Deflection limits for large-span PEMB frame

[SE2321]

So I am designing a 61m (~200ft) clear span frame for a PEMB structure with a 0.5mm roof corrugated sheet. Per the MBMA/IBC/ASCE codes, vertical deflection limits are Span/180 for live loads and span/120 for dead and live loads. This comes out to be 508mm (20in) for the latter. These deflection limits seem too high to follow. I understand that the span is large, but the almost 2ft deflection limit still does not make sense. Moreover, as per my understanding, there should not be any significant dead load deflection of rafters, which must be ensured by the cambering of rafters, but my company does not do cambering due to the additional cost and resources involved. So, what are my options here plus should I continue following code deflection limits without hesitation?? Thanks.

 

[human909]

Why doesn't it make sense? Deflection limits are normally expressed in spans or heights because they scale and the most of the problems of excessive deflection scale. Not different to a high rise that is 400m height having very large lateral deflections.

Of course not all the problems caused by lateral deflection scale so if you client has specific needs they should be considered.

So, what are my options here plus should I continue following code deflection limits without hesitation?? Thanks.

You are always free to design a stiffer structure than the prescribed limits. So if needed you could readily reduce the deflection.

In some codes you are also free to ignore the deflections 'limits' as they are simply guidelines. (I obviously can't speak for your code.) But you'd normally want to ensure that your client is satisfied with that outcome.

 

[XR250]

I thought the computer designs these things?

 

[human909]

I thought the computer designs these things?

'AI' Yo. Chat bots are now on eng-tips.

After all, we are trying to train computers to act line humans. So computer engineers surely must also post on Eng-Tips.

(https://www.eng-tips.com/threads/firm-recruiting-engineers-to-train-generative-ai-models.522665/)

 

[rowingengineer]

Presets are a cost effective way to get some dead load deflection control. Presets are created by adjusting the bevel angle of the apex, hanch and splice cuts.

 

[Tomfh]

Way too much deflection without presetting/precambering in my experience. Once the first roofer looks down the ridge and sees that half a meter of sag you will be hearing all about it.

 

[Retrograde]

At that span you may want to consider a truss.