Help Me With The Column Strip Width

[congbui93]

Hi, I am designing a building follow ACI 318. In designing slab, I used equivalent frame method. Now i am stuck with calculate the column strip width. May may you help me to find out the column strip width of line 3. It should be 8/2 m or 4.8/2 m, thank you so much! 

 

the plan is attached below.

[EngrUzair]

With reference to ACI 318-08 Section 13.2.1, column strip width is dependent on the dimensions of the two slab panels, present on two sides of the column line.

 

For line 3, two slab panels exist for each line portion;  one of the size 8000 x 8700 (upper side slab) and second of size 8000 x 4800 (lower side slab).

On the first (upper) slab side, column strip width shall be 0.25*(minimum of 8000 & 8700) = 0.25*8000 = 2000. Whereas, on the second panel side, required column strip width will be 0.25*(minimum of 8000 & 4800) = 0.25*4800 = 1200. Total column strip width for line 3, will therefore be the sum of of these two strip widths = 2000+1200 = 3200.

 

HTH.

[congbui93]

With reference to ACI 318-08 Section 13.2.1, column strip width is dependent on the dimensions of the two slab panels, present on two sides of the column line.

 

For line 3, two slab panels exist for each line portion;  one of the size 8000 x 8700 (upper side slab) and second of size 8000 x 4800 (lower side slab).

On the first (upper) slab side, column strip width shall be 0.25*(minimum of 8000 & 8700) = 0.25*8000 = 2000. Whereas, on the second panel side, required column strip width will be 0.25*(minimum of 8000 & 4800) = 0.25*4800 = 1200. Total column strip width for line 3, will therefore be the sum of of these two strip widths = 2000+1200 = 3200.

 

HTH.

Hi EngrUzair, I don't understand the way you chose column strip width. Why can we add 2000 and 1200 together to be the column strip width? May you explain a little bit more for me.

In my opinion, As 0.25*4800=1200 is the smallest number so the column strip width on each side should be 1200. Therefore, the column strip width is 2400. Is that right? 

[EngrUzair]

congbui93,

 

See the attached images. These have been taken from chapter 20 of PCA Notes on ACI 318-08, dealing with design of two way slabs using Equivalent Frame Method. First image shows the width of Column Strips, for both interior and exterior frames, when the length L2 is less or equal to length L1 of the slab panel. Whereas, the second image indicates the column strip widths for the SAME LINE, when the length L2 is  equal to or greater than length L1 of the slab panel.

 

 

Moreover, procedure for analysis of two way slabs (both without and with beams) using Equivalent Frame Method, has been explained in detail, in Examples 20.1 and 20.2 of the PCA Notes.

 

Regards.

[congbui93]

Thank you so much for your response. However, my problem is like this. How can I determine the strip column width for line B and Line 2 in the following plan?

 

 

https://drive.google.com/file/d/0Bzi4BgJnVLLcVUhCVWR0UGwzU0k/view?usp=sharing

[EngrUzair]

Column strip width for Line B = L2 / 4 + L3 / 4

 

Column strip width for Line 2 = L2 / 4 + L2 / 4

[congbui93]

Column strip width for Line B = L2 / 4 + L3 / 4

 

Column strip width for Line 2 = L2 / 4 + L2 / 4

 

Dear EngrUzair,

 

For Line B, If we choose 

""Column strip width for Line B = L2 / 4 + L3 / 4"

so the width of each side of the column will not be equal. Does it violate the requirement from the code?

 

"A column strip is a design strip with a width on 

each side of a column centerline equal to the lesser of 0.25ℓ2

and 0.25ℓ1. A column strip shall include beams within the 

strip, if present"

 

Does the code require the width on each side must be equal? .I don't know weather I understand the code properly or not?

 

For Line 2, If we choose

"Column strip width for Line 2 = L2 / 4 + L2 / 4"

Why don't we consider L3 since L3<L2 ?

 

Finally, thank you again for your help and enthusiastic. 

[EngrUzair]

congbui93,

 

I have tried to search an example, involving the design data similar to your problem, but was unable to find it. May be someone else, here on the forum, able to point to some definite source in this regard.

 

The answers to your questions given below, are based on ACI code basics and my understanding of their intent.

 

For Line B, If we choose

""Column strip width for Line B = L2 / 4 + L3 / 4"

so the width of each side of the column will not be equal. Does it violate the requirement from the code?

 

"A column strip is a design strip with a width on
each side of a column centerline equal to the lesser of 0.25ℓ2
and 0.25ℓ1. A column strip shall include beams within the
strip, if present"

 

All the examples, i have come across (Including those given in PCA Notes), use same width on each side of column line. As such, width of column strip should be equal on each side. Moreover IMHO, keeping in view basic intent of the code (i.e., using the Lesser of the spans for each slab panel, for column strip stiffness calculations), SMALLEST span of the two adjacent slab panels SHOULD BE USED for calculating the width of column strip for their common column line.

 

In line with above argument, width of column strip for Line B should therefore be equal to" L3 / 4 + L 3/4". 

 

Does the code require the width on each side must be equal?

 

IMO, yes.

 

For Line 2, If we choose
"Column strip width for Line 2 = L2 / 4 + L2 / 4"
Why don't we consider L3 since L3<L2 ?

 

You are right. We must consider lowest spans for the common column lines, in order to be on safer side.

 

In the end, following are the column strip widths, IMO applicable to your frame:

 

Column strip width for Line B = L3 / 4 + L3 / 4

 

Column strip width for Line 2 = L2 / 4 + L2 / 4

 

 

Regards.