If you are leaning toward carbon fiber (SIKA CarboDur?) be aware that
generally, it creates a fire rating issue. The carbon can't be used as
a "primary force element" because as it gets hot, it will delaminate and
you will lose your load carrying capacity. This could lead to a
catastrophic failure. You could look a fire protection systems, but I
don't have any suggestions there.
Does the soil have to be saturated?
Could you use a peat or other highly organic (light weight) soil?
What about a nonlinear strain distribution in the concrete? You could
use a parabolic stress block. May help, never done it before.
Worst case: Sister a steel plate along side (dowel and epoxy?) the
joist. Check plate bucking etc., but you could probably use the beam to
stabilize the plate.
Post tension along side the existing joist? If I remember right, by
post tensioning you get better shear values as well.
Just thinking out loud (sorta....)
Jake Watson, E.I.T.
Salt Lake City, UT
"Barry H. Welliver" wrote:
> I'm looking at possible ways of increasing shear resistance in a
> tapered concrete joist system. The original design allowed for a depth
> of soil which is less than the present proposal which also includes
> some large rocks, trees and a pond.
> So far I have been able to define a "light" weight soil (113pcf
> saturated), and am using records of test results to use a slightly
> higher in-place concrete strength.
> The results of my analysis produce shear (and some bending)
> requirements somewhat greater than allowable (15%+). I've sharpened my
> pencil enough for now and would like any comments regarding possible
> So you are aware, there is a possibility of replacing some of the soil
> volume with styrofoam (or other such material) but this may not
> completely solve the problem (nor is it preferred solution by the
> design team). Also, the rocks could become "Disney" like and
> significantly lighter if absolutely necessary. No immediate thoughts
> about using lighter water (nuclear), but I'm sure someone will bring
> it up. :-)
> I've started looking at SIKA CarboDur for possible moment improvements
> but have no hands-on experience.
> Would appreciate any comments.
> Barry H. Welliver