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Horizontal cracks in beam below slabs

Horizontal cracks in beam below slabs

RE: Horizontal cracks in beam below slabs

'Cold' joint?

RE: Horizontal cracks in beam below slabs

Inadvertent at that. Probably too long a time between two truck loads, without adequate interstitial vibrating or mixing.

Mike McCann
MMC Engineering

RE: Horizontal cracks in beam below slabs

Looks like a plastic settlement crack to me. Inadequate vibration.

RE: Horizontal cracks in beam below slabs

but i think if it was a problem of an inadequate vibration, there would be no way for this crack to appear again after plastering and painting . may be it's because of the rusting of longitudinal bars. i think there r some marks of old wetness on the surface

RE: Horizontal cracks in beam below slabs

(OP)

The designer who saw it told me it can't be from torsion because this only occurs in cantilevered slab. We don't want to chip if off yet and still discussing what it might be. Do you have any picture of how cracks from torsion looks like, or from the so called sliding failure where the T-beam flange has separated from the beam web?

RE: Horizontal cracks in beam below slabs

I have seen cracks like this due to corrosion of post-tensioning tendons. Do you have grouted draped PT tendons at the top of the beam over a cantilever? Bleed water from grouting can get trapped here and cause corrosion.

RE: Horizontal cracks in beam below slabs

[img ]
this photo displays what happens because of rusting of steel. may be it looked like your beam on it's earlier stages .at this photo we can notice that the upper part of the damaged area is the most damaged part . which can tell that may be it begins with a crack at this part ..i can't see ur photo well but i think that there is another lower horizontal crack at the right side of the photo..

RE: Horizontal cracks in beam below slabs

RE: Horizontal cracks in beam below slabs

(OP)

The beam is just one year old. It doesn't have the cracks a year ago. There is no post tensioning tendons. There are proper stirrups and bars. Perhaps in a T-beam, if the flange is very stiff and the beam twist, it can cause separation of the slab and beam?

RE: Horizontal cracks in beam below slabs

can u give more details about the static system

RE: Horizontal cracks in beam below slabs

(OP)
Here, maybe when the secondary beam rotates, the girder at horizontal gets twisted and slab restrain cause the beam to separate from it? Has this happened before in recorded history?

http://img24.imageshack.us/img24/7697/ff1g.jpg

RE: Horizontal cracks in beam below slabs

The beam width to slab thickness ratio may be too large creating shrinkage issues.

RE: Horizontal cracks in beam below slabs

(OP)

Here is a hi-res photo not compressed:

http://img823.imageshack.us/img823/6323/n0oi.jpg

The stain above is just portion not painted well since the beam is just 10 months old.

In a T-Beam, won't it be possible the flange just separate from the beam web? You have slabs forming very rigid plane, then the beam underneath it. The thickness of the slab is 4 inches, the width of the beam is 12 inches. Would this be considered too large a ratio?

RE: Horizontal cracks in beam below slabs

You will have to look into this more, I don't typically work with 4" slabs. CIRIA had publications on this.

RE: Horizontal cracks in beam below slabs

that static system tells that it's a solid slab ....if so, that may be the cause because the rigidity of the solid slab much smaller than the beam .but if it's a flat slab it will be hard to occure . or may be it's more than one cause ..i really started to believe that it could be a pouring problem. alot of engineers prefer to pour concrete in beams first to insure the equilibrium of the formwork . if u attended the concrete pouring u can tell us if that happened or not . that may cause separation between layers. they could act like two deferent materials. with torsion caused by the secondary beam they could make that problem .
anyway i think there is nothing to be afraid of . because of the similarity of the structure insures that it was an immediate torsion and with small value. may be because of some deferent settlements . another reason to be free from worry that the spans arn't too large . and from the photo, the beem looks to have a good depth.
where r u from eng. Releky?

