Allowable Deflection for DL only (RESIDENTIAL)

[ndalle03]

I'm aware that allowable LL deflection limits vary based on use, but in this instance, lets assume that L/360 is acceptable for a residential application. If say, 10ft spans, the allowable LL deflection is 0.33inches. If the floor is sagging under dead load only, at say 0.25inches, or even 0.33inches, would you deem this not acceptable? At what deflection limit would you consider acceptable for DL only??

 

[JoshPlumSE]

Deflection is a serviceability issue. So, if it negatively affects the service of the structure then it is a problem.

Looked at another way:
1) Is the 0.25 inch deflection noticeable to the occupants?
2) Can the 0.25 inch deflection be hidden or minimized in some way? Cambering of beam. Maybe some kind of thin topping to reduce the amount that it gets noticed.
3) My tendency is to use a live + dead deflection ratio of 240. If you meet that, you should be in pretty good shape.
4) Above all else, we need to be sure that the beams are sufficient for strength. If that's the case, then it's a matter of cost vs benefit. Does the client prefer to deal with the deflection, or would they like to pay for a more expensive solution.

 

[ndalle03]

Josh - Thanks for the input. I agree with your response. In this particular instance, I'm assessing a sagging floor of a 100+ year old building. Unfortunately, the floor joists are not exposed so I'll have to make assumptions on beam depth, spacing, and whether some DIY plumber notched the heck out of the joists to determine whether it's ok structurally. I was curious to receive the input from other fellow structural folks to determine at what deflection limit you would consider it a potential structural issue, given that you do not have access to visually see the joists. BTW, 0.25inch was for reference only. I'm seeing about 0.75inch deflection over a 16ft span; it is noticeable.

 

[Phil1934]

http://www.bgstructuralengineering.com/BGSCM13/BGSCM008/Deflection/BGSCM0080402.htm

says L/180 for DL+LL and L/240 for LL. For a 10' span this is 0.5" for LL and 0.67" for DL+LL. This makes DL only .17"/120"=L/700. More important

https://www.govinfo.gov/content/pkg...VPUB-C13-1e5ab865e936437ac5f6edcb597ed862.pdf

but 10' is a small span. For the large houses being built now I like to limit deflection to 1/2" per room width. This agrees with pg. 22 (30 of 34) as people walk about 2Hz.

 

[Phil1934]

100 yr. old bldg may just be sag, not load induced deflection. You need to verify sizes and condition to determine if can float floor level or need to open it up and sister joists. You should be able to get an idea of joist depth at a stairwell. Old houses here rarely saw more than 2x8 joists so probably have to sister.

 

[JAE]

Phil1934 is on target - for a 100 year old building the joists probably have long term creep deflection built in to them now.

So if you were to magically take a joist out of the floor, it would not straighten up but have a permanent bow in it.

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