Parapet deflection

[xez]

What does everyone use as the minimum deflection limit for parapets? Footnote i in Table 1604.3 of the IBC allows cantilever members to have twice the deflection of the wall. In an application where the wall deflection limit is L/180, that would allow a parapet deflection of L/90. Does that seem a bit excessive assuming it isn't affected by other factors such as siding, stucco, etc.

 

[haynewp]

If the siding is not affected by the deflection in terms of aesthetic damage or moisture I don't see why it would be excessive.

 

[BAretired]

Parapets are usually not very high, so I don't recall ever checking the deflection of a parapet. But allowing a parapet to deflect twice as much as a simple span seems to make sense.

For a simple span:
M = wL[sup]2[/sup]/8; Δ = 5wL[sup]4[/sup]/384EI
So Δspan = 5ML[sup]2[/sup]/48EI

For a cantilever:
M = wL[sup]2[/sup]/2; Δ = wL[sup]4[/sup]/8EI
So Δcant = ML[sup]2[/sup]/4EI

The main reason to limit deflection is to limit curvature which would cause cracking in brittle finishes. Curvature = M/EI, so to maintain the same curvature in a cantilever or a simple span:

Δcant/Δspan = 12/5 = 2.4, a little more than 2.0​

Another reason to limit deflection is to limit P*δ, a moment magnifier. In the case of a parapet, P is very small and δ is about Δ/3 so that does not seem to be a concern.

This would suggest that if a simple span is permitted L/180 deflection, a cantilever should be permitted L/90.

BA

 

[xez]

Great. Thank you for the feedback and explanation! This makes perfect sense.

 

[BAretired]

You are welcome.

BA