RE: Horizontal cracks in beam below slabs

sorry eng. Releky i hadn't seen the dimension when i wrote my last reply ...i think with these dimensions 30 cm for this main beam and 10 cms for the slab ...may be u have to worry ....i think that this main beam doesn't have an adequate depth especially that there is no internal column and it carries the secondary beam alone ...what is that structre for? is it a roof?
i think u have to check loads and design.

RE: Horizontal cracks in beam below slabs

(OP)
Moamen2020, the depth is 19.68 inches, width 12 inches and thickness of slab (flange of T-beam) 4 inches. I was there in the pour, the slabs and beam were poured at same time or monolithic. If there are really separation between layers. It's great cause of worry because the beam being separated from the flange can no longer make it work as one unit, the compression zone can be said to be broken (the horizontal cracks) and this may not make compression work property (just like column with vertical cracks) although there are 2 compression bars on top with a diameter of 0.8 inch and 60000 psi. The loads and member sizes and reinforcements are fully in line with design code so no problem with it.

RE: Horizontal cracks in beam below slabs

"...It's great cause of worry because the beam being separated from the flange can no longer make it work as one unit, the compression zone can be said to be broken (the horizontal cracks) and this may not make compression work property..."

But I assume you have vertical shear reinforcement passing through the horizontal crack...hopefully with fully developed hooks that engage to the top long't beam reinforcing bars.

RE: Horizontal cracks in beam below slabs

(OP)

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?

RE: Horizontal cracks in beam below slabs

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.

RE: Horizontal cracks in beam below slabs

(OP)

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)?



http://img824.imageshack.us/img824/4475/j0v7.jpg

RE: Horizontal cracks in beam below slabs

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.

RE: Horizontal cracks in beam below slabs

(OP)

Quote:

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.

Would this cause just a small crack near the interface as shown in the picture above or would this cause the entire separation of slab/flange from the beam?

RE: Horizontal cracks in beam below slabs

(OP)
This is exactly how the 0.787" (60 ksi) bars are arranged in the T-beam.



http://img571.imageshack.us/img571/1747/fb1m.jpg

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

Quote:

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.

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?

RE: Horizontal cracks in beam below slabs

RE: Horizontal cracks in beam below slabs

RE: Horizontal cracks in beam below slabs

(OP)

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...


http://img809.imageshack.us/img809/1623/2kxd.jpg

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?

RE: Horizontal cracks in beam below slabs

(OP)
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?

RE: Horizontal cracks in beam below slabs

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.

RE: Horizontal cracks in beam below slabs

(OP)

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)?



http://img594.imageshack.us/img594/2712/s3f9.jpg

RE: Horizontal cracks in beam below slabs

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.

RE: Horizontal cracks in beam below slabs

You said the spacing of stirrups is 4" to 8". So there is no cause for concern about the flange and web failing to act compositely. As JacksPanic suggested, epoxy inject the crack if you wish.

RE: Horizontal cracks in beam below slabs

(OP)
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?

RE: Horizontal cracks in beam below slabs


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

RE: Horizontal cracks in beam below slabs

(OP)
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?



http://img834.imageshack.us/img834/1993/wpsy.jpg

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
http://www.concrete.net.au/publications/pdf/Settle... where the settlement cracks form up. With bars, please illustrate how the cracks can form in the beam just below the slab


http://img834.imageshack.us/img834/4268/u4ej.jpg

RE: Horizontal cracks in beam below slabs

In the last picture you posted, from the Australian publication, there are no bars at the top to hold the concrete, so the entire beam depth settles under its own weight Without sufficient vibration, the cracks as shown can form due to the abrupt change in concrete thickness.

But in your case, there is a rigid group of bars to support the plastic concrete, and these bars also inhibit effective penetration of the vibrator, so the concrete below the bars settles under its own weight, causing the horizontal cracking below the beam bars, or probably in line with the bottom layer.

I have seen both types of cracks, both due to inadequate compaction.

