BEAM ON ELASTIC FOUNDATION ANALYSIS

[4873]

Hi, I have a question. I need to know if it is well done to apply the wrinkler model and use the Hetenyi formulas to calculate the structurals parameters such as: moment flexural, shear forces and deflexion, just in the same way established in the BOWLES, FOUNDATION ANALYSIS & DESIGN because I need to analyse an underground beam structure that it is used for installed underground electrical cables lines. I supposed you know what I mean, it is a beam of a base not so big probably we are talking about 40 in. a square size but the beam of this type are very long more than 10 ft. For the soil parameter I used the subgrade modulus of reaction K, from any table book. Because I think that the most critical soil to analyse underground structure is the clay soil type, then I used the average from any table values I reached. Also I need to know if I am Ok. in using this concepts to make an approachement analysis using the beam on elastic foundation for the above mentioned structure type?

 

[Focht3]

No, you're not all right. Soils are not like steel, concrete or electrical cables - you can't simply look their properties up in some table. (This thread reinforces my dislike of Bowles' book.)

Your post does not have enough information for a reasonable answer. We need a lot more information tin order to help you.

By the way, a 1 meter by 3 meter footing is not very big; why are you trying to perform a detailed analysis on such a small foundation element?

 

[4873]

Ok. I'm trying to analyse a concrete beam but it isn't a common beam. It's a PVC conduit underground that is totally enveloped with a concrete casement. This casement is a concrete beam laying directly as a strip footing but it was desing without steel reinforcement. The beam analyzed has moreless the following size: base 16 in, height 8 in, 30 ft long. But this is not the only size, there is other more greater and it will be cover long distance as probably 50 meters and much greater. Just depend on the electrical distribution, so the sizes of this underground beam systems can be varying depending about the amount setting of cables quantities. I'm trying to perform a detailed analysis for this kind of structure because I needed for design. I need to reinforced the concrete beam with corrugated steel rods.
So maybe and in this case the elastic foundation method can bring me more realistic results because it will be an underground structure design.
Well, another thing is the soil type. In the construction area here in my country the most is the clay type soil. But also I know it will be another kind of soil type. In all case ¿what value of subgrade modulus of reaction, K, it will be the correct selection in general? And, How can I get the more realistic value for the type common soil, for example the clay soil type or sandy clayey soil, etc? Cause you told me that I can't simply look for it in any table.

 

[4873]

I read the Subgrade Modulus Reaction Thread and there is a discussion about the value of K. Anyone can take the most representative value of K from the Bowles book table, so its no problem to take off the correct value for the clay type soil applying from the reference. I'd rather use the value took it from Bowles. I will choose the smaller value so I can get higher moments values and higher deflections values this mean "critical conditions considered in the analysis" . I think this is the quickly way for assume this parameter, ¿Right or wrong?

Please can U tell me, If this Ok?

 

[Focht3]

Hmmm, why do you want to model this beam? They are fairly short runs (50 meters) in PVC pipe encased in concrete. Why do you expect the conduit to move? Where are the loads coming from?

If you are putting a lot of cables through the PVC and are concerned about settlement, then the use of k won't do you much good. You need the help of a local geotechnical engineer.

If you are concerned about the effects of heat on the surrounding soil, then you need a local geotechnical engineer.

The modulus of subgrade reaction will not really help you - unless you want a false sense of security.

 

[4873]

The different loads acting on this beam are coming from the weight of the volume of earth that resting above the beam, the weight of others similar beam that are going to be placed just exactly over the beam, and the seismic forces. And the deflection of the beam by the acting earth pressure force and the weight of the others beam over this one. So I think that the flexural bending of the beam in tension or compression is meaningful. And of course, a lot of cables are going to be placed inside this PVC that are going to be encased by the concrete beam. Well, my interest is not about the soil surrounding, its about the beam bending (because I think) this beam needs steel rods reinforcement. And the numerical value of "K" I need it to introducing in the Hetenyi formula just to calculate the structural analysis using the beam on elastic foundation classical method.
With the hetenyi formula I can find the numerical value of the flexural bending moment of the beam just in fact to calculate numerically the amount of steel quantity that the beam requires. So that's why I ask you if its Ok that the numerical value of K can get it from the Bowles book.

 

[dlew]

4873,

It is better to have the modulus of subgrade vertical reaction, k, obtained by test on the actual site, but this is not always practical. I have used values of k from obtained from books many times. In those cases, I have checked the foundation for the two extreme values of the range given, to have a feeling of how the moments, soil pressures and deflections would vary. Also, I would verify that the actual soil fits the soil description of the reference.

I have used Hetenyi's formulas, and they require a lot of calculations. A spreadsheet or simple computer program would be useful.

Remember that uniform load along the beam do not produce moments (unless k varies.) Only concentrated loads and varying distributed loads will do.

Regards

 

[4873]

I'm going to follow Focht3 and your's (dlew), both suggestions.
Well, I tried an evaluation using the spreadsheet, applying: the "K" value from the National Structural Code table, distributed load (dead, live and seismic) applying the Hetenyi's formula, and from that, I obtained certainly, lower moment values. Perhaps, this mean that the beam structures shall be need the minimal amount reinforcement according to the ACI 318M-99. In fact, I have this ACI edition.

But if you have any other suggestion, you can tell me. So it will help me to view and do my work much better.
*****Have a Happy New Year 2003********

 

[ChipB]

4873,
Every job I've worked on that had these, refered to them as duct banks even though they were carrying electrical conduit.

I might be missing something but...

Unless the site experiences a significant differential settlement under the duct banks, you should be able to use minimum reinf. I think the most important item is to ensure adeqate soil compaction over the duct banks especially if they are on top of each other. The weight of the concrete is about equal to the weight of the soil, so as long as your first duct bank bears on virgin soil, and the finished grade elevation is equivalent to initial grade elevation, you have not increased the soil stresses any appreciable amount.

However, if there is the potential for equipment or vehicular traffic over the banks, now or in the future, you may need to re-evaluate.

 

[4873]

This mean that I have to re-evaluate the soil or you are trying to say that the re-evaluation it has deal with the concrete electrical duct bank? Well, in the case of heavy equipment or traffic, this mean that I have to include heavy loads because the duct bank always in the most general cases they are constructed passing under crossing the traffic lane pavement. So maybe I missed this...