thick transfer slab

[said the noob]

For very thick transfer slabs 1500mm and greater, do you guys find it excessive that the developement length of say negative reinforcement over columns (regular supporting columns, not transfer columns) are made excessively long due to the dvCot(theta), for very thick slabs this number is sometimes double or triple the amount of development length of the bar required, is this excessive?

dvCot(theta) will be approximately 1850mm, where as say Ld for a 35M bar would be approximately 1385mm (35mpa concrete, 1.3 factor for horizontal bar in slabs>300dp) this is about 1.33 times that of the development length.

anybody have any thoughts on this?

tia

 

[rapt]

It has nothing to do with development length. It is all about shear.

 

[rapt]

Actually, if you look at the theory and do the calculation in terms of the extra tension force required for shear (there is a method in Eurocode, Canadian Code, AASHTO, AS3600 etc for this) rather than the offset rule, then the actual length is .5 dv cot theta + development. Some of those codes do not explain it well (AASHTO has it right).

 

[said the noob]

Hi rapt I am working off Canadian code and I have a spreadsheet that calculates the required rebar development due to shear, and your right the demand is much less,


I will have to ask my boss to see if this is allowed, because our transfer slabs is 2000mm thk and just following the typical bar development and cut off, the bar is excessively long in my opinion,

 

[Tommy385]

How long are your spans? Possible to use stut and tie method?

 

[rapt]

If you look at the Collins and Mitchell theory, the extra tension force should be supplied at .5 dv cot theta from the shear calculation point in the direction of reducing moment, not at the calculation point.

If you design it by strut tie, the tension steel is full length and must be fully developed at the ends. This is the extreme of the flexural shear logic!

 

[said the noob]

My L/d >4.0 so don’t think I can use strut and tie although that would be awesome as then my top reinforcements would be much less demand,

Problem is I have recesses of 500 dp about 1.6m from a column and with my excessive top reinforcements lengths Ld+dvcot past column face it conflicts with these recesses (they are small in area but none the less conflicts with my bars)

So I will have to provide some dropped reinforcing at the recesses
And lapping these bars has become difficult and expensive,

Anybody have a detail for recesses? I am thinking just hook the bars @recess then provide dropped bars to match each discontinued bar tension lap past the recess face,

Tia

 

[rapt]

You would have to provide a lot more detail on your suggested " just hook the bars @recess then provide dropped bars to match " before I would be willing to comment., The top reinforcement has to be effectively continuous through this area.

 

[said the noob]

Hi Rapt,

see attached,

I am thinking something along lines of det 1 and 2, if I can get away with det 2 would be great, I am thinking this will be depending on the moment demand at the specific location of the recess.

 

[Agent666]

Noob, seems like a good application for strut and tie to justify the load path around the recess. Development lengths only get you so far, for 'disturbed' regions Strut-and-Tie is the recommended approach.

 

[rapt]

As long as Ld is the full development required at that point!

 

[Agent666]

If you are stopping and starting longitudinal reinforcement (or changing the moment capacities along the length of a member) then you need to also ensure that you follow the rules in your code for moment coverage (easiest to visualise this with the creation of a moment coverage diagram with an envelope of all applicable load cases and apply the code moment development provisions).

Often this will require reinforcement to extend more than a development length past the point where its no longer required from a strength perspective. For your case this means the layer of reinforcement under the recess would need to extend more than Ld past the recess.

 

[said the noob]

Hi agent,

I will snapshot the situation I am working on and also write a detailed explanation on the checks I made,

I feel like a non-contact lap can also work in this situation where the longitudinal reinforcing can even be cut right at the face of recess without hooking as I have drawn in det 1 and 2 and the transfer of loads will be through strut and tie to the dropped reinforcing below the recess, provided it is long enough to encompass enough ties (my hairpins) to transfer the loads from the results I got for the tie in my stm model.

Thx will update when off work.

 

[Agent666]

Just keep in mind that with the strut and tie that there is a vertical component that needs to be dealt with further past the step. Given the large step it's my opinion that strut and tie is the way to go, rather than direct reliance on the offset lap rules. Then develop the reinforcement past the strut and tie nodes, and as I explained above regarding the development of moment strength between the two sections. For example as shown below for the -ve moment case you will see that the development starts quite some distance from the step depending on the geometry you determine for the strut and tie model (construct similar strut and tie models for both sides of the step and positive and negative flexure and reinforce to suit and develop reinforcement to suit).