Allowable Beam Deflection 7

[packie81]

Just wanted to know how I find out what the max allowable beam deflection is? Is there a standard? Does it not matter how much it deflects? Is there a calculation?

Thanks Steve Mackie, Product/Applications Engineer
Apical Conveyor Systems, Inc.
Tavistock, Ontario

 

[Lutfi]

Yes there are many standards. It is all based on the application and what is tolerable. For floors I use span/360. These criteria came from the past and it prevents plaster from cracking. I have used it for years with great success. You can also have span/240 and can go as high as span/180 for live load alone.

Any structural engineering textbook or handbook will give their limits.

I am concerned that this question coming from an engineer!

 

[packie81]

LOL...

It's all in what you are trained to do. I am by experience a manufacturing/process engineer with a Mechanical Engineering Technology diploma. I just happened to get a job involving a lot of structural stuff. It's not that I haven't done this stuff before, it's just been a while.

You can view my resume if it interests you at

http://www.apical.ca/stevemackie/steve_resume.htm

Oh, and I'm green, only one year in the field.....

Steve Mackie, Product/Applications Engineer
Apical Conveyor Systems, Inc.
Tavistock, Ontario

 

[Trussdoc]

As Lutfi says, its all in the application. A floor more than 6 m or 20 ft long with a deflection of L / 360 will probably be somewhat bouncy and your furniture will rattle.

Increasing the allowable to L/480 will usually solve this problem, but you also have to consider the surfacing material. Ceramic tile for example, will tolerate much less deflection than carpet.

While the bending of a pure beam is generally controlled by external factors, if there is axial load on the beam as well, you will have to consider the effect of buckling and the effect the eccentric load due to the beam has on the material itself.

Sorry, but there is no easy rule of thumb on this...

 

[BantrelStructural]

Also, something missed even by many experienced engineers, is that a deflection limit of l/360 (or any other span based limit) is not always the governing limitation. Intersecting structures, appurtenances, or equipment may impose stricter limits.

A number of years ago I was working on a major modularization project where large piperack and process modules were being fabricated and assembled for transportation to a plant some 300 miles away. With many years experience under my belt it was requested that I be seconded to this project as the module yard field engineer (a role normally given to fairly junior engineers). The reasoning was that they felt that my experience would help deal with the inevitable problems with the multi-consultant/multi-contractor project.

One day I was called to one of the yards with a complaint that the steel fabricator had messed up on an underslung pipe support (all the pipes on it were 1-2" above the support beam). After examining all the relevant drawings and shop drawings I was able to quickly determine that steel fabrication was not the culprit, they had built exactly what they were told to fabricate. The nature of this module was that there was a platform level that supported 1 or 2 large, heavy pipes (36 or 42" dia).

A lower level of small pipes was supported on underhung pipe trapezes located directly under each of the support beams for the large pipes. The support beams for the large pipes were supported either directly on legs or heavy diagonal braces (in the vertical plane) that converged at the base of the legs, and so all had direct compression paths to the base. Well except for one beam. This one was supported by flexure of the longitudinal plaform beams spanning between two brace points. And of course this is the one where we had the problem. The flexural deflection of these beams and indeed the large pipe was within code limits per span, but was too much for the lightly loaded (empty) small pipes (1 to 4" dia.) to match the deflection under self-weight only. The problem was resolved by shimming the smaller pipes once the module was installed on site, but the relatively senior engineer who designed this module overlooked the deflection compatibility issues between the large and small bore piping. Easy to do when designing the trapezes for a uniform 'pipe' loading rather than for specific pipes.

I can give many other examples of where deflection compatibility will govern the allowable deflection limit of a specific element.

Another consideration in limiting deflection can be a roof beam or truss where the effect of water ponding can be to increase load concentrations above the original design value, thereby increasing deflection further, etc. Camber can be used to counteract this effect, but even here the actual limits may be determined by criteria other than span based limits.

 

[idly123]

But my doubt is in practice do u really reach these servicibilty limit states of short and long term deflections of span/350 or span/250 etc....

A beam designed for Limit state of collapse should suffice this requirement, its a check thats often made only at a design table. Raj

 

[BantrelStructural]

Well you will only achieve deflections in the order of the limit values if the loads approach the design loads, and then only if the selected member's design was governed by deflection. Research in the UK a number of years ago showed that the average floor load in office buildings was about 15 psf. Generally upper level office floors for general use are designed for between 50 and 70 psf (50 for live load plus 20 for demountable partitions).

Generally for shorter spans flexural strength (or even in some instances shear) will govern your design. As the span gets longer, deflection increasingly becomes the governing criteria.

