Braced Frame - Grade beam

[slickdeals]

Folks,
I am reviewing a design calculation for a braced frame in SDC D. The braced frame columns are supported on drilled piers and there are grade beams connecting the pier caps between braced frame columns.

However, the grade beam has only been designed for moments due to gravity loading and axial load induced by 0.1 Sds P.

The eccentricity of shear from the base of the footing to top of footing and its subsequent transfer to the grade beam has not been done. I believe that the moment due to the eccentricity of the lateral force from the brace to the bottom of the pier cap should be transferred into the grade beam. This will produce additional moments at the interface of the beam and pier cap. Am I correct in making this statement?

Also, I have looked to see if there are good examples that show how the grade beam tied to pile caps is designed. I want to do one by hand to get a better feel. Any references?

Thanks

 

[slickdeals]

Any takers for answers?

 

[BAretired]

I might if I understood the question.

BA

 

[JAE]

slickdeals, I see what you are saying about the lateral force at the base of the framed column (baseplate horizontal shear). That force might be resisted by the whole grade beam assembly pushing against earth (passive pressure) as well as possibly lateral stiffness of the piers themselves in the ground.

The relative stiffness between the piers-in-ground and the gradebeam-passive resistance would determine how much load gets into each. You could possibly check the gradebeams with 0 load going into the piers and/or visa-versa.

As far as bending moment getting into the gradebeams, I'm not sure I totally see that as the gradebeams are sort of on-edge shearwalls to some degree. I can see horizontal shear in the gradebeam as the load is transferred from the top to the bottom of the beam. But if passive pressure from perpendicular grade beams is involved, there may not be that much moment. I sort of see how you could get moment...just not sure I've ever seen it incorporated into calculations.

 

[ishvaaag]

I think basically you are right, the effect of eccentricity must appear, but, as JAE says, there's the coercion of the soil restraining the beam going down unto it and the bracing effects. Anyway, if CBF (with the grade beam), the axial brace force will be resolved unto one in the grade beam and one in the piles, etc.

Respect how to get the effects right the best way is with one model that includes the soil, foundation and structure. To mimick such without the structure above you will need to use ports of the reactions in a per hypothesis basis and still have a foundation-soil interaction model; the result will be more or less fortunate in being accurate depending on how good the insight, particularly the restrictions at support points have been in the model or models for the structure above.

 

[slickdeals]

Sorry if I did not make my question clearer.

I was trying to ask if pile caps interconnected by grade beam should be designed as a moment frame based on the pile flexural length (as an analogous column length). Also what happens to the eccentricity between the base plate and the centroid of the grade beam (for deep grade beams)

 

[ash060]

If the baseplate is on the pile cap why would any moment want to go into the grade-beam being the pile cap is almost certainly stiffer.

JAE

If the piles are being used because the surface soil is no good will there be any passive resistance?

 

[JAE]

I would say yes, there's be passive resistance (and maybe some of our geotechnical members could weigh in). Piles are indeed used when soils are poor - many times because the vertical load would cause excessive settlements.

But even with poor soils, there is soil mass and some cohesion or friction that does develop passive resistance...maybe not as much as a good soil, but some.

 

[BAretired]

When a pile deforms due to a bending moment or shear applied at the top, it creates stresses in the soil. The magnitude of those stresses are a function of the pile stiffness and the soil properties. The effective length of the pile is substantially less than its overall length, i.e. the pile is stiffer than you would calculate using the flexural length. It could be considered a beam on an elastic foundation.

The eccentricity between the base plate and the centroid of the beam produces a moment which must be resisted by the beam, pile and column according to their relative stiffnesses.

BA

 

[paddingtongreen]

To try to realistically model the effects of the bending of the piles in the soil with increasing stiffness and passive pressure capacity, the bending of the beam, and the possibility, but not certainty, of passive pressure under the beam (the degree of compaction of backfill under a beam is problematical) would be so costly in manhours that it is better to dummy up the worst case for each member and provide that strength.

