folded slab in flat slab

[bratty]

Hi,

I am troubled by folded slab ie a step down in flat slab. The step down or drop can exceed the a couple of times the thickness of the flat slab. I have seen in the RAM modelling, the reinforcement of the step down section looks like a beam with top slab on oner side and bottom slab on the opposite is the same the slabs on both sides ie continuation of the slab reinforcement.

Are there any structural design and detail guidelines when the slab fold? What is the behaviour of the folded slab? I have seen when the folded slab is supported directly by columns, it is still detailed like slab with no increased in size for longitudinal bars and no links but bend Z bars. What happens when the fold is getting further away from the columns?

Appreciate if one can explain when the depth of the fold increases

 

[KootK]

This is surely the thread for you: Link

 

[bratty]

Thanks kootk

Looking into the Z shape section, I would thicken the width of the stem to 1.5 to 2 time the slab thickness especially when the slab is PT and the span is long.

The discussions you had previously, I assumed is just a "Z" shape RC element. But in slab, we have to consider the fold in longitudinal direction. In PT Slab, the fold has has post tension strands pulling on top and bottom which in essence trying to open the corners. Since the fold EI is stiffer in longitudinal direction, it will attract more load. Will it become a beam in the longitudinal direction? I have seen ordinary slab rebars are used in this direction. The step down can be as much as 2 to 2.5 ft with ordinary slab rebars.

 

[KootK]

The discussions you had previously, I assumed is just a "Z" shape RC element.

Nope, we were absolutely discussing slab steps.

But in slab, we have to consider the fold in longitudinal direction...Will it become a beam in the longitudinal direction?

It seems to me that you more or less answered this yourself in your originating post. If you're miles away form vertical support element, then it's just weird flexure and no longitudinal beam action. If you're fold is on or near vertical support elements, then there will be some longitudinal beam action. In most practical situations, you're bound to have some as both. As long as you've got adequate strength in one mechanism or the other, you should be fine for ultimate load capacity. That said, some consideration of both effects is often prudent in order to prevent serviceability cracking issues.

 

[bratty]

Kootk

I have been asking around my friends and none has a clue. Your reply reinforced my confidence but the question on your reply is remains

"as long as you've got adequate strength in one mechanism or the other" - It is a checken and egg. How to achieve adequate strength without knowing the adequate loads or moment taken by the folded section? What is the best and more accurate way to determine the amount of attracted loads and moments onto the folded section of a large panel slab?

 

[rapt]

Bratty,

Terminating the PT like that is not a good detail.

I would make the overlap much longer (10 - 15 times slab depth) and transition the tendons through it.

How you lay it out depends on the location in the span. Often I would extend the lower slab soffit all the way to the next support (depending on distance), to simplify the detailing.

I would not normally detail a PT slab step as you would an RC slab step.

In An RC slab situation, you would not normally assume it attracts any more loads. But you have to design the vertical part of the step to transfer the slab moment through it. If the thickness of this is the same as the slab depth, then the reinforcement required vertically in the step is the same as is required in the slab either side and you have to provide full continuity.

For PT it will be different. The slabs either side will not need as much reinforcement due to the PT. But vertically in the step you still have the full moment but no PT capacity, so a lot of reinforcement must be supplied for continuity through the step. Also, if you terminate the tendons as you are suggesting, you will have to determine the extra twisting effects and add more reinforcement.

You should be working under a mentor who knows all of this and can advise. If you are not, I would suggest letting your client know so they can find someone who can sort it out.

 

[bratty]

Kookt,

I am refering to deep fold in the slab where the PTs have to be discontinued and terminates in the fold. Thanks for your explanation that the web face of the fold has to be heavily reinforced with Z bars to take the continuity and twisting.

As for the fold slab in longitudinal direction, for easy undersrand and ingrained with beam and slab design, I always think the fold behaves like a beam. But correct me, the slab panels behave more like a plate membrane and with PTs on either side of the fold in the slab helping to support?

 

[SteynvW]

Kookt

You your brain serves as the registry for the Eng-tips archive.