But the key point I was making is that no span based limit means anything if an attachment, intersecting or proximate item cannot tolerate an absolute deflection above a certain limit. For example meeting a sidesway serviceability limit of say h/200 could be meaningless if the building next door is located close enough that your structure will be banging into it in heavy winds or a seismic event.

Serviceability criteria are often overlooked or quickly glossed over, often because necessary evaluation criteria are not readily available, and also because computing the necessary deflections is often not trivial. In many cases this is not an issue, but I'll be quite willing to bet that in real terms there are far more serviceability "failures" than pure structural failures.

These serviceability failures are generally non-catastrophic, but can result in cracked walls or windows, or other costmetic issues. While lives generally are not lost, money certainly can be wasted doing repairs, and it is still a failure of the structure to be fit for use.

And deflection assessment errors can actually cause potentially catastrophic effects. A number of years ago I was helping an associate with the design of a new grocery store. The basic layout of the store was identical to a store located in another community, so I was using the other store's structural steel drawings as the base sizing for our store. Snow loads were close to being the same, and while our rain accumulation loads were slightly higher resulting in some increased sizing of main line girders in a couple of locations, one area caused me considerable concern because I was getting a significantly overloaded member when using their beam size.

This was an area located at the rear of the store where there was a depression in the roof designed to hide the HVAC units. Conceptually the depressed roof beam (one side only, the other was supported on a masonry wall) had two hanger supports from the much longer span roof girder above, ending in an extra column that did not go all the way up to the upper roof. In execution the lower beam was a single continuous member with the hangers bolted to the top flange rather than three seperate members with only shear connections to the hangers. Since it was shorter and stiffer than the upper roof girder, and now a continuous member, instead of transferring load to the upper girder, it actually took load from that girder, resulting in a flexural "failure" as a 30 ft. member rather than the 3 simply supported 10 ft. members that it was intended to be.

Now the key issue here was the detailing of the under-hung beam as a continuous member, but the mechanism that would have caused a problem was the deflection incompatability between the two beams (the upper beam was within the L/240 criteria that was allowable for a roof beam). I don't believe that the roof would have collapsed, because the overloaded beam would have developed a plastic hinge which would have relieved the excess load and transferred it back up to the upper girder through the hangers. It would have looked ugly probably and scared customers and staff, but collapse was unlikely. I designed and detailed my beams differently (as simply supported members), and passed word back to the other consultant about his situation.

 

[dpa]

Lutfi,

I was surprised to see this question coming from an engineer also but I am kind of sorry that you said it. I would hate to discourage anyone from asking questions for fear of being ridiculed. After all "the only stupid question is the one you don't ask".

The prime importance of this forum is for the more experienced guys to help out the newbys.

Dave Adkins

 

[IJR]

I must emphasize dpa's last comment, without attaking lutfi's feet. It is an opportunity to send an important message. If a question lacks time and space, the response need not lack time and space.They need not be one to one.

I often ask the same question several times and I am always surprised at the scopes of responses I receive everytime I reask the question. No harm interrogating the rules of thumb anyway.

Instead of taking a question lightly we should expand the scope of our responses.

Serviceability is a design criterion worth spending years to understand properly I guess, by learning,experience and constant questioning. As lighter but stronger materials show up in the market we have more responsibility to understand this criterion.

respects
IJR

 

[steve111]

Fascinating question. Ultimate Limit States vs. Serviceability Limit States. Well definitely one will govern the design solution. L/360 (LL only) type criteria is a gross over simplification. Vibration, dynamic loads (read lateral), storey drift, P-Delta, differential settlement, compatibilty equations, temperature, creep, long term, short term, axial shortening, camber, etc.... you could go on for hours determining serviceability limit states criteria also the most prevalent source of envelope failure and then rapid deterioration and then problems (read $$$$$). Very complex problem. Give me a couple of steel columns and a pinned connected beam between them and i'll make a living.... wait a minute better speak to the Geotech. guy first ... could be a problem here, may have to adjust my bay spacing for the footing size, ... now what is it Bouissineq figured out...., what was it Hetenyi figured out, beams on elastic foundations, better wip out Bowles, it all ends up in the dirt anyways, if you haven't got that figured out then everything above it is garbage. Ok then couple of columns, single storey, pinned connected beam between them, founded on bedrock, i'm good. L/360 forget it you haven't even started. However what a fantastic subject! This is what structural engineers do! Mechanical Tech., get a Structural Engineer. We'll turn a two or three dimensional problem into a static load so fast it will make your head spin. ah well anyways.....