Strap Beam In Raft For Columns Punching Shear Problem Counter

[Yousuf Ali]

Can anyone can tell me is that possible to provide strap beam in Raft for columns to counter punching shear of columns, if yes then how to do in SAFE? restriction on increasing Raft length and width.

[WR1]

Just draw a line element for punching. It will not calculate the punching and design shear reinforcement for beam for required forces. Otherwise just increase localized thickness under the column for punching. Its a common practice in raft construction.Top of these isolated footings and raft will be at one level.

[UmarMakhzumi]

Can anyone can tell me is that possible to provide strap beam in Raft for columns to counter punching shear of columns, if yes then how to do in SAFE? restriction on increasing Raft length and width.

Do design the raft for punching reinforcement if you cant provide enough thickness. Just because you have detailed a portion of raft as a beam doesn't mean it will behave as beam. Such concepts are a total misconception. Similarly, a lot of people go for 'concealed beams' in slabs which is totally useless. Like Rana said, increase your thickness locally or design it accordingly.

Thanks.

[EngrUzair]

Assalam-o-alaikum!

 

Punching shear in raft footings, is often tackled by providing thicker slab portions under heavy loaded columns, as already mentioned in previous posts.

 

Generally length and width of raft can be increased suitably keeping in view allowable soil bearing capacity, provided space is available. However, in many cases this area extension might not be possible beyond the outer faces of exterior columns (being at the property line).

 

 

Regards.

[WR1]

Yes, As Umar pointed out, in order for the beam to behave as a beam with raft or flat slab, it should have sufficient thickness.

[Yousuf Ali]

@ Sir Rana and umar, incresing localized thickness under coulmn u mean i increase raft thickness from 0.7m currently to beyond may be 1m.. But, by doing this there comes -ve moment over column which should be +ve over otherwise...Dont understand...only punching problem is on Edge columns near the periphery...can i show u picture possible?

[EngrUzair]

Assalam-o-alaikum!

 

If you want to get to-the-point advice, you should provide some basic information about your structure (like building type, number of stories, bay spacing, seismic design category etc.)  along with some sketch, to understand the situation and nature of problem.

 

Regards.

[Waqas Haider]

regarding locallized thickness increment, i am very much confused. I have done this. I drew a slab of 9" thickness with column size of 24". I decided to increase thickness in critical zone upto 24" and hence my critical perimeter would be at a distance of d/2 (22.5/2 = 11.25 inch) from face of column. But i tried, for the sake of checking, for 24" square column, i just drew another slab of dimensions 2.1ft x 2.1ft. Now it does not even cross d/2 critical section but SAFE takes effective depth for 22.5 inch. I was studing SAFE MANUAL titles "SAFE KEY FEATURES AND TERMINOLOGIES" where in section 6.3, in SLAB PUNCHING SHEAR CHECK, point 2, says,

"Check that each slab element in the area enclosed between the face of the column and the critical section for punching shear has the same slab property label.  If this is not the case, the minimum slab thickness within the punching shear perimeter is used.  "

 

Now I m confused, SAFE should take effective depth 7.5 inch instead of 22.5 inch because i have not provided 24" till d/2. My version is 12.3.1

[WR1]

@Yousaf

You will not increase entire raft thickness. Just draw a raft of minimum thickness. I dont know why you require 0.7m thickness. It could be only 350mm. After having drawn this, draw isolated foundations over the already existing raft foundation slab. Make its thickness as per punching requirements. May be 1m. At the edges or peripheries you could increase the raft thickness to 0.7m but inside 350mm should work for usual loading. Yes please provide the sketch.

 

@Waqas

I did not get what are you trying to say. You are talking about slab or foundation? In slabs you provide drop panels for punching and negative moment and invert this for foundations and you get localized footings with raft.

[Yousuf Ali]

how to attach document no option i find?

[EngrUzair]

Yousuf!

 

Click on 'More Reply Options' button, provided at bottom right corner of 'Reply to this topic' window, before writing your post.

 

A new window, with 'Attach files' option provided below the window, will appear. Now, you may write your post, and attach your files as well.

 

Regards.  

[Yousuf Ali]

Thanks uzair bhai... here it is plz reply me the dimension of one span is 11.4 m along x-axis and 4m along Y-axis....all are rectangular columns..design need for raft? periphery columns having punching problem.

[EngrUzair]

Did you try to use raft thicknesses, suggested by Rana Waseem?  Result?

 

Which SAFE version are ypu using?

[WR1]

ok ignore my comments in post#9. I thought its a big raft. You might need more thickness here (less continuity). Anyway take a screenshot by right clicking on perimeter column showing punching so I can see why it is failing these columns in punching.

[ahsun]

I am designing a footing of column for seismic loads. Is it permissible to increase the allowable soil bearing capacity by 30% for seismic design of footing? Can anyone provide me with book or code reference?

[WR1]

I am designing a footing of column for seismic loads. Is it permissible to increase the allowable soil bearing capacity by 30% for seismic design of footing? Can anyone provide me with book or code reference?

please check this topic

http://www.sepakistan.com/topic/61-33-increase-in-bearing-capacity-in-transient-load-condition/#entry808

[Yousuf Ali]

@ sir Rana , sir Uzair, as u can see in columns 6th and 7th of table attached..max shear stress exceeds the capacity of shear stress..making raft columns vulnerable to punch. suggestions? Thank u.

[WR1]

Take a screenshot of calculations by rightclicking on punching ratio of any failed location. Not this table 

[Yousuf Ali]

check these please sir rana.Thank u

[Yousuf Ali]

ACi punching and shear design detail

[WR1]

Calculations are okay. You are calculating punching as corner columns. So you end up with less bo. For edge columns you should select edge columns while calculating punching. 

Either increase the raft thickness or column size or try reducing moments at edge/corner columns by changing the system or may be all of these three options. At the end you need more bo.d.