Hollow core slab 4

[JanMac]

Hi
I have hollow core slab with design strength Mn < Mcr but greater than the Mu factored load.Is it automatically considered as failed and will crack in the future? Hope to receive clarification to this matter. Thank you.

 

[KootK]

The US precast industry gives themselves a little leeway on this. They apply that provision at the point of maxim moment and nowhere else. Usually this is an issue with heavy loads near plank ends. Strand develops kinda slow...

 

[JanMac]

Many thanks KootK. In my case the Mn is greater than 1.2 Mcr at the location of maxim momemt. Hence i think the HC slab is safe and accepted.I still do welcome new commemts.

 

[rapt]

There is another area where I have a problem with this. Shear design requires that the tension force be developed into the support.

The force to develop and where it has to develop varies in different codes, but it is necessary.

Pretensioned strands do not develop any force at their ends and for about 6 - 8 db from the end and then only increases slowly as Koot said. Depending on the length of the support, often extra reinforcement needs to be added at the end in the bottom to provide this tension force.

May precast designs seem to ignore this requirement!

 

[Agent666]

NZ code recognises exactly what Rapt is talking about, we are required to cast bars into the cores on site to enhance the shear capacity at the ends to prevent a loss of support, also helps with retaining the planks during seismic events considering the effects of frame elongation/dilation and preventing premature failure of the planks when accommodating all the seismic movements being imposed on them.

They take this approach because you cannot obviously put any other reinforcement into the planks as the hollowcore is 'extruded' (pretty sure this aspect of how hollowcore is manufactured will be universal round the world?)

Edit - picture for clarity

 

[JanMac]

Thank you guys for your comments. I have read also that when Design Strength Mn is 1.33 greater than Mu then provision for 1.2 Mcr is waived. In my case Mu is 28kN and Mn is 41kN. Is it correct? What code use for 1.33 because as I know aci use 2Mu?
Many thanks for your help.

 

[BridgeSmith]

AASHTO has the exception for Mn > 1.33Mu, and has dropped the 1.2 multiplier for Mcr. Article 5.6.3.3 of the AASHTO LRFD 8th Ed. requires minimum reinforcement for the lesser of Mcr or 1.33Mu.

 

[JanMac]

Thank you HotRod10..To all of you many thanks. I really appreciate your help.

 

[rapt]

Australian code dropped the 1.33 rule in 1988! Because it is logically wrong!

ACI has a 2 * rule - at all points in the member, so even at the maximum moment point. It is cheaper to design it properly!

 

[JanMac]

Hi rapt. The doubt is that the aci not clearly mention at which section should be checked for 2Mu minimum. On the other hand PCI states that this provision is applied at critical section. In the case of having straight strands the critical section is considered at midspan.

 

[BridgeSmith]

So AASHTO has kept a provision through several decades and a dozen editions of its spec, that is "logically wrong"? I'm going to have a hard time accepting that without a fairly convincing explanation.

 

[Celt83]

The doubt is that the aci not clearly mention at which section should be checked for 2Mu minimum. On the other hand PCI states that this provision is applied at critical section. In the case of having straight strands the critical section is considered at midspan.

The term critical section in my opinion is taken the wrong way by everyone myself included, all sections are critical sections if they do not meet the code capacity requirements. Yes typically your maximum moment for a uniformly loaded single span plank will be at midspan but this does not mean that you are not required to verify that you meet the stress requirements at any other point within the span.

Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

 

[rapt]

Different versions of ACI say the 2Mu either applies to the "slab" or the "member". I have taken that to mean what it says. The whole member. Not just at the cross-section under consideration.

Supplying it at critical sections would satisfy this as Celt83 says as all sections are critical sections especially if reinforcing is terminating or the section depth is changing.

HotRod10
ACI318 used to have 1.33Mu many years ago, it has increased it to 2Mu and for the whole member while the old 1.33Mu rule applied at the cross-section under consideration if my aging memory is correct.

 

[IDS]

I am really surprised that AASHTO has dropped the 1.2 factor on Mcr, if it was my decision I would increase it.

Cracking moments are often higher than the design value, and the consequences of cracking with inadequate capacity of the cracked section are potentially sudden failure of the section.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[rapt]

IDS,

Agreed, the current Mcr uses the lower bound tensile strength so it is underestimating the possible cracking moment.But it seems to work.

Interestingly, AS3600 uses 1.2Mcr but only requires that Mu be greater, not phi Mu.

ACI uses Phi * Mcap, so 1.2 * .85 in the latest AS3600 = 1.02 Mcr. So now similar to AASHTO

Not saying I agree with it, but that is what is in AS3600. I have pointed out the error in logic previously and I think it was put in the "too hard to change now and admit our mistake" basket!

Eurocode uses 1.15 but also uses material factors so capacity reduction is built into the calculations as well as the 1.15 Mcr.

Personally I think a strain limit needs to be added as well. With T-beams, 1.2Mrc puts in more reinforcement in the flange than the web due to the higher cracking moment, but the strain in the reinforcement in the flange is a lot higher than the strain in the web reinforcement after the section cracks. With less ductile steels, the cracking moment approach does not put sufficient reinforcement in the web to limit the steel strain to an acceptable level.

AS3600 added a fudge to help account for this with a reduced phi factor, but that does not affect minimum and could have been handled better with a strain limit!

 

[Toby43]

I tend to only permit the use of the 1.33Mu concept for footings, where other requirements can lead to excessive amounts of reinforcement if the cracking moment is to be reliably "developed" by the reinforcement. Never would use it for anything "overhead". Although I have seen many a tilt panel wall 150 thick with central SL82 mesh (227mm2/m as opposed to the required 407mm2 that satisfies <8.1.6.1(2)> of AS3600:2009.)

 

[Settingsun]

The AS3600 commentary says why they're happy with using f'ctf (lower characteristic flexural tensile strength) instead of the mean or even upper characteristic value so I doubt it's an accident, though it is a bit of a fudge. For those who don't have the commentary, the reason given is that restrained shrinkage causes additional tensile stress which reduces the cracking moment. There's an implicit assumption that this additional stress is the same as the difference between f'ctf and the real flexural strength.

This isn't the first time I've seen capacity reduction factors ignored when determining whether the failure mode is ductile or brittle. I came across it in a Hilti presentation (IIRC) regarding use of chemical anchors in earthquake areas. A bit uncomfortable compared with our usual conservatism but apparently this is how it's done.

Cracking moments are often higher than the design value, and the consequences of cracking with inadequate capacity of the cracked section are potentially sudden failure of the section.

This is true but falls into the same category as permitting beams to have no shear reinforcement if the shear force is small compared to Vuc. The ACI's 1.33 factor is also a version of this.

 

[rapt]

steveh49,

I was only suggesting the lack of the phi factor was an accident. Not the lower bound tensile strength.

There was a mismatch between the percentage reinforcement version of the rule and 1.2Mcr that could be explained by the lack of the phi factor!