HOLLOW CORE SLABS

[Muhammad Hashmi]

Assalam O Alykum 

Small question

The hollow core slabs are defined as a membrane or shell element ( In the direction of hollow core panels)

As a hollow core slabs are rests by bearing on the beams with grout and screed on the top with no composite action between beams and hollow core slab.

Thank you 

[Badar (BAZ)]

This structural element has shear, flexural and axial stiffness. It is not a membrane. Having said that, you can model it as membrane; You need  to decide about the modelling approach based on your design assumptions for supporting members and structural system as whole.

[Muhammad Hashmi]

Thanks Muhammad for your reply

Dowel bar will be provided to make it as connection between the hollow core slab and cast in place beams.

Beams supporting the hollow core slabs will be designed by us and hollow core slabs will be designed by the specialists.

My question is that this dowel bar will really create a composite action between hollow core slabs and cast in place beams

& considered enough to define the slab as shell in our E-tabs model.

To design the slab as membrane will increase the amount of reinforcement too much.

 

 

 

 

 

[Badar (BAZ)]

Yes, membrane assignment will do the job assuming that you are only concerned abut estimation of demands on the frame elements and not the hollow-core slab itself.

With your shear connection detail between beam and slab in the form of rebar-dowels, it would be wrong to model the slab with flexural stiffness (shell) , as it would lead to an underestimation of flexural demands on beams.

[Muhammad Hashmi]

Thanks Muhammad 

What if I can define slab as a shell and reduced the bending M11 direction ,  bending M22 direction and bending M12 direction = 0.01. 

Or apply the linear load on the beam in the direction of one way slab or in other words as per the below snap shot 

 

[Badar (BAZ)]

2 minutes ago, Muhammad Hashmi said:

What if I can define slab as a shell and reduced the bending M11 direction ,  bending M22 direction and bending M12 direction = 0.01. 

This approach will give you the same behavior as membrane.

4 minutes ago, Muhammad Hashmi said:

Or apply the linear load on the beam in the direction of one way slab or in other words as per the below snap shot 

I am not sure about your intent. The UDL is not the only effect of connection that you intend to use.

Let me explain it this way: The beams supporting the ends of one-way ribs of hollow-core floor system will need to be loaded with the line load and torsion. See the attachment.

 

[Muhammad Hashmi]

Thank you Muhammad for clarification

1. You mean linear load plus torsion on the beam will be the effect on the cast in place beams. Shall I understand torsional constant must be applied on the program along with stiffness modifiers of beams 0.35?

3. Diaphragm I applied as a semi rigid.

4. Building components such as columns , shear walls will be designed based on the membrane as well or can say shell with stiffness modifier 0.01? shall I say like that

[Badar (BAZ)]

My name is Badar. You are addressing me with the wrong name.

2 hours ago, Muhammad Hashmi said:

You mean linear load plus torsion on the beam will be the effect on the cast in place beams. Shall I understand torsional constant must be applied on the program along with stiffness modifiers of beams 0.35?

It seems that you are not able to realize that the eccentric nature of connection between  cast-in-place beam and pre-cast hollow core slab will transfer torsion in addition to the UDL in the beam.

It is better, in this case, to design the beam for the torsion due to eccentric transfer to shear between slab-beam interface. Do not use 0.35 modifier for torsional constant.

The practice of using a modifier <1 for torsional constant is for monolithic construction involving combability torsion, which is due to the transfer of negative moment from slab end to  the supporting beam (you do not have these moments because of your connection detail).

[Muhammad Hashmi]

Thank you Badar for your continuous support.  

You are right I am unable to realize How much will be the torsional constant shall I consider design the beams. 

apologies if you advice about the below as well 

Diaphragm I applied as a semi rigid.

Building components such as columns , shear walls will be designed based on the membrane as well or can say shell with stiffness modifier 0.01? shall I say like that

[Badar (BAZ)]

14 hours ago, Muhammad Hashmi said:

Diaphragm I applied as a semi rigid.

I do not have enough information about your framing to comment on that.

 Generally speaking, this assignment, in most cases, has a limited effect on  elements of Vertical Lateral load resisting system.

When Structural Engineers  intend to check forces in chord members of diaphragm, for the purpose of estimating its suitability to transfer lateral forces to vertical elements, they use this semi-rigid option. The rigid-option does not let you check these  diaphragm forces.

The assignment also depends on your structural system. CSi has a simple video on it. 

