Point Loads on continuous footing

[mocon11]

My client would like to remove posts beneath two beams to make some space in the garage. I plan on using two steel channels to stiffen the beam. I have two questions:

What is the appropriate way to connect the channels to the post?

For the new, larger point loads on the 8" wide perimeter footing - how is the bearing pressure distributed with the point loads? There is enough footing along the length of the wall if I distribute the point loads along the length of the footing, but I feel this is unconservative and introduces quite a bit of eccentricity. Of course, the contractor says he has done this several times and never added spread footings beneath the new posts.

 

[NKT]

The design for the beam will be similar to a flitch beam, assuming that the existing beam is continuous over the posts that will be removed. Go to this document for a good description of the design process:

http://www.destefanoassociates.com/tech/flitch.pdf

The channels don't need to connect to the post directly; you can transfer the reaction from the channels to the beam end through a bolt group, then to the post through direct bearing, just as it was before. I'm assuming that you have adequate bearing area in the existing connection to handle the increased reaction. Make sure to check.

If you know what reinforcing is in the footing, you can run an analysis and check if it will work. Treat it as a rectangular footing with enough length to provide the required bearing area, and check bending and beam shear. If it works, that's great. If not, design a supplemental spread footing to be poured under the exising foundation.

Please explain your concern about eccentricity a bit more.

 

[mocon11]

Yes the beam is continuous over the posts that will be removed.

My concern with eccentricity is over the length of the footing. The wall on top of the footing is a non-bearing wall. So with a point load at the end of the 36' long footing and another point load 13' from the end of the 36' long footing, and with a relatively small uniform load acting on the rest of the footing, I feel that the end of the footing with no point loads might tilt, or lose contact with the soil.

I do not know the existing steel configurations of the footing, but the house was built in the 70s so it is probably not up to current code.

If I treat the footing as a rectangular footing with enough length to provide adequate bearing, how can I check that the soil pressures are ok. I am using 1500 psf for bearing because I have no soils information. With a total point load of 18 kips / (8"x36') =746 psf < 1500 psf. How can I know that the footing is actually bearing over the entire length?

Thanks for the link. That is a good resource.

 

[DaveAtkins]

You will get trapezoidal or triangular bearing pressure under the footing. Calculate the location of the resultant force on the footing--if it is within the Kern limit, bearing pressure will be trapezoidal and
q = P/A +/- M/S. If it is not within the Kern limit, the centroid of the triangular bearing pressure will be directly below the resultant force.

Also, I would be OK with using 2000 psf allowable bearing pressure (that's what I use when I don't have any other information).

DaveAtkins

 

[jheidt2543]

Just a couple of points:

mocon11: I assume in your OP that you meant the existing footing is 8" THICK, not 8" wide. If it is really 8" wide, that is too narrow and you should put a spot footing under the column. Also, without some reinforcing in the footing, I would not think the point column load would distribute the full lenght of a continuous strip footing. The most I would feel comfortable with for plain concrete is maybe 4' each side of the column, but calculate the concrete stress. Also, an 8" thick footing is not much depth on a concentrated load, I think it is overstressed in shear.

Dave: I too use 2,000 psf for the bearing pressure when there is no geotech report. In most of my area the soils are good for 3,000 - 4,000 psf from past experiance. However, the IBC calls out 1,500 psf to be used if no geotech report. I justify my decision by saying I'm using engineering judgement and the plan examiners haven't taken exception to it - yet. Is this your reasoning too?

 

[DonPhillips]

For the 3 jurisdictions that I do plan reviews for, I would accept 2000 psf if there is someone doing special inspections, otherwise the code requires 1500 psf without a geotechnical investigation.

Don Phillips

http://worthingtonengineering.com

 

[DaveAtkins]

I actually did not know you must assume 1500 psf without a geotech report! The codes seem to always get more conservative...

I am assuming there is a 4' high foundation wall on top of this wall footing. Otherwise there's probably no way the load can spread throughout its length.

DaveAtkins

 

[DonPhillips]

The assumed value of 1500 for clays and silts changed in the 2003 codes.

Don Phillips

http://worthingtonengineering.com

 

[mocon11]

There is a 16" high stem wall that the framed wall rests on. The contractor is telling me that the footing is 8" wide and 16" deep (below the slab). The house was built in the late 1960s or early 1970s, so I am not surprised.

 

[mocon11]

It turns out the 8" wide footing is a stem wall, not the actual footing. The contractor has not excavated the existing footing to see what is really down there. The client doesn't want to pay for adding spread footings, or excavating to see what the actual foundation's dimensions are. The contractor is pressuring me to just design the beam and posts, and the foundation is adequate as is. Does anyone have any advice how to handle the client/contractor in this case? Would it be beneficial to hire a geotech to see if the soil has greater bearing pressure than the code minimum 1500 psf?

 

[youngstructural]

Hey Mocon;

You have my sympathy: It is never easy to tell a client 'no', particularly when a contractor is telling him something's going to be fine. The reality is that there are times we MUST say no, and no matter how experienced the contractor, they do no know everything. I always remind myself how we must design for 50years and almost no contractor has that length of experience.

Some possibly productive opinion: The analysis can be done to determine what minimum capacity the soil would need to be in order to permit your contractor to build the design he/she wants. Issue your design with the minimum permissible soil bearing capacity to be verified prior to contruction. Then let the city building inspector force the client/contractor combination to get the require Geotech verification.

Two caveats: This is only possible if you find that only a reasonable level of SBP is required, and further only if your local authority is sharp about this type of thing.

Perhaps not the most professional of approaches, but I will call the council on occation to spell out my concerns in order to ensure that the critical details are caught.

Good luck and update again if you get the chance,
Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...