# How to determine the allowable delta max for a beam

1. Dec 27, 2013

### CWIL

I am struggling to remember how to determine the allowable delta max for a beam.

I am currently modeling the bending forces upon a piece of square tubing, and though developing delta max seems rather straight forward, I can not seem to remeber how to determine just how much deflection I am allowed in that beam.

Essentially... I have my delta max, but how do I know if that much deflection is OK in my design (failure due to Plastic Deformation)

Any input is appreciated

Thanks.

2. Dec 27, 2013

### CWIL

I have seen on another PF thread that 1" of deflection per every 250" of beam is an accepted approach. Is this accurate? What is the basis behind this estimation?

Thanks

3. Dec 27, 2013

### SteamKing

Staff Emeritus
Depending on the design code being used, there are several different rules of thumb like you cited in Post #2. However, most of these are for beams which are supported at both ends.

In order to check how much max. deflection is allowed before failure (which I don't recommend, since you should always have a factor of safety > 1) using tubing which is cantilevered, I think you must consider shear and bending at the point of fixity, since this is probably the most critical location in the entire beam. The bending is pretty straightforward, but shear stress calculation for closed sections is a little more complicated than for open sections.

4. Dec 27, 2013

### PhanthomJay

As noted, codes may have deflection limits for a number of reasons, but I have often designed 100 foot long closed tapered multiple sided tubes with a whopping 10 foot deflection at the free end at max yield stress at the fixed end. But bear in mind that this is with a safety factor of about 2.5 for the max design load case (snow or ice and high winds ), so actual deflections for that case would be about four feet or so, for a design condition that is infrequent and of short duration. Actual everyday load without wind and ice might only be about 1 foot or so. I use a deflection limit of about 1 percent or so for that 'everyday ' typical load case.

5. Dec 29, 2013

### 256bits

Based on the application, you would have either a maximum permitted stress or a maximum permitted deflection.

People, for example, do not like walking on a platform that deflects too much, from psychological factor coming into play, even though the platform would be design safe from failure due to stress..