RE: Horizontal cracks in beam below slabs

(OP)
In other words, the T-beam flange and beam web has separated already (full crack) as far as concrete is concern? And what's only holding them together is the stirrups? In your experience, this would cause reduction in ultimate capacity or nominal strengths of the members? Which of them would be reduced in the cases where such separation indeed occur? Truss action?

RE: Horizontal cracks in beam below slabs

As I said before, there is not a strength issue...in my opinion.

RE: Horizontal cracks in beam below slabs

(OP)
Can the cracks from plastic settlement form or become manifested later or does it appear immediately? Please see the picture of the beam below one year ago compared with at present. There is no cracks one year ago 3 weeks after removal of formworks. Could plastic settlement cracks be just inside and the normal movement of the beam made cracks manifest outside much later?

http://img545.imageshack.us/img545/4990/6tyb.jpg

RE: Horizontal cracks in beam below slabs

Is it possible that the two photos are taken in different locations?

BA

RE: Horizontal cracks in beam below slabs

(OP)

Im sure they are exactly the same beam (the latter one with crack is one already painted white that's why the stains in the original picture gone). So I wonder if it's settlement cracks.. it could just be inside virtual the whole year then it slowly cracks outside because of concrete tension

RE: Horizontal cracks in beam below slabs

Is the crack visible on the opposite side of the beam?

BA

RE: Horizontal cracks in beam below slabs

(OP)

No, it's not visible. Just one side, so hopefully the flange-web is still at most half crack.

Can low viscosity epoxy get inside a crack as small as 1mm?

RE: Horizontal cracks in beam below slabs

Plastic settlement cracks form while the concrete is plastic...thus the name. But that doesn't mean the cracks won't become more apparent with time.

Yes, you can inject 1 mm (or smaller) cracks. You have to drill and install injection ports, etc. Suggest you consult with a specialist in this area.

RE: Horizontal cracks in beam below slabs


"Can low viscosity epoxy get inside a crack as small as 1mm?"


Yes, assuming an experienced operator with correct pressure-injection equipment and resin, cracks less than 0.1mm can be successfully repaired.

RE: Horizontal cracks in beam below slabs

(OP)
Ok.

Let's say the neutral axis is one half of the beam depth at service load. So the cracked flange and web is in the middle. One can think of a column with vertical crack in its half. With ties that hold it together, won't there be transverse pressure as axial strain increase (or compression strain in the case of a beam)? This means the ties or stirrups would be more stressed than usual if there is concrete crack as they have to resist the tranverse strain, or do you believe that even in uncrack concrete, the ties are working to resist transverse strain (or the beam stirrups resisting transverse compression strain in the compression zone of the beam)?

RE: Horizontal cracks in beam below slabs

(OP)
We invited one of the country's top structural engineer to examine the horizontal cracks. He said it could be bleeding or cold joint or formworks related as there is no deflection in the beam. I told him whether I should chip the crack deeper to see extend. He said what's the point because cold joint can cause it. I told him about epoxy injection. He doubts it can even reach the inner part (if the inner part is indeed bridged).

Here's the weird part. In over hundreds of buildings he inspected. He has never seen horizontal crack in beam one inch below slab. Most of you haven't seen it too except Hokie66. So how come only I and my crew and Hokie has seen such cracks. Is it so unique in the annals of structural engineering history that we should let the photograph appear in structural books?

RE: Horizontal cracks in beam below slabs

releky,

Curious, what country do you practice? The very short span slabs with very thin slabs with 2-way beams reminds me of framing concepts prevalent in the Philippines back before 2000.

I have repaired a lot of cracks on concrete structures, including several horizontal beams cracks caused by cold joints, plastic settlement, restraint to shrinkage, incorrect PT drapes, etc etc. For a RC beams that is only 12" wide, low viscosity resin via pressure injection with an experienced operator will indeed fill the crack width and length, if deemed structurally necessary.

