huangyhg

options for property line type footings
i'm working on an in-fill building project that is about 30'x60' (lot and building size). it is a multi-story concrete floor/cmu wall building and due to the total wall loads involved along the pl, a typical pl type wall footing will not work. i'm leaning towards specifying a mat / raft foundation but concerned about the cost. is there a more cost efficient system out there in lieu of the above? thanks.
it would help if you explain what a typical pl footing means to you.
your idea of typical is bound to differ from those of us who don't live local to you.
spread footing, trench footing.
don't you need/have basement?
yes, there is a much more cost efficient system.
ba
a typical property line (pl) type footing is a continuous wall footing with the wall offset to the edge of the footing due to the property line.
the concern is due to the heavy total gravity loads that the wall needs to transfer to the footing. the eccentricity of the wall away from the center of the footing is inducing rotation and extreme pressure. normally for lightly loaded wall footings, pinning the stem wall to the slab will solve the rotation problem. the resulting moment from the eccentricity is simply converted to a couple to determine the force at the slab on grade level.
no basement for this building and the soil allowable is only 1500 psf.
i was thinking about this problem on my way home and got an idea about adding tie or grade beams at x spacing to tie the pl type footings at each side of the property line. the walls that require the pl type footings are about 30' apart, so i'm thinking a grade beam perpendicular to these 2 footings to tie them together. this is typical for pre-engineered metal building foundations but i have not used it before on a conventional building.
material wise, it will be a lot less than a full mat but not sure if it will be cheaper.
well, i think you have hit on a plan! whether or not it is cheaper than a full raft foundation is difficult to say in the absence of full geometrical information. but, it seems to me that your idea has merit.
call it a raft foundation with openings in it or a series of strip footings with strap foundations to stabilize them...it might just do the trick. good for you...you thought of it all by yourself. now that is what i call engineering!
ba
i've used your idea, but tied to internal column pads.
as the opposite pl is only 30' away it should work.
baretired:
you had too many beers!
mike mccann
mmc engineering
mike,
you, of all people, should know there is no such thing as having too many beers.
racingaz,
i also agree with you and the beer drinkers. but then i have had a few glasses of red.
for the straps to rectify the load across the entire 30' width, keep them quite stiff.
mike,
that was just a lucky guess.
ba

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