huangyhg

minimum reinforcement on grade beams
hi i wanted to know about the minimum reinforcement on grade beams. we ussually use the minimum reinforcement for shrinkage and temperature. i wanted to know if this is ok or if we´re missing something. the amount of steel with this minimum reinforcement is huge compared with the requiered reinforcement per calculations.
as always,thanks a lot
lg

the minimum reinforcement required for grade beams is the same as any other reinforced concrete beam per aci 318. are you talking about continuous footings, which are called "grade beams" by some people?
first consult your geotechnical report. in foundation recommendations section it will tell you if the foundation should be designed as continuous footings or structural beams. the report should provide bearing pressures for continous footings, if this is the case minimum steel for footings is 1/2%. if report recommends void forms below grade beams use minimum steel for beams designed for flexure.
archeng59: no, im talking about a grade beam a beam below of a moment frame that is used to resist the moments at the bottom of the columns.
i would like to know what is the minimum reinforcement for this elements.
by the way,we ussually design continuos footings as a grade beam and we specify the minimum reinforcement requiered by temperature or shrinkage. i´d like to know if we could specify only the requiered by calculations or which is the minimum percentaje of steel requiered for continuos footings
lg
by the design standard i use, the reason for minimum reinforcement is to ensure ductility. as your 'grade beam' is part of the structure, subject to flexure and shear, it should be treated as any other rc beam and comply with minimum reinforcement rules.
by as3600 this basically means that the ultimate strength in bending should be 1.2x the concrete cracking moment.
if it's a beam, aci 318 governs minimum reinforcing requirements. in my opinion, it doesn't matter if is a grade beam spanning between two piers or an elevated concrete beam spanning between two columns.
i'm curious, are the moment frame columns supported by the grade beam or do those columns bear over piers?
if the flexural capacity is required for structural stability then minimum reinforcement should be based on that for a beam. if the reinforcement is only there to hold the footings together (e.g. strip footing) then it should be based on the minimum for a footing.
csd
archeng59
the columns pass through the beam and are supported by a pad footing. the grade beam takes the moment of the column with a the couple generated by threaded rebars welded to the flanges of the steel columns and the compresion on the concrete.the footing takes all the axial load and both with the passive earth pressure and friction takes the shear forces
lg

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