Analyzing structural capacity of old concrete beams 4

[VBI]

How does one go about analyzing an old concrete beam without knowing what size/amt of rebar was used? (No drawings are available). I am a civil engineer with some structural background. Do I simply assume minimum steel reinforcing ratio was used for the particular cross-sectional area of the beam?

Thank you in advance

 

[UcfSE]

You should probably test some core samples and look into using x-ray techonology for determining rebar. You never know what someone actually put there and you wouldn't want to bite the bullet for someone else's mistake or oversight.

 

[JAE]

VMI - there are numerous ways to add to your knowledge of a "mystery" beam or joist. ACI has some excellent literature on this. A few things to consider are:

1. You can use chipping hammers at midspan to reveal the bottom bars. The lower half of the concrete beam generally doesn't do anything but provide fire resistance so temporary removal can be done - with a re-patching.

2. Same goes for the top of the beam at the supports - to reveal top bars running over the beam's end supports - but this requires invading the floor area which may not be possible.

3. Chipping along the sides of beams - just to remove the side concrete to reveal the stirrups can also be done - but for older beams, shear reinforcing was not always provided as aggressively as today - so assuming only phi-Vc for shear capacity many times controls your recommendations - as it is a conservative lower limit on strength.

4. Load tests may also be performed if you have a rough idea of what the overall psf capacity is of the floor - The IBC has some descriptions on this in Chapter 17 and the ACI code has some as well (see Chapter 20 - Strength Evaluation of Existing Structures).

5. CRSI has some literature on old reinforcing bars - See CRSI Engineering Data Report Number 48 - Evaluation of Reinforcing Bars in Old Reinforced Concrete Structures.

 

[VBI]

Thank you for your prompt response.

I am evaluating a concrete/concrete beam garage deck that has 24" of soil over it. The garage deck needs to be waterproofed and they want to know if they can drive a payloader to clear the dirt off the deck.

My thoughts are the only way to evaluate this is to evaluate the beam capacities. OR perhaps simply recommend supporting the beams during construction to allow for the added weight.

Incidentally, the underside of the concrete appears to be in good condition.

 

[Bbird]

If there is no possibility of finding out the rebar information then a good start is to assume the minimum reinforcement corresponding to the age of the beam, bearing in mind at some time in the past people could use exclusively mild steel rebar too.

The bending capacity of a RC beam can be easily assessed by elastic theory (with rebar converted in area times the elastic modulus of steel over concrete to form (1) reinforcement tension balances with concrete compression and (2) the couple created by the rebar and concrete under compression balances with the applied moment)which forms a pretty good staring point. Ultimate state analysis can also be computed but the process is more complicated.

The beam will be held in equilibrium or would have collapsed already. Thus it should be just a matter to find out where about the maximum stress and strain position of the rebar.

Old concrete would creep to the full extent and would probably reflect the modulus ratio=15 rather well too.

 

[SlideRuleEra]

VBI - I suggest that you recommend shoring, even if you calculate the existing beam capacity. Estimating the load applied by the equipment (payloader) is very dependent on how it is operated.

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