Maximum Allowable Beam Deflection Reference

http://www.eng-tips.com/viewthread.cfm?qid=292822

The steel construction manual does not provide deflection limits as those are a serviceability limit state, not a strength limit state. The IBC specifies a few basic minimum deflection limits for floors and roofs (of which L/360 or 1" is one); however, these are minimum code limits. The final limits chosen for any project are based on the specific project, situation, materials used, and owner requirements based on engineering judgment and past performance - not code limits.

For a cylindrical aboveground steel storage tank with a uniformly supported bottom use API Standard 650 for petroleum liquids or AWWA D100 for water. Both limit you to ASD design and neither have deflection limits, but both have other requirements. Get the one for your tank or contract with an expert.

thanks, the problem is,.....

To begin with, I have to establish a reference. And this L/360 is a good place to start. (of course I could have started with L/240, but I didn't)......

...... what is existing doesn't even meet the "minimum code limits."

If you're desperate, you can use AISC Spec. for the Design, Fabrication & Erection of Structural Steel for Buildings February 12, 1969, Section 1.13, Par 1.13.1 Deflections, "Beams and girders supporting plastered ceilings shall be so proportioned that the maximum live load deflection does not exceed 1/360 of the span."

The actual allowable deflection varies with code and application. The IBC allows different criteria for live and total loads at different locations, and there is an additional calculation for natural frequency since long-span floors can tend to bounce.

Not so much desperate, just tying down details for my documentation.

Thanks,

This is what I was looking for,

I use it for misc. Items such as Catwalks, Platforms, I so I'll referenced, I see it used in reference/ Text books, but I never really saw the source.

I would calculate the actual deflection. And I use the L/360 as Maximum Deflection for expectable Maximum limits. when there is not limits, you really have nothing.

The Project I'm working on, (On top of a existing Oxidizer) we are replacing the stack falls under repair.....

I was told to put this aside (bending stress on the Oxidizer supports for stack) and just use the bending Stress.

I want to replace the Oxidizer because its a POS. Company does not want to go through the environmental licensing and just wants to go with a Maintenance repair instead to the stack It can be replaced, and be considered maintenance/repair but they don't replace the Oxidizer along with it...... poor foundation.

And was told to ignore this with a verbal reprimand, I should have asked for a written reprimand....

If I can't make them come to their senses, may be time to dust off my resume because of this.

ANSI Standard Allowable Deflection Rule

Maximum Allowable Deflection = L/360 = 88"/360 = 0.25"

Actual Deflection: 1.54"

Is 1.54" observed deflection or calculated? It looks too high to me in either case. Also it's surprising for something 88" long as deflection rather than stress being limiting tends to be more likely as length increases (stress varies as L^2, deflection as L^3). What is the stress?

Its calculated,

Its a C4x7.25 Channel with the stress being 14,241 lbs being applied at the center.

sorry I got pulled away and my editing time ran out.

here's the full response"

Both, calculated and observed

Its a C4x7.25 Channel with the stress being 14,241 lbs being applied at the center. (there are (2) channels running the full length on the roof, separated with channel spinner.

And this is what is existing. even though they look at my calculations, and say, its good enough. I point out that the Oxidizer is actually 20+ years old. So I the material is compromised due to weather (rust) and heat.

By replacing the stack which is applying the force, it would actually be greater.

The last time this oxidizer refractory was replaced (about 7 year ago), the contractor would not step foot into it, until it was reinforced.

I'm pushing the risk present here, and now there is talk that there should be reinforcements. I'm pushing for replacement of the whole unit, the oxidizer itself looks like a crumpled pop can with stiffeners.

What I was not aware of, we are currently replacing the stack on our oxidizer in one of our other facility located elsewhere.... with the oxidizer being of the similar design, but newer (15 years old) they are having problems. This was just revealed to me yesterday, and reinforcing my concerns.

Having actually design oxidizers shells, these oxidizers have be made on the cheap.

The roof where it should be normally plate (1/2"-1" thick) is 1/4" thick.

some typo's it separated by channel Spanners

And total force (mostly the stack) is 28,482 lbs.

I started out with a question, I'm glad it was followed up with more details needed..... gives me a chance to reflect, and as well as vent....

second codemasters response, your channels will be way overstressed if manage to get that much deflection. also make sure you check buckling along with bending stresses.

Thanks,

What I am doing, is reverse engineering what is there already.

To get am accurate representation is to do a FEA, I just recall, I do have an excel sheet for doing a little more details. If you can make it out, there are warnings. (red) Which confirms my original calculations.

(Ignore the cantilever picture)

Unfortunately, I am pushing against the grain of "why fix something that isn't broken mentality", because they have been using it of a long time. even though some contractors refused to work on it until they reinforced it.

This I see is getting ugly, but the way I see it, where I have resources (data, calculations, observation) on my side. They only have luck.

I just keep pushing my recommendations. And from the gist from the my original post, I was just getting my documentations in order.

The bending stress is 122,483 psi

your typical channel will be A36 steel which has a Fy of 36 ksi and an allowable bending stress of 36/1.67 (omega factor using ASD) = 21.6 ksi

You're correct, thanks .... your post prompted me to do a reverse calculation of an acceptable loading, which I should have done already....

It comes down to this.

Continuous loading beam is 34.6 lbf/in for a total of 3045 lbs. (the roof itself with the refractory weight)

That leaves only about 650 lbs for the stack for the mid force.

for a an acceptable bending stress of about 21.1 ksi

650 lbs is a far cry from the actual 11,200 lbs the stack is currently exerting.

I'm surprised that this was how it came new........ 25-30+ years ago.

Someone's luck is running out.....

Without going into it in complete detail, it looks way over-stressed, as malarrrrkee says, #9. And according to the bit of XL in #13, that's with a load of ~ 14241 lb, not the total 28482 mentioned in #8.

Just one point, the only way I get close to your stress and deflection is if the channel is lying flat. There's presumably a reason for that, but it would obviously take higher vertical load on edge. I suppose there is just one channel? That seems a bit unusual, but we don't have all the details.

Yes, as I mentioned earlier, I was just looking for a reference for my data, which I long since received. I never originally intended to go into detail, other wise, I would have included graphics...... since I don't what to leave you people hanging.... how's this for more details.

As you can see the cross spanners channels between. Load point application is off, as will as any strength (stiffness) gained by geometry, but I ignored that as well as the branches off the stack and went with the simplest route with out going in to FEA.

Not shown are the (2) blowers mounted on the outside of the channels.

The Channel length is 120" but the channels contact on the oxidizer roof is 88"

I went in more detailed calculations.

Initially the total weight of stack and refractory on a single point on oxidizer roof is 28482 lbs

divide that by (2) channel supports = 14241

in the excel sheet I put the oxidizer as a distributed load on the strut and the actual weight of the stack divide by 2 on a single point on the channel.

OK thanks I see it now.

Sounds unsafe, it may be time to drop a dime to OSHA.

They're starting to think,... considering the same attitude they took on the other locations and the problems that they felt "that suddenly occurred".

The good thing about that is, it validates my recommendations.