Just trying to visualise the shape of the deflected pile boggles the mind, it is tough enough thinking of the infinitely stiff model used for transmission line poles.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[slickdeals]

If I am understanding correctly, the common practice appears to be designing the grade beams for axial load transmitted between pile caps (if a single pile cap can't take the shear). In addition, only gravity load moments are added on to the axial load and it is designed as a beam-column?

The grade beams are designed as though no soil existed below them?

 

[slickdeals]

I don't want to make it into a research project, but strive to understand the correct way of designing them, without missing important parts.

 

[asixth]

You will need to make some assumptions on the stiffness of the grade beam (I,eff) and the soil-structure interaction which is a theory in itself and would not be fully understood by most structure engineers who have not undertaken a great deal of study in geotechnical engineering.

If the grade beam is monolithic with the drilled piers it will want to act as a moment frame. The grade beam will experience axial load as well as moment because the grade beam will want to distibute the shear force between piers depending on the stiffness of the system. This depends on how much reinforcement is in the grade beam and the ground conditions best assessed by a geotechnical engineer.

1. RIGID GRADE BEAM

The piles will want to act as a group and resist lateral loads by push-pull in the piles (i.e tension piles will be required)

2. FLEXIBLE GRADE BEAM

Less moment is transferred from the grade beam into the piers, passive soil pressure provides more resistance to overturning.

The big unknown in this question is the soil conditions and that will have an outcome on how the structure will behave. Will the soil be able to develop the passive resistance behind the piers and will the soil adhere to the piers to develop skin friction.

To answer your original question, yes, I would design the ground beam for additional moments from lateral forces (in addition to selfweight moments) but how much moment will depend on how flexible you want to make the structure and the soil conditions.

 

[paddingtongreen]

I don't see how we can discuss this in hypothetical terms, so much depends on the actual member stiffnesses. This problem tends to solve itself, if the beam is designed for only it's own weight, it probably is not stiff enough to attract much moment from the piles. On the other hand, if you assign it part of the moment it will become stiffer and perhaps attract even more of the moment... It tends to become self-fulfilling prophesy.

Again, without knowing all about the structure, we are flailing in the wind, but if you do apply fixed end moments, will they change the magnitude of the maximum moment. Consider, if you apply a fixed end moments to the ends of the beam, the line joining them crosses the beam near the center, it's value is zero at the point where the gravity bending moment is maximum.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[slickdeals]

I must have mentioned real stiffness terms in the post.

The piers are 6ft diameter drilled shafts (going 60+ feet deep). The grade beams are 4' deep x 7' wide spanning 50 ft.

The shear capacity of the piles is about 350 k assuming a fixed ended top.

 

[westheimer1234]

ok, i dont know if this is correct method but we have done a lot of this, we dont do it manually..

we model the structure and foundation (including grade beam) using staad.. plate element for the gradebeam and pile cap.. spring support for the pile..

then get the peak moment of the plate element on the interface between grade beam and pile cap and design the rebar of the grade beam based on that moment..

another method is use "beam" between two pile cap (instead of plate element) for grade beam and again let staad design the grade beam.

 

[asixth]

westheimer1234

How do you determine the magnitude of the spring stiffness for the soil. Does the geotechnical engineer give you a modulus of subgrade reaction and do you assume the spring stiffness increases with depth?

 

[slickdeals]

Also how do you model the lateral stiffness? Or do you assume your pile to be like a column with no soil around it?

 

[asixth]

Our general procedure is to take full base fixity at some dimension below the ground level (1.5*pile diameters). This allows for some rotation at the base increasing which increase lateral drifts of the superstructure. The reactions are then taken from the superstructure model and used as input in the sub-structure design.

Lately I have been investigating into modelling the soil as an elastic spring however it is difficult to get a geotechnical engineer to give me the required soil properties that I am after.

Bowles textbook on foundation engineering goes into the use of elastic springs for soils briefly.

 

[BAretired]

One of the problems is that soil is not an elastic material, so geotechs are understandably wary of giving out spring constants which vary with soil type and also with depth.

The assumption of 1.5 pile diameters to full fixity seems low to me. I would have expected it to be considerably more.

BA