One project that had significant horizontal beam cracking caused by a cold joint, "leaked" cement grout during the PT grouting operations!

RE: Horizontal cracks in beam below slabs

(OP)

I practice in the Philippines. Why, have you visited it prior to 2000? Can you repair the horizontal cracks in the project?

Have you seen horizontal cracks near slabs or is it in middle of beam? The one who inspected it said he only see horizontal cracks in middle of beam, not near slab.

What Hokie66 mentioned about plastic settlement forming beneath bars made me realize one thing. Why you mustn't place beam and column concrete at same time, because after the column settles, a gap would form between it and the beam bars concrete. And the building can suffer moment magnification. In the Philippines, most contractors do column and beam pour together even in 50-storey buildings. Isn't it scary. In my neighborhood, an 8 storey buildings just falls to the ground (it's good the tipping is noticed at morning and all evacuation), then falls in the afternoon hitting another building. See video at:

http://www.youtube.com/watch?v=h5Z6itwKE0M

Maybe this practice of casting beam and column together must be outlawed? Almost all do it that way.

RE: Horizontal cracks in beam below slabs

(OP)

In case anyone wondering when the collapse occurred, here:

http://www.pinoy-pinay.be/news/20040723.htm

I think the bad effects of gaps in columns from beam-column placed together can only occur during strong earthquakes (as we don't have falling buildings often).

Does anyone know how to compute for settlement? Supposed the concrete has height of 400 mm and width of 300 mm.. what is the settlement distance in mm for a concrete of 4000 psi compressive strength, ready mix with admixture and fly ash. Does this settle more than normal cement mix? No one in my country knows how to compute for settlements so they don't separate column and beam placements but do them at same time.


RE: Horizontal cracks in beam below slabs

Foundation failure is the most likely cause of the collapse in your video.

As to plastic settlement, adequate compaction of the concrete and adherence to proper construction practices will eliminate these concerns. Prediction of crack thickness, when you can prevent the cracks from occurring, is poor thinking and not what engineers do.

RE: Horizontal cracks in beam below slabs

The practice of pouring a floor or roof together with its supporting columns is not done in my area and should never be permitted because of plastic settlement.

The Canadian concrete code, A23 uses a factor k1 of 1.3 when calculating development length of horizontal reinforcement with more than 300 mm of fresh concrete below the bar. This recognizes the fact that bond in such bars is reduced because concrete settles, leaving a small gap on the underside of each bar.

BA

RE: Horizontal cracks in beam below slabs

I agree with BA on this one, Always have a construction joint between the columns and floor plates,

"Structural Engineering is the Art of moulding materials we do not wholly understand into shapes we cannot precisely analyse, so as to withstand forces we cannot really assess, in such a way that the community at large has no reason to suspect the extent of our ignorance." Dr. Dykes, 1976

RE: Horizontal cracks in beam below slabs

(OP)

The invited structural engineer suspects the formworks may also be a factor. In your country. How exactly do you do your formwork? The following is what is done in my country, leaks always occur at the interfaces. The second url below is the exact picture of the beams with formworks and leaking cement.


http://imageshack.us/a/img844/5611/jzcv.jpg

The following is actual picture of the leaking formwork. Standard in my country. How do you handle the leaks?

http://img835.imageshack.us/img835/8545/61l2.jpg

RE: Horizontal cracks in beam below slabs

"The following is actual picture of the leaking formwork. Standard in my country. How do you handle the leaks?"

Steel bearers and bamboo joists for formwork. Been a while since I have seen its use. I was in Manila in 1999 and it was prevalent, and of course Hong Kong, but more so for scaffolding rather than formwork.

Those form "leaks" look about similar to what I have experienced in AU, US and CA.

I do note that is some parts of world, formwork chamfer strips for "external" edges are not as common as other areas. Correctly installed chamfer strips can assist in making the forms leak less.

