In-Situ Load Testing of Beams with In-adequate Shear Strength

[n93s412]

Has anyone ever heard of performing a load test on an existing beam that has insufficient shear capacity. ACI 437R-03 states that “Load testing to evaluate the shear capacity of structural elements is not addressed in ACI 318 (Chapter 20), and should not be recommended except under unusual circumstances.” Maybe there are further guidelines available elsewhere or someone has actual experience with this circumstance?

 

[SlideRuleEra]

We did, but only because of calculation errors, I'll explain.

In 1989, we (the Owner) obtained a new concrete building that had poor quality construction, by the Contractor, plus discrepancies in the Engineer's design. We hired an independent national Engineering firm to determine if the design was adequate to allow a safe load test of the construction. Result of the design review: perform the load test.

The full scale load test was performed, by an independent testing lab, in strict compliance with ACI; results were acceptable. A few days later, our senior vice president received an "emergency" phone call for the independent Engineer that reviewed the design and gave the ok for the load test: Don't perform the load test; a calculation error during the review had been made, the concrete beam design was inadequate in shear. We were all VERY lucky!

My advice, DO NOT load test if the beam is known to have insufficient shear capacity. There could be a sudden, dramatic, and catastrophic failure.

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[Ron]

The reason you don't normally do a load test for shear is that there is not a convenient way to measure shear accurately during a load test...you are more likely to fail the beam in bending than in shear.

Yes, you can apply strain gages and measure deflections, but a shear failure is more likely to be catastrophic without warning, whereas bending gives lots of indication prior to failure.

 

[n93s412]

Thank you both for your input. It was concluded that this type of load test should be avoided for a good reason. In-depth analysis was completed for this beam inclusive of strut-and-tie analysis, which still rendered the beam insufficient. Repair looks like the only option.

 

[gobsmacked]

I have tested a number of concrete beams that were inadequately reinforced for shear. They generally gave warning with cracks appearing at the neutral axis, before flexural cracks at the edge. They were never sudden failures because of flexural reinforcement, but I have heard that, at worst, they can be sudden, as in a corbel, for example.

 

[nyoko1215]

Can anyone recommend a company that performs the load test described in ACI 318 Chapter 20 "Strength Evaluation of Existing Structures?" Thanks

 

[Ron]

nyoko...you don't give your location, which would be necessary to give a recommendation; however, most testing laboratories can do load testing. If in the eastern US, try AMEC, Froehling and Robertson, Wiss-Janey, Simpson-Gumpertz, etc.