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Re: Grade Beam Design - HELP!

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Dennis,

 

Since the column axial loads will go to the erection blocks directly, the grade beam will be practically experiencing only the bending moments transferred by the columns. Other than the end moments, M (and resulting shear of 2*M/L), the grade beam will be carrying just the self weight and any triburary floor weight long the span whose effects could be neglected since the grade beam has a continuous support from the soil. This could be one way to simplify the structure.

 

Suresh Acharya, S.E.

 

 


 

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I have an old building that I am retrofitting – a 1920’s reinforced concrete building (a fire station) with a soft-story (open front). I need to resist lateral movement in the open front and limit deflection to 0.0025H where H is 9-feet to the bottom of the concrete lintel (that is also the parapet above).

 

I designed the frame (W14x38) with the base of the columns fixed through a grade beam and all axial loads transferred to erection pads below the grade beam.

 

The left column will develop 72.4 ft-kips and the right column 71.1 ft-kips (the weight of the lintel above is using one column to replace a damaged concrete column which helps resist uplift – the other column is going to be tied by epoxy anchors through the flanges to the concrete wall (8-inch concrete with ½” square rebar at 12-18 inches on center horizontal and vertical).

 

My problem is that I have not designed a f’c=3000 psi concrete grade beam to handle only the moment and a shear at the base of column of 12-kips in more than ten years. I had a shortcut method that I used at the time, but I am relying on a design library to design a combination footing. However, the software assumes that the columns are connected at the top of the foundation rather than down through the foundation to the erection pads below.

 

The foundation that was design extended 10-feet to one side of the column and flush (1-foot)) on the other end – essentially the foundation is 22-feet long, 30-inches wide and 24-inches below grade. The soil is silty sand with a bearing pressure (worst case) of 1,500 psf and with a 20% increase allowed for each foot below grade and for each foot in width over the initial 12-inches. I ended up with #6 rebar at approximately 8” o.c. top and bottom and will install shear ties #3 steel at 12” on center until it approaches the columns and will reduce the spacing to 6-inchs on center within 12-inches of the columns and then 3-inches on center from 6-inches away to the face of the flanges.

 

I am not confident in this design but can not find the MathCAD file I wrote years ago to design a grade beam and erection pad. Do any of you have a hand calculation based on ACI 318 that I can follow and verify the results I came up with in this calculation that is part of the library? I would feel more comfortable if I had a better understanding of how the grade beam works not only in bending but against the resisting soil pressure. The ends of the grade beam will be dowelled into the concrete foundations for the 8-inch reinforced concrete walls at each end. The top of the grade beam will be 6-inches below grade to allow for a 6-inch slab with #3 rebar at 12-inches on center each way above the grade beam and as a slab on grade. The erection pad worked out to be a 30-inch square by 12-inch thick pad (with 10.5 kips as the worst loading) w/ (3) #5 each way.

 

The shear transfer will occur by installing threaded rods through the flange of the beam into epoxy holes at the bottom of the 12-inch lintel/parapet. The flange of the beam (compression flange) will be flush to the bottom of the lintel.

 

I hope this provides you with a clear picture of the moment frame (a W14x38 column and beam – beam is 13-feet long and the columns are 9’-0” high above finished slab). Please write me privately if you can fax me or e-mail in PDF format (or a MathCAD template if you are using one) to help me verify the design of this grade beam.

 

Thanks for any help you can give.

 

Regards,

Dennis

 

Dennis S. Wish, PE

California Professional Engineer

Structural Engineering Consultant

dennis.wish(--nospam--at)verizon.net

http://www.structuralist.net

 


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