 

 

14 hours ago, Muhammad Hashmi said:

Building components such as columns , shear walls will be designed based on the membrane as well or can say shell with stiffness modifier 0.01? shall I say like that

You cannot assign membrane behavior to column.

You can assign membrane behavior to shear walls. It is reasonable simplification for a majority of design scenarios. 

[Muhammad Hashmi]

Thanks Badar 

Framing plan is attached for one floor almost same for other floors as well.

How much will be the torsional constant shall I consider to design the beams. 

[Badar (BAZ)]

Straying away from your questions, I am curious how will the construction of floor-system with these connection details take place?

Will they cast the frame with rectangular X-sections of beams first, and then install the panels?

Will they drill the dowels afterwards? Will the dowels be left in place during pouring of concrete for RC beams.

How will they adjust the dowels at their right position in precast hollow-core panels?

For these kind of construction, I have seen following construction methodologies:

1- Hollow-core panels supported on ledges of RC/Pre-case beams.

2- Hollow-core panels supported on horizontal joint of RC beams. In this case, they do not pour concrete on the full depth of beam, leaving room equal to the thickness of floor on top of horizontal joint with-in beam's depth.

[Badar (BAZ)]

21 hours ago, Muhammad Hashmi said:

Framing plan is attached for one floor almost same for other floors as well.

How much will be the torsional constant shall I consider to design the beams. 

Rigid diaphragm is reasonable assumption for this kind of arrangement for vertical members of lateral-structural system. 

I already answered the issue of torsion constant. In the first run, analyze the section with out using any reduction in constant for torsion (use 1).

Make changes if you are redistricted in the choice of section size for beam. In this case, how will you know if the additional flexural demands in hollow-core panels, due to torsion-related cracking in RC beam, will be taken into account?

[Muhammad Hashmi]

Make changes if you are redistricted in the choice of section size for beam. In this case, how will you know if the additional flexural demands in hollow-core panels, due to torsion-related cracking in RC beam, will be taken into account?

Eng Badar I could not get this point 

[Badar (BAZ)]

Do you understand that the supporting beam is subjected to torsion, in addition to shear and bending moment, due to connection detail chosen for transferring forces from the precast hollow-core slab to RC beam?

Can you imagine what will happen if beam has not been designed for the interaction of torsion and shear?

Can you imagine that the beam will tend to twist or rotate about its longitudinal axis if the adequate amount of torsional reinforcement is not present?

What will be the result of that rotation?

In this case, can it lead to an increase in connection forces that may not have been accounted for?

[Muhammad Hashmi]

Assalam O Alykum

Do you understand that the supporting beam is subjected to torsion, in addition to shear and bending moment, due to connection detail chosen for transferring forces from the precast hollow-core slab to RC beam?

yes i understand. Final connection detail is not yet received from the designer of HCS. The detail which I shared is preliminary. 

Can you imagine what will happen if beam has not been designed for the interaction of torsion and shear?

Can you imagine that the beam will tend to twist or rotate about its longitudinal axis if the adequate amount of torsional reinforcement is not present?

What will be the result of that rotation? 

Yes I can understand. 

In this case, can it lead to an increase in connection forces that may not have been accounted for?

Any possibility of sending any recommended connection details to avoid any torsion or reduced the torsion on the cast insitu beams.

[UmarMakhzumi]

7 hours ago, Muhammad Hashmi said:

 

In this case, can it lead to an increase in connection forces that may not have been accounted for?

Any possibility of sending any recommended connection details to avoid any torsion or reduced the torsion on the cast insitu beams.

Baz replies are very detailed, and here is one item I can re-elaborate on since you have asked it again.

Because the way your slabs are being connected to your beam (The slab is connected to the face of the beam), you will always have torsion and you need to deign for that torsion as that is the only load path from slab to beam with the provided connection concept. 

If you want to reduce or avoid torsion, your slab to beam connection philosophy needs to be revised. For example, if this hollow slab was "casted above the beam" you will not have this issue (torsion will reduce at the center beams but will still have torsion at corner beams), but you will need to connect your slab and beam through some system and in that way your beam depth will be much smaller. 

Thanks.

Edited March 21, 2022 by UmarMakhzumi
Updated the response to make more detailed. See underlined and italic.

[Muhammad Hashmi]

Assalam O Alykum

Dear Badar 

Do you have any authentic ledge design spread sheet for the cast in place ledge for L and T ledges both.

Small confusion regarding the torsion on cast in place T Beams. If we are talking about torsion on the T-beam or the middle beams where the Hollow core slab is resting on both sides. We are saying that torsion occurs during construction right?