Metal Deck Ip and In 2

[slickdeals]

Why is there a difference in the moment of inertia of a metal deck for positive bending (Ip) and negative bending (In)?

I can understand section modulus being different based on the fact that the metal deck is not symmetric.

 

[msquared48]

Good question for the Steel Deck Institute!

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[ash060]

It is because of where the tension (or compression) is in the deck. In the negative bending the top of the deck is in tension therefore it is fully effective. In the positive case the compression is in the top of the deck and(which usually has the wider rib) it will not be fully effective because of local buckling.

If you look at the heavier gauges the I's come closer together because local buckling is less of an issue and more of the deck becomes effective.

If you check AISI S100 Chapter B there are Strength and Serviceability sections for calculating the effective properties. The AISI Manual has some examples.