Allowable deflections for Residential Floor Beams

[Delchi]

I have been pondering allowable deflection limits for our residential design standards for high-end residential *FLOORS BEAMS*(wood framed with LVL, glu-lam, and steel beams). I am trying to develop a list of criteria for younger engineers, and I would appreciate some input from some of y'all. Over time, I have tended toward the following limits for these expensive homes (using the minimum of any of the following):

Limit 1: Live load deflection less than L/360 for L < about 8 ft, L/480 for longer spans.

Limit 2: Total load deflection less than L/240 for L < about 8 ft, L/360 for longer spans, but never more than 3/4".

Limit 3: Beams parallel to a wall below: Total load deflection < 2D/240 max, where D is the distance from the beam to the adjacent wall.

Limit 4: Any beam supporting masonry: Total load deflection less than L/600.

I would appreciate some of your thoughts on these criteria, and any experience you've had with other types of deflection limits. Thanks!

 

[SlideRuleEra]

Limit 4 does not meet the complete MINIMUM requirement of the Brick Industry Association. Their recommendation is L/600 or 0.3" (whichever is less).
See "Technical Note on Brick Construction, 31B" at this link:

http://www.gobrick.com/html/frmset_thnt.htm

For an upscale structure you may want to consider L/1000 or 0.3" (whichever is less). See this article:

http://findarticles.com/p/articles/mi_m0NTA/is_7_16/ai_106028138/

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[apsix]

Attached are deflection limits as defined by AS1684, the Australian Standard for residential timber framed construction.

Your Limit 3 is an important check that is often overlooked.

 

[Delchi]

Excellent masonry info, SlideRule. Thanks!

 

[msquared48]

Beat me to it Sliderule. I would have recommended a .25" limit for #4.

Mike McCann
MMC Engineering

 

[msquared48]

Another thought here - for floors with a concrete overlay, ceramic tile or terrazo, I would recommend a LL deflection of L/600, nte 1/4".

Mike McCann
MMC Engineering

 

[Delchi]

Mike, The tile/terrazo deflection limit makes sense to prevent cracking, though I never thougth about it before--is that limit just borrowed from standard masonry rules? Usually my joists are sized for that type of stiffness (for comfort levels, using TJBeams's Prorating system), but not always the bigger beams...

Also, what is the reasoning behind the LL deflection of L/600, nte 1/4" for the concrete overlay? Some cracking would be OK, I guess...are you just trying to keep the overlay from cracking too much and peices working loose?

Thanks!

 

[msquared48]

Maybe Sliderule has a specific code reference. This is just what I have used with success over the years.

Mike McCann
MMC Engineering

 

[SlideRuleEra]

IBC 2003 does specify L/600 in Paragraph 1405.9.1.1, Interior Masonry Veneers. For the most part, IBC just refers to ACI 530 for details (IBC Paragraph 1604.3.4, for example). Like so many other parts of most building codes and reference standards, the requirements are really just adopted from established industry standard practice (such as BIA, in this case). This is especially true with "time-tested" materials and practices such as masonry construction.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[Lion06]

For what it's worth, the 0.3" minimum has been ommitted from ACI 530-08. The new limit is just L/600.

 

[asixth]

Span/600

 

[aggman]

I would also highly recommend analyzing floors for vibration. I have been involved in several designs that I ended up with L/600 live load deflection limits to correct vibration problems, particularly with TJIs. Also I never design my sleeping areas for 30 psf, always use 40 psf, again because of vibration. I realize that this adds some cost to the structures but it can be easily sold to the customer if you explain the issues. When you start talking about hearing dishes rattle in a china cabinet when someone walks across the room it usually gets their attention. TJI now has their floor performance criteria that can be used to help with analysis.

 

[Delchi]

Aggman--I'm with you there. We've also used the TJI vibration analysis forever for all of our floor designs for that very reason. Thanks for the comment.

 

[racookpe1978]

What is the psf rating needed for bedrooms with a waterbed? Still 30 psf? Or back more than 40 psf?

For the "newer" megabath tubs and inside suana's - same psf limit? Or do you design that floor uniquely knowing what the spa/pool weight of people + water + spa weight will be?

 

[msquared48]

I would beef up the area under the bed to the weight of the depth of water seen in the bed. If it is a foot, then I would use a minimum of 65 psf, maybe 70 considering the weight of the bed structure.

Mike McCann
MMC Engineering