Has anyone encountered horizontal cracks in beam right below the slabs just like in the picture above? What could have caused it?
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Horizontal cracks in beam below slabs
- Thread starter releky
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- #21
Yes, there are adequate vertical shear reinforcement spaced at 4 inches to 8 inches.. so the tension of each is enough to carry the concrete.. but the worry is the compression zone. The more loads there are and greater the compression, the horizontal cracks may be the weak part as it's above the neutral axis. How common is this case in practice, where the flange of beam has separated from the beam web?
VoyageofDiscovery
Structural
This is not a common problem, more details of where this is would be helpful, not to beat up the design but for me I would not design a 4" slab as I need space for 4 layers of steel, again that's just me and my bridge background speaking. Perhaps this was better suited to be all slab designed with drop panels at columns, given the span lengths, or adjacent precast t-beams framed into headers.
Still looks like a restraint issue to me.
To add some value now, fill the cracks, see what happens over time. One more thing, now that this is no longer monolithic, composite behaviour of slab and beam requires horizontal shear resistance, check if the shear reinforcing is capable of this under design load, if not start busting up concrete.
Still looks like a restraint issue to me.
To add some value now, fill the cracks, see what happens over time. One more thing, now that this is no longer monolithic, composite behaviour of slab and beam requires horizontal shear resistance, check if the shear reinforcing is capable of this under design load, if not start busting up concrete.
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- #23
We may eventually chip a bit to explore the extend of the crack, whether it cuts the entire slab/beam or on the surface like in the following (is this what you meant by restraint issue, what could cause the following)?
the cause could be that the slab does not deform simultaneously with the beam producing to much compressive stress in the slab that the stirrups (acting as shear studs) are insufficient to resist the shear stress being produced between the junction of the slab and the beam.
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- #25
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- #26
This is exactly how the 0.787" (60 ksi) bars are arranged in the T-beam.
The reason there are many bars is because it is used for negative moments and it is L/3 in the beam span. For the rest of the span, there are just 2 compression bars to hold the stirrups. And oh, the stiirrups (0.4" (40 ksi) are distant 10 pcs at 4" and 8 pcs at 6".
Since the T-beam width (12") is 3 times the thickness of the slab (4") and the depth is 19.685", can the whole flange just separate from the beam web?
For perspective.
Dan97
Would this cause just a small crack near the junction as illustration earlier or would this cause the entire separation of slab/flange from the beam?
The reason there are many bars is because it is used for negative moments and it is L/3 in the beam span. For the rest of the span, there are just 2 compression bars to hold the stirrups. And oh, the stiirrups (0.4" (40 ksi) are distant 10 pcs at 4" and 8 pcs at 6".
Since the T-beam width (12") is 3 times the thickness of the slab (4") and the depth is 19.685", can the whole flange just separate from the beam web?
For perspective.
Dan97
Would this cause just a small crack near the junction as illustration earlier or would this cause the entire separation of slab/flange from the beam?
Moamen2020
Structural
i assume that slab reinforcement is as shown in this pic ....i think it's sufficient to prevent separation
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- #31
But in actual, the blue you draw is above the top longitudinal bars. The following is exactly how the slab bar frames into the beam.. it's spaced 1 foot apart...
But I think the more bars, the more it can cause separation because you are introducing rigid plane into the slab/flange of the t-beam. Why, how do you think are slab bars should be arranged to prevent or promote separation?
But I think the more bars, the more it can cause separation because you are introducing rigid plane into the slab/flange of the t-beam. Why, how do you think are slab bars should be arranged to prevent or promote separation?
JacksPanic
Structural
Is this perhaps the same beam as mentioned in this post?:
No transverse top rebar over those beams?
No transverse top rebar over those beams?
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- #33
Perhaps another engineer wrote it as we have about 7 engineers in the management company handling different projects and many look alike. Does one need an additional tranverse top rebars over the existing closed transverse stirrups which it has already? When you said transverse top rebars.. do you mean just a horizontal bar hooked across just the top bars?
I said it before, but I'll say it again. The horizontal crack just below the slab is a plastic settlement crack. The plastic concrete in the beam settled, and the concrete at the top was held up by the bars. You may not have noticed the crack at first, but it was there. This crack has no structural implications if within an enclosed structure.
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- #35
We tried to chip off hoping it was just surface but the crack is deeper. If it is settlement cracks and since there are longitudinal bars just as the interface between T-beam flange and web (as shown below), won't this just be equal or literally cause breakage of flange from web (with only the stirrups holding the two together)?
JacksPanic
Structural
Concrete in deeper sections (your beam) will settle more than in thin sections (your slab).
I agree with hokie66.
injection grout it if it will make you sleep better.
I just dont like this thin slab and heavy beam system and rebar ratio that can be seen on the photo. The amount of top rebar for the negative moment at the beam is unrelated to this crack, but i would be surprised if there's no crack in the length of the beam in the top of the slab.
I agree with hokie66.
injection grout it if it will make you sleep better.
I just dont like this thin slab and heavy beam system and rebar ratio that can be seen on the photo. The amount of top rebar for the negative moment at the beam is unrelated to this crack, but i would be surprised if there's no crack in the length of the beam in the top of the slab.
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- #38
Yes, only logical solution is to inject all cracks with epoxy and hopefully the low viscosity is enough to get inside in between the flange and the beam (should they have really separated). And epoxy is even said to bind concrete stronger than their original tension strength. Would you disagree with this?
Moamen2020
Structural
that's the analysis of the beam .... there is no torsion at all on the beam even i put dead load and live load on just on side ( i expected some torsion when i do that but for my surprise there was no torsion. i don't really know why)
may be it's just a plastic settlement crack as the guys said
about the best arrangement of bars of the slab i prefer to put one bar below the longitudinal bars of the beam and the other above them.to resist any potential negative moment.
but there is a notice about no. of bars: for AS' the minimum number is 5 bars/m that means 1 bar @ 20 cm for assuring distribution of loads. u put 1 bar @ 30 cm i think it's quite little
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- #40
Tomorrow we will let other engineers who haven't seen such horizontal cracks go to our project site and see it. But I'd just like to understand how exactly plastic settlement with many restraining bars form. If what hookie described is so, shouldn't it form between the bars and concrete inside (shown in red in sketch below) and not outside just below the slab?
Hookie phased it: "The horizontal crack just below the slab is a plastic settlement crack. The plastic concrete in the beam settled, and the concrete at the top was held up by the bars. You may not have noticed the crack at first, but it was there."
And if this is common, how come you guys haven't seen one??
Also
where the settlement cracks form up. With bars, please illustrate how the cracks can form in the beam just below the slab
Hookie phased it: "The horizontal crack just below the slab is a plastic settlement crack. The plastic concrete in the beam settled, and the concrete at the top was held up by the bars. You may not have noticed the crack at first, but it was there."
And if this is common, how come you guys haven't seen one??
Also
where the settlement cracks form up. With bars, please illustrate how the cracks can form in the beam just below the slab
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