Girder/Secondary Interface Cracks 3

[howarts]

1. Flexural crack occurs normally in midspan of a beam
2. You have a secondary beam framing into the midspan of a girder
3. Flexural cracks occur in midspan that that is nearer to the top portion of the beam (when the neutral axis is higher up). This is common in such interface.
4. A little movement of the building (like a little torsion) is enough to make existing vertical flexural cracks reaches even the top portion resulting in the secondary beam only supported by the longitudinal bars below (this is even if the load is just within the service load and not anywhere near the ultimate strength (bars not yielding from tension).
5. Irregardless of whether the midspan has internal hanger stirrups, the whole secondary beam is supported only by the longitudinal bars. .
6. How do you see this? Even though the secondary beam is like detached from the girder, can we treat it as a normal beam where the top of the beam is still in compression and the longitudinal bars below are intact.. only it's not only handling tension but also the weight of the entire secondary beam.

 

[dik]

Other than making your secondary beam slightly less deep than your primary one to avoid a conflict in placing reinforcing steel, I'm not sure of what your question is.

If mid span, then your primary beam is taking approx half the shear unless loaded from each side. I often provide added stirrups to help transfer the shear from the secondary beam to the primary one and rely on the anchorage of the ties around the top reinforcing of the secondary beam.

Dik

 

[howarts]

Other than making your secondary beam slightly less deep than your primary one to avoid a conflict in placing reinforcing steel, I'm not sure of what your question is.

If mid span, then your primary beam is taking approx half the shear unless loaded from each side. I often provide added stirrups to help transfer the shear from the secondary beam to the primary one and rely on the anchorage of the ties around the top reinforcing of the secondary beam.

Dik

The obvious question is. Since the bottom longitudinal bars of the girder is taking the entire load of the secondary beam due to the flexural crack on either side of the girder/secondary beam interface reaching into the top (see illustration above). Would the bars rupture? Remember the girder bottom tension bars were supposed to take tension from the beam and not carry the load of another beam.

 

[KootK]

Recognize that normal flexural cracking does not compromise shear capacity. If girder reinforcing dowel action was the only shear mechanism available you might be in trouble but that is unlikely the case here.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[howarts]

Recognize that normal flexural cracking does not compromise shear capacity. If girder reinforcing dowel action was the only shear mechanism available you might be in trouble but that is unlikely the case here.

I know aggregate interlock is the second mechanism of shear action, the third is shear resistance of uncracked concrete but since the whole beam has let's say full vertical flexural crack, there is no more shear resistance of uncracked concrete. Most of the case of aggregate interlock mentioned in references is inclined due to diagonal shear crack.. but for fully vertical crack.. are you aware of any research of shear resistance of such vertical aggregate interlock?

Is full vertical flexural crack that reaches into the compression zone not common? But then even for flexural crack that is 3/4 upward (with say neutral axis that is 3/4 up). A little torsion or just the weight can make the flexural crack continue upwards cutting the beam in half due to the small remaining thickness in the compression zone.. so this should be common occurences (why is it not if you disagree).

 

[hokie66]

How wide are these cracks? KootK is correct...flexural cracking is not very significant in regard to ultimate strength of beams.

 

[howarts]

How wide are these cracks? KootK is correct...flexural cracking is not very significant in regard to ultimate strength of beams.

The crack is not due to the beam reaching its ultimate strength but simply the case when you have hairline flexural crack that is half the height and some unforseen torsion, etc. twist it that cause the vertical crack to reach all the way to the top cutting the beam into two.. in this case, the entire weight of the secondary beam depends on dowel action and aggregate interlock of the primary girder midspan with no uncracked concrete left to resist it (see again the illustration above if you can't visualize it). But dowel action is not supposed to be a normal occurence and a beam tension bars is only built to resist tension. So can the bars rupture?

 

[hokie66]

The resistance of concrete beams is complex, but I will just point out one flaw in your reasoning. Hairline cracks do not mean that that aggregate interlock is compromised.

 

[KootK]

Through thickness cracks are not all that uncommon. Axial stess due to temperature and shrinkage can produce 'em. If such cracks seriously compromised shear capacity, we'd be in all sorts of trouble.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[JAE]

but since the whole beam has let's say full vertical flexural crack, there is no more shear resistance of uncracked concrete

That is not a true statement.

Flexural cracks only go up so high and you have significant compression and shear capacity through the section in any case.
This is similar to a recent question on Eng-Tips where someone was asking about the shear resistance of a retaining wall at its base. (thread507-418612)



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