Diaphragm Design

[Muneeb Badar]

Dear All,

Just a brief introduction about the Diaphragm Design for Lateral Forces in case of Major Earthquake Areas:

Please correct me where I am wrong:

There are two types of forces in a member

1- Out of Plane Forces (out of plane behavior)

2- Inplane Forces (inplane behavior)

Followings are the major components for design purpose in any structure

a- Foundation

b- Shear wall

c- Column

d- Beam

e- Slab

Foundation : We design it mostly for out of plane forces

Shear wall : We design it for purely inplane forces because we neglect its out of plane stiffness

Slab : We design it for out of plane as well as for inplane bending.

Slab:

Out of Plane

In slabs, we normally provide the flexural reinforcement and check the thickness of slab which is out of plane behavior. This design should be conducted on gravity load basis even the building is located in severe earthquake areas. In case of Slab supported on beams we need to design for out of plane forces based on gravity loading. In case of Flat Slab we also need to design it on gravity loading but we just need to satisfy one requirement of ACI code 21.13.6 (b ). Actually code asks this condition to be satisfied due to the rotation limit of slab due to punching at these joints. One option is to satisfy this requirement (ACI 21.13.6 (b ) or second option is we can check the actual D/C ratio of these junctions by using PEER/ATC 72 guidelines. PEER/ATC 72 guideline is attached here. Both are equally reliable just the later is a guideline not a codal provision.

Inplane

The second design for a slab which is MUST in severe earthquake areas and normally nobody perform is inplane design. As we know, earthquake acts on a structure laterally, and diaphragm is used to transfer lateral forces to vertical members. We need to assign the proper diaphragm to the slab. Proper means the diaphragm which can distribute the forces to vertical members as well as it transfer the forces through slab. So we have two options. a- Rigid Diaphragm and b- Semi Rigid Diaphragm. So we ll assign semi rigid diaphragm. There is one question why, we ll put this question to some other topic.

So when we assign semi rigid diaphragm it will transfer the forces through slab member and in ETABS we can see the forces in the slab.

Followings are the reinforcements which we need to design for inplane forces

1- Shear Reinforcement at basement Slab Level and Ground Floor Slab Level.

2- Shear Reinforcement at Podium Levels

3- Tension Reinforcement

4- Chord Reinforcement

5- Shear Friction Reinforcement

All of these reinforcements are used to guide the inplane forces from retaining wall to shear wall at basement levels and from shearwall to slab at upper levels. for example tension or collector reinforcement collects axial inplane force and transfer to shearwall. Slab shear reinforcement is used to avoid lateral cracking of slab in case of earthquake.

So we must design the slab for these two forces and both have different design practices.

The attached NEHRP file is very use full guideline to understand diaphragm design.

In ETABS, we can check the inplane forces from F11, F12 and F22 and then making the section cuts from slab. From these section cuts we can obtain shear forces and axial force and flexural force.

In addition to slabs, we need to check the retaining wall shear reinforcement (distribution reinforcement) and flexural reinforcement (vertical reinforcement) for these inplane forces. We also need to provide the reinforcement at the junction of basement slab and retaining wall in the form of U bars. This is also based on inplane forces

Thanks

Muneeb

PEER-ATC-72.pdf

NEHRP Guideline for Diaphragm Design.pdf

[WR1]

Very well written Muneeb! Nice effort! Lot of new things for me

[Muneeb Badar]

Very well written Muneeb! Nice effort! Lot of new things for me

 

Dear Waseem,

 

This is the minimum brief which I can write, otherwise if you start elaborating this you could end up with 20 or 30 pages.

 

Anyways thanks for your appreciation......

[UmarMakhzumi]

Its a nice short basic level summary. I was actually looking for someone who could contribute an article about Design of Cord and Collector elements and other topics- you can check the list athttp://www.sepakistan.com/topic/1329-sefp-consistent-design/#entry4430

Let me know if you are interested. I can help you with any article if you plan to write for SEFP.

Thanks.

[Muneeb Badar]

Dear Umar,

 

What do you mean by Chord and Collector Elements. Is it from diaphragm design. If so then why only chord because in diaphragm, there are many other type of reinforcements.

 

Anyways I am interested in anything in which I can contribute.

 

Thanks

 

Muneeb

[UmarMakhzumi]

Muneeb,

These elements are part of diaphargm design.. Collectors are drag stuts (not always required). Anyway, if you have any topic up your sleeve, we can write about it. If not then lets detail an article about diaphragm design.. Send me an inbox about whatever suits you..

Thanks.

[Muneeb Badar]

Dear Umar

 

Thanks, I can write on collectors or any detail related to dynamic static, linear or non linear...........its no problem. But to which level you want me to me to write on collectors.............boring the bookish type or the practical the design type. ..?

 

For book type, there are many literature available which we can write while the design type we can do it by ETABS.

 

Thanks

 

Muneeb

[UmarMakhzumi]

Muneeb,

Great to hear.. Lets start with something that gives introduction and can be used as a design go-bye.

Thanks.

[Muneeb Badar]

Ok I ll compile the material and share it as its ready........

[EngrUzair]

Muneeb,

 

IMHO, your write-up would be very helpful for us all, if you could include following items in it:

 

a. General concept and brief theoretical background (should be be very brief, and limited to practical aspects only)

 

b. Reference to applicable design code & code clause

 

c. A comprehensive example, elaborating step-by-step calculation procedure for the analysis and design of structural element being discussed manually.

 

d. If possible, procedure of  carrying out the example referred in para 'c ' above, using one of commonly used structural software (e.g., ETABS, SAP2000 etc.)

 

Regards.

[UmarMakhzumi]

I can do proof reading and editinh before we post here..

Thanks.

[Muneeb Badar]

Dear Engr Uzair,

 

You gave me alot of work man..... .....I am not too good to write such kind of details. But I ll try my best to do good.

 

Dear Umar,

 

I ll send you the article before posting and you can modify and proof reading as I am not good in writing....... .....Secondly, this will only be for Chord Reinforcement of diaphragm design?

 

Thanks

 

Muneeb

[UmarMakhzumi]

It could be about diaphragm design and it can include cord reinfio as a subset. I will be available for proof reading.

Thanks.

[mhdhamood]

Thanks Mr.Muneeb