me a lot.. i highly appreciated for the suggestions and comments...