Expert: Arshad Khan - 5/6/2008

Question
QUESTION:  i need to check for opening in post tesion slab.slab in 7.5*7.5 m. opening size is about 1m*.6m this lies at column strip. had are the parameter i need to check. shear is obiviously. Please suggest me in which literature i find about opening in slab.

ANSWER: Hi Pawan,

This depends how far away it is from the column- if it is closer than about 1m, then you will have a serious punching problem. Also, you did not say whether this opening is to be cut into an existing slab or designed for a future slab. If the former, again you will need to ensure that no tendons are cut. If the latter, then you can detail around the opening. Check out this website:

www.cementindustry.co.uk/PDF/BPG%20Flat%20Slabs.pdf

---------- FOLLOW-UP ----------

QUESTION: Hi,
I'm using post tension slab and precast column. All column are pinned connection at top and bottom(dowel).I want to design for tie force. how i 'am going to design it. as code says.  1.Internal floor ties ------ which i can achieve in post tension slab i think.
2.Peripheral floor tie ----which i can achieve in post tension slab i think.
3.for vertical column and edge column.I'm using pinned connection. if i use ties here. it is going to developed the moment in the slab. so this moment should be used incorporated in design of Post tension of slab.
Could you please suggest me what all i need to consider for stability or robustness ,I am using lift core for stability.
With Regard,
Pawan

Answer
Hi Pawan,

Although you are designing columns as pinned, in reality they will attract moments from the slab due to the lever arm between the central dowel and the edges of the columns. So, the best way is to ensure you ahave sufficient X-section area and of dowel and also sufficient anchorage to meet the ties requirement and then also design the slab supports and column ends for the induced moments.

The other horizontal ties you can provide within the slabs, as you have noted.