the allowble deflection for 2.5m cantilever

[johnstephard3]

i use safe programe and after i have represented the wall loads as aline loads on the cantilever (2.5 m the thicknesse is 30 cm ) the deflection is 2.2 cm and i don't know is this safe or not?

 

[KootK]

It doesn't sound too terrible to me. That said, you haven't given us much to go on. Deflection is generally a serviceability issue, not a safety issue. Are there sensitive finishes below or above the cantilever that might be damaged by the deflection?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[johnstephard3]

no there is not any sensitive finishes below or above the cantilever

 

[KootK]

So what then are your concerns from a deflection standpoint? Sensitive medical equipment that might not tolerate the slope? Bowling alley?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[johnstephard3]

i want to know about the allowble value that the deflection shalln't exceed at the two cases: 1-existance of sensitive structures ,2- normal brick wall above the cantilever

 

[KootK]

That's why I've been asking about the circumstances that surround your cantilever. I've used values as high as L/90 where there were no consequences associated with the deflection. In multistory transfer structures, I've used values as high as L/1200. To identify an appropriate deflection limit, you need to know what matters in the vicinity of that deflection.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

#1) need to know more about the sensitive structures.

#2) how many stories of brick will occur above the cantilever? How high is the next relief angle in the brick? 22 mm is probably a bit much if it will cause a horizontal joint beneath a brick support ledge to open up. I'd think that you'd want to keep that under 10 mm at the most.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[johnstephard3]

l/90 wich code mention that ?...i have read in ACI 2008 that the allowable deflection for the cantilever is L\240

 

[johnstephard3]

i think that the brick wall isnot asensitive structure

 

[johnstephard3]

i cann't undertand 22mm or 10mm

 

[msquared48]

It's less than an inch (7/8") in just over 16 feet doubling the length of the cantilever. That's about L/222. Might want to stiffen it a little. It is marginal depending on the useage.

Mike McCann, PE, SE (WA)

 

[hokie66]

My question would be "How does the computer program calculate deflections?" There are several ways in several different programs, so most of them are not reliable. Putting your trust in a program which you know little about is hazardous. A lot about deflection in concrete depends on what is done on site after the design is long since completed, so knowing what the actual deflection will be is a guessing game.

 

[johnstephard3]

Thank you all for the advice.

 

[rapt]

The Australian code would limit, for a cantilever

total deflection - long term including creep shrinkage cracking and tension stiffening effects to L / 125

brittle finishes/masonry partitions - total deflection after addition of finishes etc - L/ 250

Also depends on manufacturers specifications for anything on attached to the slab eg glass curtain walls

For transfer members, depends on support settlement considerations above.

As Hokie said, make sure you know what is being considered in the calculations. Safe gives different deflection calculations depending on designer data. The limits above are for properly calculated long term deflections. NOT short term uncracked.

 

[rapt]

The concept of "live Load" deflection is an interesting one in concrete members. What do you define it as?

The problem with concrete is that a load like LL might cause an increase in the SW and SDL deflection!

Then you get creep which is normally considered to be caused by the permanent load which includes some live load, but most of which happens after the SW and most of the SDL is applied.

Then you get shrinkage which is technically not load dependant except that it depends on the amount of reinforcement and the neutral axis depth among other things, and most of which occurs after the SW and SDL are applied.

So which of these deflections are part of LL deflection?

Where does BS code talk about LL deflection (ACI does but the logic of LL deflection comes from elastic member design, and concrete is not elastic and it is about time some design codes figured it out!).

if you are going to deal with real deflection calculations, you need to understand the logic before using computer software to calculate them. There are far too many buildings around the world with deflection problems caused by the lack on understanding of design engineers of real deflection calculations and what they are getting from their software. And you should not be getting your education on this from this website, get a good book on concrete deflection calculation.