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Deflection ratio explanation

BossHog's picture

The question of what deflection ratios means comes up every once in a while, and it can be confusing. So I thought it might be a good idea to start a thread about it.  then when the question comes up I can refer back to it rather than re-typing it when the question comes up.

 

When you see span charts for a floor joist, you'll often see something like "L/360".  What that means is that given the PSF loading specified in the chart, the live load deflection of that joist is limited to 1/360th of the span. To use an oversimplified example, a floor joist that's 360" (or 30') long would be allowed to deflect a maximum of 1".

But that's only half the story. The span charts typically only show the deflection limit for the live load. There's a 2nd number for the total load that usually isn't shown. (Total load is the live and dead load added together)

 

If the floor live load deflection limit is L/360, the total load deflection limit is typically L/240.  That means on the example above the 30' joist would be allowed to have a total load deflection of 1.5".

 

For this reason I don't recommend using a span chart for any kind of floor member that uses L/360 as the deflection criteria. I prefer to go with a span chart that uses L/480 for the live load.  That means L/360 is implied for the total load deflection, and it will result in much better floor performance.

With roof trusses, the typical deflection limit is L/240 for live load, and L/180 for total load. So using the 30' span we used above, the allowable total load deflection would be 2".  That's a heck of a lot.

 

Deflection in roof trusses isn't a problem in most cases.  But if you get long span trusses or those with a shallow depth you can run into problems.

One instance would be a flat truss for a commercial roof. If you get too much deflection water ponding can be a problem.

Another example would be a long span residential truss.  In this area 50' trusses are common on duplexes. They can have a lot of deflection.  Especially if the trusses have vaulted ceilings.


I hope this explains the subject fairly well.  If not let me know and I'll take another crack at it.

Ron (post #207410, reply #1 of 3)

I mentioned in that other thread that the 3/4" deflection in the beam would be not up to my idea of good, however it is allowed. 

When I went looking at beam deflection and some explanations, I found alot of letters and symbols and calcs. and formulas..............that invariably led to the L240 being acceptable.  I think it odd when sizing floor joists and using L360 and putting those joists on an L240 beam.

Seems to me that these code minimums might be cheaper to build but what price is cracked sheetrock/plaster?

And I've always relied on plans as drawn, never wanting/needing to get into the equations and formulas.

 

This whole thing has at least got me to understanding-not so much the math as the reasoning behind it. 

I don't know if understanding is the correct word.

 

I don't use a Construction Master calculator, but are these equations in one or another calculator or program that the common man could insert the size and spans in order to figure beam / size-rather than just going to the tables listed in most code books?

thanks.

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


http://www.quittintime.com/

 


"I think it odd when sizing (post #207410, reply #2 of 3)

"I think it odd when sizing floor joists and using L360 and putting those joists on an L240 beam."

I think you missed the point of my original post - Floor joists and beams are typically designed to the same standard - L/360 for live load deflection and L/240 for total load deflection.

Fortunately that's changing to some degree. I-joists and floor trusses are no more often designed to L/480 Live & L/360 total.

 

 

"I don't use a Construction Master calculator, but are these equations in one or another calculator or program that the common man could insert the size and spans in order to figure beam / size-rather than just going to the tables listed in most code books?"

 

I learned all the formulas, and they aren't too terribly difficult. But a few years back I threw out all my notes. The new design programs are so good and so fast I hadn't used the formulas for several years.


The people who supply our I-joists and LVL beams provide an excellent program for designing those. For steel beams and 2x headers I used to have a program called "BeamCheck".  The company I work for doesn't sell steel beams any more, so I didn't get a copy of the program.

Ron (post #207410, reply #3 of 3)

I think you missed the point of my original post - Floor joists and beams are typically designed to the same standard - L/360 for live load deflection and L/240 for total load deflection.

I didn't miss it, I responded in reference to the other thread on the sagging beam.  The reply (not yours) to me there mentioned the 360 live/240 total as being acceptable which meant a 3/4" deflection was ok.  Certainly not my idea of a good frame. 

 

I usually take what's spec'd and call it done.  Now I might start shortening my span ratings in my own head.

What am I talking about, now I'm going to start thinking of retiring and lightening my work load-I don't want to keep lugging those [JOBSITE WORD]s around any more.

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


http://www.quittintime.com/