For concrete members of depth of more than 300mm it is usual practice to re-vibrate concrete placement, to minimize cold joints, plastic settlement, etc. The concrete vibration task is often passed to the least qualified/experienced site laborer, and his/her duties are paramount to the success of otherwise of the hardened concretes properties.

RE: Horizontal cracks in beam below slabs

(OP)

So leaking formworks as shown in the illustration is normal? What country or place where the practice put putty around the formworks?

Ingenuinity, could you confirm if you have seen horizontal cracks in beams just below slab (one inch below) or is it more near the bottom or center of the beam?

RE: Horizontal cracks in beam below slabs

If the project specifications call for Class 1 architectural off-form finishes, then it is possible that all form edges and joints are sealed, but for a standard structural finish then sealing of joints etc is not undertaken.

Yes, we recently had a project where there was a horizontal crack about 50mm below the slab soffit on a PT beam that was 1000mm deep adjacent to a column - very congested top rebar and PT tendons and the concretor failed to adequately compact the plastic concrete. It was a localized crack to one beam of a total of 6. Was repaired via injection.

RE: Horizontal cracks in beam below slabs

(OP)
How about horizontal cracks not caused by a PT? Like the cracks I shown earlier. Have you never encountered one before?

RE: Horizontal cracks in beam below slabs

releky,

I do not believe the PT influenced the cracks in the situation Ingenuity has described. You have a large congestion of reinforcement, I would agree with several other posts here that it is lack of compaction and poor placement.

Regards,

"Structural Engineering is the Art of moulding materials we do not wholly understand into shapes we cannot precisely analyse, so as to withstand forces we cannot really assess, in such a way that the community at large has no reason to suspect the extent of our ignorance." Dr. Dykes, 1976

RE: Horizontal cracks in beam below slabs

Exactly as stated by "aaronPTeng" - PT as such was not the cause. Just congestion and concrete placement technique.

For your stated case, 1) your beam is deep (therefore more probable to be subject to plastic settlement), 2) is narrow (300mm), and 3) the top rebar is congested over the beam width - then 4) add in some inattentive concrete placement and it results in your horizontal crack. $hit happens. Point this out to future field construction supervisor or concretors and make them appreciate the consequences.

RE: Horizontal cracks in beam below slabs

(OP)

Can plastic settlement be avoided by compacting below it or is plastic settlement inevitable in spite of all efforts in the case of the beam I shown with horizontal cracks one inch below slab?

Or to rephrase it. If you will construct the same beam using exactly the arrangement of bars and beam width and depth (I shown). Is plastic settlement inevitable or can you do something that can totally avoid the plastic settlement underneath the 8 bars?

RE: Horizontal cracks in beam below slabs

It may be inevitable if you can't get access to compact the concrete. Some of those bars could have been located outside the beam web. Constructability should always be a consideration in the design, and if you can't compact, it is not constructable.

RE: Horizontal cracks in beam below slabs

(OP)

I'm a quality control engineer who checks finished construction.. so maybe I should take more part directly in the construction. Our ready mix concrete pumpcrete hose is 4" in diameter. We don't have smaller one. What smallest size have you use?

RE: Horizontal cracks in beam below slabs

It is not the delivery hose size that is important. It is the space for the concrete to get through the reinforcement, and for the vibrator head to penetrate into the concrete.

RE: Horizontal cracks in beam below slabs

(OP)
My last question especially for moamen2020, hokie66, Ingenuity, Baretired, etc.

Below is the stirrup spacings of the 6 meter girder beam.


http://img844.imageshack.us/img844/5080/61q5.jpg

From face to face of columns, closed stirrups spacing starts at 1 stirrup at 50mm from column, then 10 stirrups at 100mm (4") spacing, 8 stirrups at 150mm
(6)" spacing and middle at 200mm (8") spacing. The beam is 500mm in depth, 300mm in width, there are 3 200mm (8") spacing stirrups at center (see picture) where 6-meter secondary beams on both sides framed into it. My concern is the contractor move one of the midpsan stirrup closer resulting it in forming 200mm, 100mm, 300mm spacings (see picture)..

