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Re: Grade Beam Design - HELP!

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Dennis,
 
I am not sure I completely understand the picture.  But a couple of comments:
 
Grade Beam Width:
The flange of the W14x38 column is bf = 6.75" wide (assuming strong column axis for the moment frame. The width of the grade beam should probably not be less than 20" wide(3" cover each side, 1/2" for tie diameter (add 1/8" to rebar actual diameter (3/8+1/8 = 1/2")for detailing to account for deformations on the rebar) + 7/8" for #6 rebar each side of the col. plus at least one inch clear to column flange so you have some room to work, recommend more though, closer to 2"). Therefore total width = 3"+1/2"+7/8"+1"+6.75"+1"+7/8"+1/2"+3" = 17.5" use 18" min, but recommend not less than 20" min.
 
Grade Beam Depth:
I would make the grade beam at least the same depth as grade beam width, probably deeper by 6" than the grade beam width.
 
Top of footing:
The top of the erection pad for the column, I call it a footing, should be 4" to 6" below the bottom of the grade beam, this allows room for the base plate and anchor bolt projection to remain below the bottom of the grade beam reinforcing.
 
Grade Beam Reinforcing:
You call out for the grade beam rebar to be 8" o.c., I don't think that works, since you want the rebar to extend past the sides of the steel column, so you are probably limited to 1 or 2 top and bottom bars each side of the column flange, confined by the ties and terminated with 90 degree hooks at the end of the grade beam.  I assume you are extending the grade beam to the far end of the footing beyond the steel column. 
 
Are you welding any rebar to the flange of the column in the grade beam to help transfer the bending moment from the column to the grade beam (this may not be an issue if you can show that you can transfer all the bending by column flange bearing on the concrete.).  I would use #4 ties, and use the same spacing for the entire length of the grade beam, the grade beam span is not that long and the cost difference probably isn't worth your time to detail the different stop and start points for the reinforcing, let alone verify that it was placed correctly.
 
Top of Grade Beam:
I would hold the top of the grade beam down 12" from top of finished slab, this allows for piping etc to pass over the top of the grade beam. Also do you want to provide dowels from the grade beam to the slab-on-grade to help transfer the seismic force to the slab-on-grade from the grade beam.
 
Footing Size:
I would increase the footing size to 48" x 48" minimum (if you have the room), give yourself some room to work. Also dowel the grade beam to the footing each side of the column (2-#4 L dowels each side of the column), so you have the footing and grade beam will tied together to help resist uplift, not just the column base plate anchor bolts..
 
Hope this helps.
 
Mike Cochran
 
In a message dated 6/4/2004 10:49:03 AM Pacific Standard Time, dennis.wish(--nospam--at)verizon.net writes:

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.