beam deflection L/360

[Sergio76]

thread507-37706
Hello Everybody after reading the reference thread i just want to figure out the meaning of 360 on the deflection parameter, where this number came from? it has any units? if my deflection calculations are in metric units i use the same value? what are the conditions to compare this parameters... i undestand that in imperial system the value of L must be in inches, what about in metric?

thanks a lot for any comments.

 

[jayrod12]

It works the same regardless of units. It is an value obtained (through testing I assume) that keeps the effect of beam deflection within noticeable limits. I.e. the typical user of the system will not notice the beam deflecting.

Some of the other deflection limits are also set by the type of finish applied to the member. Brittle finishes need tighter deflection limits to prevent cracking etc.

 

[msquared48]

No units, so it is applicable to both metric and imperial units.

All it means is that you are limiting the vertical deflection to 1/360th of the span of the beam, a common limitation for sheetrock and plaster ceilings to limit cracking of the joints.

Mike McCann
MMC Engineering

 

[Sergio76]

thank you guys, i appreciated.

 

[KootK]

I believe that L/XXX is generally a means of limiting curvature rather than absolute deflection. As MS^2 stated, the goal of limiting the curvature is generally to obtain compatibility with finishes such as drywall which are prone to cracking.

Recently, a senior colleague told me that he feels that L/XXX is intended to limit the maximum slope occurring along a member for occupant comfort. I'm not sure that I buy it but it's a novel take on it.

At roofs, of course, there is ponding to consider.

When I've been aggressive with deflection limits, I sometimes wonder about the impact of movement on my connections. Is a single plate shear tab still a moment free thing with end rotations commensurate with, say, L/90 deflection? I'm not sure.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[tony1851]

Deflection is a serviceability issue, and is limited for a number of reasons, such as appearance, possible cracking of brittle finishes etc. For timber floors, it is also limited to reduce excessive bounce.
If you are working to a specific figure, you need to establish whether it is just under imposed load, or imposed plus dead load

 

[rapt]

KootK,

It is probably more rate of change in slope that is being limited, so curvature as you first suggested.

 

[Ingenuity]

I had always assumed that the denominator in L/180 or L/360 etc was based upon its divisibility by "12", with it's origin from imperial inch and foot units.

Some of us in metric countries have adopted L/250 and L/500...

 

[MikeHalloran]

An L/360 beam or floor will not deflect enough for a normal person walking on it to notice the slope.

... but it will resonate around 4Hz, which they will describe as 'a little bouncy' when they dance on it.




Mike Halloran
Pembroke Pines, FL, USA

 

[Archie264]

>>>I had always assumed that the denominator in L/180 or L/360 etc was based upon its divisibility by "12", with it's origin from imperial inch and foot units.<<<

Yep, I always assumed the same. With the span measured in feet and the deflection measured in inches L/360, 240, 180 become L/30, 20, 15 respectively.

It's similar to the reason concrete compressive strength is measured at 28 days: that's divisible by 7 so the testing cylinder break won't have to occur over a weekend.

 

[jvvse]

Chiming in. Posts above are correct, it's meant to limit the visible slope or curvature and not the total deflection.

Keep in mind these are minimum and there are many reasons to have stricter deflection limitations.

 

[Bagman2524]

Deflection is discussed in AISC-360 spec chapter L commentary. Traditional limit of L/360 has worked well for controlling cracks in plaster ceilings during 1st half of 20th century. More modern ceiling systems can get by with a higher max. deflection limit. Deflections greater than L/200 may impair operation of moveable components such as doors, windows and sliding partitions.

"Look for 3 things in a person intelligence, energy and integrity. If they don't have the last one, don't even bother with the first 2. W. Buffet