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What does L/360 deflection mean?

Shanghaied's picture

I am trying to figure out what L/360 means in terms of deflection in wood beams. When I am looking at the criteria for engineered wood beams these different ratings are used (L/360, L/480, etc). I have searched the web to find a definition so that  can get some idea of what these beam tables are telling me. Anyone out there know?

(post #60570, reply #1 of 5)

Length of span divided by 360. For instance 12 feet divided by 360 is 0.4 inches.

(post #60570, reply #2 of 5)




A loaded beam deflects by an amount that depends on several factors including:
the magnitude and type of loading, the span of the beam, the material properties of the beam (Modulus of Elasticity), the properties of the shape of the beam (Moment of Inertia), the beam type (simple, cantilever, overhanging, continuous)

A general rule for limiting deflection of simple spans , is that the deflection should not exceed the span (in inches) divided by 360 (max D= L/360). The deflection for exposed ceiling beams at the roof is often allowed to be 50% to 100% greater (l/240 or l/180). Codes usually specify that these deflections are based on live load only, but experience shows that this is sometimes excessive.

 A conservative approach is to limit the deflection to these values for total load in lightweight construction (such as wood and steel). These guidelines are general and apply in most cases, but certainly not all. For example, the springiness of a floor is influenced more by the mass of the floor than by the total deflection; more dead load could cause more deflection, but probably less springiness.

Best yet, see an Engineer and get the right one for the job





(post #60570, reply #5 of 5)


Good to see you posting back again. I had this explained once awhile back but like butter off a hot noodle, eh? This time I am printing this stuff out. I am still amazed by the workers here expecting engineering knowledge from a framer/builder. Back home, like you said, we get a real engineer to do this and use our expertise to install it. Learning curves are like roller coasters, the ride is fun, but you sure feel it in your gut.

You had asked previously for pics of the job here and I tried to post some using breaktime but found it frustrating to use. I have posted work pictures on our fotopage that you can puruse at your leisure.

Thanks one and all for the input.


(post #60570, reply #3 of 5)

Just for the record, L/360 is the maximum deflection for most tile floors, Nateral stone (granite, marble) is L/720.


Can't I go 1 day without spilling my coffee?


Can't I go 1 day without spilling my coffee?

(post #60570, reply #4 of 5)

............. What Uncle Dunc said. Length in inches divided by 360 is the maximum allowable amount of floor deflection. Get yourself a copy of Simplified Engineering For Architect and Builders  by Parker/Ambrose, published by John Wylie and Sons. Book has been around for ages and one you may find helpful.