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# To calculate long term deflection

## To calculate long term deflection

(OP)
Hello Guys!!
Currently I am working on a project. Can anybody tell me how to calculate long term deflection for flat slab. I am working on SAFE12. As per BS 8110 what is the limit for long term deflection. I want to check if the thickness that I have provided is safe or over design. Please help me out.

### RE: To calculate long term deflection

I generally apply span/360 for the secondary span direction and span/500 for the primary span.

The calculation is a difficult process and requires correct application of longterm (permanent) load. Correct analysis of deformations from creep and shrinkage models.

### RE: To calculate long term deflection

(OP)
What do you mean by secondary span & primary span. The formulae that you have suggested is for immediate deflection rite? I read in Reynolds handbook, that the deflection should not be more than 20mm. So shall we follow that.

### RE: To calculate long term deflection

If it is a two way beam system and the criteria was for longterm deflection.

The primary span is the element which spans from support to support and carries the most load. The secorday span is the one that spans onto the primary span.

20mm would be appropriate for some cases, such as residential spans about 20 to 25'.

### RE: To calculate long term deflection

(OP)
Thanks...
I am doing flat slab with perimeter beam. so I guess you are suggesting to go with considering 20mm deflection.

### RE: To calculate long term deflection

The only design code that suggests a 20mm maximum is BS8110 and seeing Reynolds is normally writing about BS8110, he would suggest that.

Most other design codes only suggest a span/deflection ratio, which is really a limit on change of slope rather than an overall deflection limitation.

You also need to check on deflection limits with suppliers of what is on top of the beam. If it is a glass curtain wall, the manufacturers will specify limits which will be more stringent than design code limits.

So check you design code and manufacturers information where appropriate.

Some consultants also have preferred maximums but they are personal choice.

### RE: To calculate long term deflection

(OP)
Well its blockwork that is coming above the slab. so I guess to go with 20mm deflection and here we have to follow BS code..

### RE: To calculate long term deflection

For blockwork, it would normally be L/500 or 20mm, but BS8110 may have L/350. Check in BS8110 part 2. It gives the limits in there, not Part 1.

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