Design of Grade Beam Foundation without Drilled piers or spread footings underneath

[oengineer]

I am designing grade beams sitting on a soil with 1500 psf allowable bearing capacity for total load. The beams are bearing directly on the soil and are NOT supported by drilled piers or spread footings. The total length of the beam runs 64'-8" LF. I have 34'-8" LF of the grade beam supporting a 13'-4" tall CMU wall, a trib. width of 10'-8" carrying DL =25 psf & LL=20 psf. I have attached a plan view of the foundation layout:

http://files.engineering.com/getfil...-36b8-4e8e-930f-5e9c0001e891&file=S4.03-4.pdf

. Currently I am designing it as a simply supported beam, but I am getting that the beam is failing as a 12x24 grade beam with (3) #6 bars. I have been only been able to get it to work using 18x36 w/ (4) #8, 18x36 w/ (5) #7, and 12x36 w/(4) #7. How should I go about designing the grade beam? Do I design it as a simply supported beam? Any suggestions and/or comments are appreciated.

 

[Shotzie]

Isn't what you're describing the same as a strip footing?

 

[delagina]

why is the grade beam designed as simply supported at ends when it's uniformly supported by the soil?

 

[oengineer]

@Shotzie - Well, I guess it acts like a strip footing but it is not that wide, only 12". Wouldn't the reinforcement be different? Right now the grade beams have stirrups in them but a wall footing would not, right?

 

[oengineer]

@delagina - When I researched the design of grade beam i found to design the like a regular beam. I believe in those cases it is when they are sitting on the soil but have either a drilled pier or spread footing underneath them.

 

[oengineer]

@Shotzie - Here is a detail of the proposed grade beam. My employer say it should work but my calculations are showing otherwise when I design it as a regular simply supported beam.

http://files.engineering.com/getfile.aspx?folder=ee6ab5a6-03a8-499d-9999-16b086dd90df&file=S5.pdf

 

[dik]

The width of a grade beam/strip footing has an impact on the allowable bearing stress as well as settlement... any geotechnical support? Early stone rubble foundation walls often had no 'footing' under them.

Dik

 

[jayrod12]

What span length are you using if there's no piers or footings? That should flick the light on that you're analyzing it incorrectly.

With the applied loading you have (roughly 1400 plf assuming partially grouted masonry) there's not much capacity left for a reasonably sized grade beam that's only 12" wide.

As for the reinforcing in the grade beam, in your case it's more like a footing than anything. You can put T&B bars and stirrups if you want. It makes sense to be sure it has some spanning capability in case there are any soft spots below areas.

 

[oengineer]

@dik- When you say geotechnical support do you mean if there is select fill underneath the grade beam? I don't follow.

 

[oengineer]

@jayrod12 - I am using a span length of 34'-8" LF. Here is a picture showing my reasoning:

http://files.engineering.com/getfil...ff6e-4fcc-9d2d-c6b509f7cd93&file=S4.03-4_.pdf

. Is this correct? I used this length because it is where the grade beams intersect with one another.

 

[oengineer]

@Shotzie & @jayrod12 - When checking it as a continuous spread footing I run into an issue with the shear check due to the width. My C=(b(width of the footing)-t (thickness of the wall))/2 - d, C=(b-t)/2 -d, is becoming a negative length number.I don't think I should continue the calculation if I have a negative number for the critical section for shear.

 

[jayrod12]

This is going to sound quite rude,

IT"S FULLY SUPPORTED AT THE BASE! There's minimal moment or shear applied to the grade beam.

 

[oengineer]

@jayrod12 - I appreciate all comments that are helpful, so I am not offended. So how would I design the grade beam? What is the best approach to analyze the detail that i have previously posted. I was taught many years ago to treat it like a simply supported beam and use the drilled piers/ spread footings as a pined condition. After seeing the grade beam detail , how should I determine it is able to handle the loads I mentioned at the beginning on my post?

 

[jayrod12]

size it to provide enough bearing width you don't exceed the bearing capacity of the soil below.

Design the reinforcing to span some extent of soft bearing material. How far you choose to design it to span is up to you, but something like 5-10ft is like more than adequate. If your grade beam has to span further than that over a soft spot, you're going to exceed your bearing capacity beyond the soft area.

 

[oengineer]

@jayrod12 - Thank you for the help.

 

[delagina]

back to your original idea, if you assume it as simply supported "grade beam", where do you think the concentrated reaction at the ends of the beam will go if you don't have drilled pier or spread footing?

 

[oengineer]

@delagina - I was using the junctions where the grade beams intersect as the ends of the beams where the concentrated loads go

 

[jayrod12]

But then where would it go from there? Somehow, somewhere, the load needs to get into the soil. and concentrating it would require spread footings at those places.

 

[oengineer]

@jayrod12 & @delagina - Yes, be that as it may, I needed some length to use. I would have the same issue for any length I arbitrarily used (even if I just check a 1 ft section/strip). This is why I asked for clarification on the matter. I am new to this type of work. I did something similar awhile back but it has been many years. I appreciate both of your input.

 

[hokie66]

Are you by any chance an engineer? I don't think so, if you are asking this sort of question. This site is for engineers.