Shear in concrete T-sections

[Alvarobg]

Hello,

I have something similar to the following structure:

In order to verify bending ULS, I calculated the effective width of the web, compounded with flange and took its Md and Nd.

My question is: When verifying Shear ULS, do I have to use the same section (flange + effective width) or can I take only the flange + web using only the width of the web?

Thank you!

 

[MIStructE_IRE]

I would just design links using the web dimensions.

 

[Agent666]

Most codes state that you use the web width only, but you didn't say what country/code you are working with to be able to comment further.

 

[ukbridge]

Keep it simple and check it as a series of rectangular beams.

 

[Tommy385]

@Agent666: are they any Codes that state otherweise? I dont see how it could work?

 

[KootK]

1) I'd go with using only the thickness of the web. That, based on technical merit, precedent, and simplicity.

2) In a fundamental, mechanics of materials way, I'm not sure that it would even be correct to utilize the flange outside of the web for shear. Your vertical shear capability generally needs to be compatible with your horizontal shear capability which enforces strain compatibility between the parts of your beam in compression and the parts in tension. This is why we use the beam depth to the reinforcement rather than the overall height of the beam in our shear calculations.

3) I don't know that #2 is strictly true in a cracked, inelastic, non-linear, non-homogeneous material. There are precedents out there for somewhat separating vertical and horizontal shear capacity. One example is studs on composite steel beams. Another is using the average thickness of a tapered web concrete joist rather than the minimum thickness. These are, however fairly minor deviation from strict elastic theory which requires vertical an horizontal shear to be complementary at all locations.

 

[JAE]

For shear you would only use the webs, not the slab/flanges.

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[jayrod12]

I believe he's asking about depth of section. You can use the total depth of concrete, i.e. slab plus rib depth. But as indicated many times already, only the rib width can be used.

 

[JAE]

You would use the width of the web plus the vertical distance, d, used in your flexural design as the shear "area". In other words, do not include the flanges outside the webs (between the webs) for use in shear capacity calculations.

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[Tommy385]

P.s. when you are calculating shear reinforcement, width of the beam doesnt really matter.

 

[JAE]

For concrete shear design per ACI, the width of the beam does affect the required steel shear reinforcement - since ΦVc + ΦVs > Vu.

So the wider the beam, the larger ΦVc is - and the smaller ΦVs needs to be.

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[Agent666]

Tommy385, in Australia AS2870 (residential slabs & footings) uses the full slab plus ribs/Web in Waffle type slabs, its a standard for residential construction and is based I believe on matching test results & using the tee section worked best. Usually there are no shear ties/stirrups required and there is no requirement to provide ties until shear is greater than the full shear capacity of the concrete tee. Most other standards by comparison need shear reinforcement when you are over 0.5 x shear capacity (or similar ratio).

 

[Tommy385]

Something new for me, To combine shear links with shear capacity (actually tension strenght) of concrete. I guess some friction of 2 cracked sections is taken into consideration. In my euro(Code) its not allowed. (But there is similar concept with punching shear in eurocode).

 

[Agent666]

I think the fact that it's sitting on the ground goes some way to accepting the approach, even if normal logic says you shouldn't do it. If it fails it cracks and simply sits on the ground and no harm done.

Having said that these things are very stiff and strong. I've heard of plant picking up the slabs and dropping them to try demo them, and they literally are able to lift up 7-8m width the building slab in an effort to break the slab up, but they survive intact. Sometimes we used to put steel fibres if more shear capacity than just the concrete was required, as this was more cost effective when compared to all the time to tie the shear ties on site.

 

[rapt]

Tommy385

Eurocode is the only code that does not do it. Instead it lowers the minimum strut angle to 21.8 degrees to get more stirrups engaged instead of a minimum of about 30 degrees in the Canadian MCFT and in the old Australian code and a fixed 45 degrees in ACI. Looks like a fudge to get the same result!

When the method in the next Eurocode changes (Model Code already includes an option for MCFT) you will be doing it to!

 

[KootK]

in Australia AS2870 (residential slabs & footings) uses the full slab plus ribs/Web in Waffle type slabs, its a standard for residential construction and is based I believe on matching test results & using the tee section worked best.

Fascinating, thanks for sharing that. If you've any idea where to get the testing results, I'd love to see that. I design precast, inverted tee beams on a regular basis and am often tempted to try to claim shear capacity for the portions of the beam that are not vertically continuous from the tension reinforcement to the compression block (parts under plank bearing ledges), even if that is not kosher with conventional elastic theory.

Intuitively, I think that most engineers have the sense that you pretty much have to crack the whole cross section in order to truly mobilize a vertical shear failure plane. So it's hard to accept that there are parts of the cross section that contribute nothing to shear resistance. On the other hand, for beams with shear reinforcement, I suspect that the lion's share of Vc takes place via aggregate interlock / shear friction the compression zone. Obviously, concrete way from the compression zone won't help too much with that.

 

[Tommy385]

Rapt thanx for the tip. Actually it goes up to 18.4 degrees in some ec countries. Yes it really reduces shear links significantly but it causes sometimes issues with ancoring of the reinforcement on the end support.
P.s. i think i will retire before new Eurocodes are implemented in all european countries.

 

[Agent666]

KootK, unfortunately all I have is the word of someone who developed/contributed to the standard that this was the approach taken, we queried the general design approach with Standards Australia and this was part of their response. The approach of taking the full slab area + web area was an ongoing debate in our office, should we/shouldn't we/goes against everything we have been told about shear/is this what they really mean, etc.

I was in the just take the web camp, even after getting the code members response, but what partially convinced me for this type of application and made me feel a little better about leaving out the shear reinforcement was the company I worked for tested some single slab/ribs assemblies, as they represented one of the concrete suppliers who licensed the systems here in NZ. As I understand it they were unable to break the test units, the test loads achieved were well in excess of the capacities standard shear equations predicted (but we never got to find out the exact failure mode or capacity). So there was possibly some other mechanisms going on that contributed to a much higher load carrying capacity. It still feels weird not putting in any shear reinforcement, but you have to stay competitive if everyone else isn't using any.

Like I mentioned earlier, apart from cases where there are expansive soils (and the slab edges end up cantilevering), or there is a loss of support condition due to liquefaction (again slab required to be designed to cantilever) these things spend 100% of their time sitting on the ground in a virtually unstressed state, and the consequences of failure are very low with respect to life safety. I wouldn't feel comfortable extending the logic to a suspended building structure!

 

[rapt]

Tommy385

Unintended consequences. I would be interested in your interpretation of the development requirements as they are not well defined in the code. Basically the point at which the extra tension reinforcement is required and how far it needs to be extended/developed. We have been doing some work on the development requirements for MCFT for AS3600 and found that CSA23 is as badly written as Eurocode2 on this! AASHTO has done a better job. Theoretical analysis would suggest they need to be modified and made more specific in some of the requirements.

You could work for another 10-20 years in that case. They have only just reformed the committees and from what I have been told by a UK representative on the committees, they have been told to start from scratch again. So everything could change. Hopefully they will see some sense in fixing the current rather than reinventing from the start.

 

[rapt]

Agent666,

Remember those rules are for residential slabs on ground. Stiffened rafts! Ones that cannot fall very far!