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What to do when Deflection in Beam Is slightly more than specified in code?
5

What to do when Deflection in Beam Is slightly more than specified in code?

What to do when Deflection in Beam Is slightly more than specified in code?

(OP)
So,I'm 3rd Year Undergraduate studying Steel structures for 1st time.I know I am treading a dangerous path here But Here i go-
So I was to design a Steel beam of 4 m length and UDL of 40 KN(Total).
I choose a section,Checked for Bending,Shear,Web Buckling and Crippling.Everything was fine.

But On applying deflection heck ,I found that Deflection was about 15 mm While the permissible deflection (I.S Code) is 13 mm
What to do in these type of Situations.
The Reason I ask is
1) I don't want to go For Cambering or Heavier section which will be uneconomical.
2) In Limit State design, Characteristic Loads are used as Service loads.They are 95 percentile loads and chances of their occurance are rare.Even if They Occur for Short time, Deflections will be elastic and beam will rebound after some time.
3)I considered Simple Support.But,There should always be some fixidity at support irrespective of Connection Type.So Actual Deflection will be less than my Calculated Deflec.
What do Design Engineer Do in such situations?Do they take into account the above factors and Design the section as such ignoring Codal Provisions?

RE: What to do when Deflection in Beam Is slightly more than specified in code?

Normally you try and keep it within accepted limits, eg by heavier section, chambering, etc.

The things you are proposing are things you might consider when it’s too late, and you need a bit extra to justify something thats struggling.

You really don’t want to sail too close to the wind like that at the outset of design, or you’ll get into trouble pretty soon. You can only cut it to the bone so many times before you cut too far.

All that being said, not many people would care about an occasional 15mm deflection on a 4m beam.

RE: What to do when Deflection in Beam Is slightly more than specified in code?

Quote (Yashish Kamar)

I choose a section...
I found that Deflection was about 15 mm While the permissible deflection (I.S Code) is 13 mm.
Heavier section which will be uneconomical.
What to do in these type of Situations.

The wrong beam was selected. This is NOT a mistake, it is part of the design process. You made the appropriate checks and immediately discovered that the beam has excessive deflection.

In these situations, select a heavier beam that meets all requirements... that is not uneconomical.

BTW, if you are being taught that heavier sections are always a waste of money... that is just plain wrong. Extra material (steel) is "cheap", what is uneconomical is unnecessary engineering time to justify an undersized beam or extra field construction labor to compensate for an undersized beam.

www.SlideRuleEra.net idea

RE: What to do when Deflection in Beam Is slightly more than specified in code?

Deflection requirements can be pretty important. Sure your beam might be strong enough but if the floor bounces like a trampoline then that could be problematic for the client. Just recently we had a non engineer post on this forum about his new house build having a concrete floor that vibrated underfoot. Engineers need to make some sensible judgement about deflection and serviceability. Most codes have some suggested reduction of live loads in checking serviceability.

I recently learnt a lesson on this with beams supporting a walkway and conveyor a good 25m up. The span ended up longer than intended due to site construction issues but a quick recalculation still put it well within strength and within deflection requirements so I let it pass. However being up there and walking on the gantry was far too springy. And with the conveyor running it really just had too much vibration. Fortunately the fix was quick, cheap and easy. However it would have been better to get it right in the first place. In many other circumstances the fix wouldn't have been cheap or easy.

Quote (SlideRuleEra)

Extra material (steel) is "cheap", what is uneconomical is unnecessary engineering time to justify an undersized beam or extra field construction labor to compensate for an undersized beam.
Exactly.

RE: What to do when Deflection in Beam Is slightly more than specified in code?

To piggy back on SRE's economy statements - it's also more economical to keep consistent beam sizes in a project. If you specify lots of different beam sizes on a project, you're going to waste money. The steel supply chain in that case will go Mill -> Service Center -> Fabricator -> Project. If you can specify just a few beam sizes for the project, the fabricator can occasionally skip the Service Center and do direct Mill orders. That saves the fabricator having to pay the overhead and profit on the steel passing through the service center. It's also easier to ship the material when it's all the same size. Think about playing a game of Tetris vs. stacking a 3 year old's alphabet blocks.

Something to consider when you're doing limit states design (we call it LRFD here in the US) - do your deflection checks first. If you take your deflection equation and solve for Ix using the maximum allowable deflection, you can keep that in mind when you select your first trial beam size. It'll cut out an iteration or two in your design process more often than not.

RE: What to do when Deflection in Beam Is slightly more than specified in code?

Is it live load deflection, dead load deflection or a combination of the two that is too much?

The reason why I ask is because there are a few things that are commonly done with deflection:
1) Beams tend to have a natural camber, so you tell the contractor to face this upwards (so that the installed beam sheds water). You don't usually account for this in design though. Because you don't have the exact piece of steel available to see what kind of natural camber it has.

2) Is there going to be a topping slab of some sort? If so, it is common to ignore the portion of the dead load deflection that occurs before the slab material sets. This is because the occupants will never notice it. Usually this is for steel beam systems with a composite floor deck over them. But, even if the slab is non-composite the concept is still valid.

Other than that, I echo the other responses which are that you probably need a stiffer beam.

RE: What to do when Deflection in Beam Is slightly more than specified in code?

(OP)
Thank you Everyone!I appreciate you taking the time to answer my question.
I learnt a ton from all the comments.I couldn't have learnt these things from Books or from my School.I also realised that i need to intern at Structural Engineering Firm ASAP.

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