Since the depth of the beam from tension bars to compression is 460mm.. half of it is 230mm... so one of the beam spacings at center didn't satisfy D/2 or 460/2=230mm because it has one 300mm spacing.

The following shows the shear and moment diagram at that location at 1.2 DL + 1.6 LL.


http://img38.imageshack.us/img38/457/gdp6.jpg

The concrete section + 300mm spacing stirrup has shear capacity V(c+s) more than the Vu (factored ultimate load).. which fulfills the spacing requirement but it doesn't satisfy D/2. According to the designer team and head, no problem with it since shear at midspan is not much. But I still have worry because it doesn't satisfy D/2. I told him what if diagonal cracks form there. He said he hasn't seen it happened. Now the reason I asked this is because I can have the 2 inches floor topping removed and change it to mere 3/4" topping to remove some Superimposed dead load to get even bigger margin.. the designer team and head said I can decide whether to do it or not. Isn't not satisfying D/2 in one of the stirrup a problem especially it is on midspan?

RE: Horizontal cracks in beam below slabs

No, I don't think it is a problem. You have asked the question of the design engineer and have received an answer.

RE: Horizontal cracks in beam below slabs

(OP)

Designers in my country are not experts in physics. Most of them forget lessons back in the school days. My name is one of the signatories for liability. All codes require d/2 for the following reason (from http://site.iugaza.edu.ps/aalmadhoun/files/Chp5-Sh...):


http://img14.imageshack.us/img14/6434/479e.jpg

The reason for d/2 is to intercept any possible cracks. In the case of the one 300mm spacing of the 460 effect beam depth where d/2=460/2=230mm.. 300 exceeds 230mm. What if steeper cracks from 45 degrees form inside the 300mm spacing like in the following:


http://img809.imageshack.us/img809/8950/tk63.jpg

How do you think when working with this problem? Do you put close stirrups near midspan for concrete load to intersect steeper shear cracks than 45 degrees?

RE: Horizontal cracks in beam below slabs

Maybe stirrups at 45 degrees would make more sense.

BA

RE: Horizontal cracks in beam below slabs

The steep cracks are flexural cracks, not diagonal tension cracks. The bottom steel intersects those cracks. No stirrups are theoretically required, but code rules do apply.

RE: Horizontal cracks in beam below slabs

Not sure if you are using ACI318, but does the required ultimate shear force exceed 1/2 the factored shear capacity?
i.e if Vu < 0.5*phi*Vc than shear reinforcement is not required.

EIT
www.HowToEngineer.com

RE: Horizontal cracks in beam below slabs

(OP)
madmantrapper, here's the detailed file photo of the girder beam bars taken directly from top position.


http://img194.imageshack.us/img194/8761/e69l.jpg

Note the top layer is in the flange part and second layer is just below the flange like in the bar arrangement illustration shown earlier. There are one inch hole in between the bars as required by code so the aggregates should flow in between them.

RFReund, Vu is > Vc, that is why stirrups are required.

hokie66, Is it not that steep cracks are combination of flexural cracks and diagonal tension cracks called flexural-shear cracks. Near midspan, I can imagine the flexural cracks forming vertically then it gets at an angle signifying start of diagonal tension cracks. Have you ever used Respond-2000? It can model how the crack exactly form and even width but I'm preflexed why in the following with the sections and bars data inputted, it only shows up to 119 KN. Who has tried Respond-2000 before? (freeware at http://www.ecf.utoronto.ca/~bentz/home.shtml)


http://img543.imageshack.us/img543/3206/ee7f.jpg

RE: Horizontal cracks in beam below slabs

Respond 2000? No, I haven't used it, and am not interested.

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