Concrete Truss

[mijowe]

I am evaluating a column transfer condition by another engineer that I am equating to a concrete truss. I am simplifying the condition for this post, the shape is essentially shown below, with both the top and bottom chords reinforced and confined with column ties. The panels(all three) are poured solid and reinforced with vertical and horizontal steel, no angled webs in the middle panel. Two columns are transferred out at quarter points. The span is about 60ft and the depth is 14ft. The span to depth ratio does not fall under the strut and tie rules.

I have some other concerns, but my question is do I have a shear problem at the supports? If it acts like a truss I don’t. But as a beam with tapered ends shear is not even close.




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[hokie66]

So this is just a deep beam or wall, and you are proposing to use a truss analogy, commonly referred to as a strut and tie model...but why do you say that the "strut and tie rules" do not apply? Which rules are they? The ratio that matters is 20/14 < 2.0, so strut and tie does apply.

 

[mijowe]

You are correct, the clear span is not less than 4 times the depth (although it is close) but the loads are within twice the depth of the support. so it does satisfy ACI 10.7

However, going back to shear, the limits set in ACI 11.7.3 (10 (f'c)^1/2 x bw x d are not met. This could be resolved by not tapering the ends, there is room to do this.

I know that a truss was modeled using software, and that the chord members were checked for their respective axial loads. With the tapered ends both beam shear and deep beam shear do not work. So for this model to work, it has to act like a truss.

 

[woodman88]

"So for this model to work, it has to act like a truss."
To act like a truss, you will need to design the joints and members for shear also.
How are the truss bearing joints being modeled for the shear?

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[mijowe]

Joints are simply detailed as rebar cages, one in compression one in tension intersecting at a working point, with the one in tension being terminated with standard hooks. The cages are confined with ties.

 

[hokie66]

Look closer at the anchorage for the tension ties. Standard hooks are not good enough. There are good references around for strut and tie design...I think one is published by the PCA.

 

[mijowe]

anchorage of the ties is high on my list of concerns as well (for this thread I was hoping for thoughts on the shear)

ACI A4.3 allows among other things standard hooks and even straight bar development as acceptable ways to anchor ties, as long as they are developed past the "extended nodal zone" which is pretty well defined in the code as well.

 

[asixth]

Why are standard hooks not good enough. I thought the rules for strut and tie design are that at least 50% bar development must occur by the start of the nodal zone and the bar stress developed by the node point is what is used for design.

 

[hokie66]

You are correct...I overstated about the anchorage requirement. What I suppose I meant to say was that the anchorage is often difficult with hooks and extensions which depend on bond, more difficult than with other types of mechanical devices which depend strictly on bearing.