Cambering: Deflection Check or Mask Deflection

[Struct123ure]

Hi,

Cambering a structural member simply masks deflection from the viewers eyes OR after cambering a member can we subtract the camber from the deflection requirements?

I have seen both approached such as “purpose of camber is to mask deflection, it does not reduce the total deflection” pg3 NRC research see attached paper; and have seen the opposite argued in for instance this thread (Link).

To highlight the difference between the two approached:
1. Masks deflection: means you have a member which you calculated the SW+SDL+LL deflection as 3”< L/240=3.1”. Deflection passes code requirement but it’s ugly because the client wants a flat floor. You camber 2” and now only have 1” as LL deflection that can be visibly seen.
2. Mitigate/reduce deflection: means you have SW+SDL+LL deflection as 3”> L/240=2.5”. You camber 2” our now you have 1” deflection left which is < L/240=2.5”.

 

[Tomfh]

To maintain a flat appearance, camber can offset deflection in the design. As you say it masks the deflection. But for preventing damage to brittle materials like brick walls, camber is inadequate as these materials experience full deflection regardless.

Where in that thread does it say otherwise?

 

[Struct123ure]

Hi Tomfh,
The first response at my previous link says:
"Non-Composite:
If your deflection due to total load is say 3", then cambering the beam 2" will cause a final deflection of 1" (L/480). Cambering does not override the deflection checks, it only helps in keeping it within the code requirements."
The 2nd part of that is relatively obvious that you still have to check deflection and I have no issue.
The 1st part: I take that to mean that you are subtracting the 2" camber from the total and then checking deflection against the remaining deflection(Total-Camber). I see this as still failing the code limit. You haven't contained the deflection within allowable limits since total deflection is still 3". But others may see it differently?
You bring up the point regarding materials which has a role to play in deflection, but my question is regarding the approach to checking real deflection vs. code.

 

[Tomfh]

I'm not certain which specific code limitations you're mentioning. However, in cases where these limits are imposed purely to preserve visual aesthetics, it's important to include the camber measurement as part of the total deflection calculation. This is important because the end user sees both camber and deflection combined.

Where deflection is to protect brittle material etc, then you need to check the full deflection that will occur after the material has been added, excluding camber.

 

[Luceid]

I agree with Tom here - it depends what you want out of your deflection limit of L/XXX. As a younger engineer I always find it difficult to feel confident about this since I don't have much experience with when I should set my own deflection limits beyond whatever the code says - although I hardly know the rationale for the code's deflection limits either....probably time to do some reading

 

[driftLimiter]

Often times brittle finishes are attached after a majority of the dead load are applied. A prudent designer could include the sequence of material application and deflection to make a proper judgement on the amount of deflection that is allowed.

Also review AISC Design Guide 36 there is significant discussion.

Remember the code limits are based on a maximum allowable rotation for varying finish materials.

The code deflection limits were determined based on an acceptable level of rotation in the beam. These limits are designed specifically to protect the attached materials.