Braced Frame - Grade beam
Braced Frame - Grade beam
(OP)
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
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






RE: Braced Frame - Grade beam
RE: Braced Frame - Grade beam
BA
RE: Braced Frame - Grade beam
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.
RE: Braced Frame - Grade beam
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.
RE: Braced Frame - Grade beam
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)
RE: Braced Frame - Grade beam
JAE
If the piles are being used because the surface soil is no good will there be any passive resistance?
RE: Braced Frame - Grade beam
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.
RE: Braced Frame - Grade beam
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
RE: Braced Frame - Grade beam
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.
RE: Braced Frame - Grade beam
The grade beams are designed as though no soil existed below them?
RE: Braced Frame - Grade beam
RE: Braced Frame - Grade beam
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.
RE: Braced Frame - Grade beam
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.
RE: Braced Frame - Grade beam
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.
RE: Braced Frame - Grade beam
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.
RE: Braced Frame - Grade beam
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?
RE: Braced Frame - Grade beam
RE: Braced Frame - Grade beam
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.
RE: Braced Frame - Grade beam
The assumption of 1.5 pile diameters to full fixity seems low to me. I would have expected it to be considerably more.
BA
RE: Braced Frame - Grade beam
subgrade modulus is based on geotech..