Beam deflection limit visually perceptible to humans? 1

[abusementpark]

Does anyone know at which beam deflection level is the sag of the member visually perceptible to humans? Has this been studied?

Obviously, if you look closely at any loaded beam, you might notice deflection. But most people aren't doing that.

 

[WillisV]

Around L/400 is imperceptible for typical beam spans

 

[MikeHalloran]

You can detect a pretty small deflection if you have a frame of reference.

Sort of along those lines, 'we' have conjectured in discussions here that flatbed trailers are pre-cambered to straigthen out at their load limit, so the only measuring tool needed to detect an overload is a taut string.

On a separate but related topic, I used to buy production gages for machining operations. Some of them were 'flush-pin' gages, where a round pin either projects from a surrounding flat shoulder, or is recessed within it. ISTR that a minimally skilled person using such a device can rapidly resolve a deviation of as little as .005" from flush, entirely by feel, with no delicate mechanism and good lighting not needed.



Mike Halloran
Pembroke Pines, FL, USA

 

[civeng80]

L/500 I think would be more appropriate.

 

[asixth]

I think beyond span/250 would raise concerns. I was looking at a cantilever the other day from the soffit which was measured to span/100 and it was noticeable across the street.

Also had a slab with a measured deflection span/150 and that was very noticeable from standing on the surface.

 

[JStephen]

Perhaps you would notice vibrations more than deflection.

 

[Ron]

Public perception of deflection is somewhat irrelevant. Serviceability is relevant and must be complied with. Thus, the limits.

We (collectively, geeks) are the only ones likely to notice such....the general public does not. As Jstephen noted...vibrations are much more perceptible than deflection.

 

[EdStainless]

I know of a house where no one would walk across the large open living room, the 1/250 deflection is so much that it makes people nervous. We ended up adding steel channel sisters to the floor joists.

Just sag, think of many old buildings with uneven floors. It is fairly easy to detect 1/8" just walking across them.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[IDS]

It depends where you look from, and how hard you look. Any specified limit is essentially arbitrary, and I don't know how any more defined limits can be set by considerations of serviceability. I'd say the current limits are set on the basis that they have been found to give a reasonable control of vibration and the magnitude of dynamic effects, based on the historical performance of actual structures.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[IDS]

I think the following from the commentary to the Australian Bridge Design Code (AS 5100.2) is relevant here:

"C6.11 Deflection
The specification of limits on live load deflections and span to depth ratios have
traditionally been used in road and railway bridge design codes to attempt to control
vibration, prevent fatigue, limit stresses in secondary members and allow for dynamic
loading. Although this Standard provides specific clauses aimed at addressing these
serviceability limit states, it has also retained limitations on live load deflections.
Relaxed deflection limits have been introduced in this version of the Standard as part of the
transition to designing bridges for SM1600 loading and the uncertainty about appropriate
serviceability limits for controlling vibration.

For bridges with walkways, the criteria for limiting vibration in Clause 12 of the Standard
will generally control.

For other bridges, the magnitude of allowable deflection limits have been increased from
1/800 of the span or 1/400 of the cantilever projection in ABDC-1992 (HB 77.2—1996)
(Ref. 1) to 1/600 and 1/300 respectively and the method of calculation has also been made
less conservative, as per AASHTO LRFD (2004) (Ref. 15).

These limits are similar to the values that have been used for railway bridges for many
years and reflect the magnitude of the SM1600 loading. They are also representative of the
magnitude of live load deflections in typical prestressed concrete bridges designed for
SM1600. These deflection limits, together with the fatigue criteria specified in Clause 6.9
of the Standard, are likely to prove to be controlling serviceability limit states for steel
beam bridges, particularly when high strength steel is used."

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[abusementpark]

Well, let me know frame the question this way: if you were designing a roof structure with prominent, exposed glulam beams, what would your deflection criteria be?

 

[MikeHalloran]

Just make sure the sprinkler pipes and lighting conduits bend with the beams, or are not close to them.




Mike Halloran
Pembroke Pines, FL, USA

 

[dhoward26]

I think a lot of it has to do with the length of the beam itself. I see a lot of home garage door headers with deflection so I would say L/240.