topping on precast roof members 2

[smvk3]

I'm designing a storm shelter where the gross wind uplift is on the order of 225 psf. We will be utilizing either precast hollowcore planks or double tees as the roof system with a 4" topping to resist in-plane diaphragm forces.

How is the topping slab anchored to the roof members for net uplift loads? By inspection, the 50 psf weight of the topping won't be close to resisting the gross uplift loads so how is it typically anchored to the roof members? I have seen hairpins embedded into the stem of the double tees but I'm not sure if that is for composite action or providing a positive connection between the roof member and topping.

 

[EZBuilding]

Don't interpret this as me claiming to "know" anything on this. It's just me spit balling a little devil's advocacy in search of some insight.

I am not confident enough on the subject to claim to know my thought process is correct either. It is more a question of me not being able to sell myself on a story and preferring to erre on the side of conservativism. I can not nitpick at any aspects of your alternate interpretation. I would be interested, however, to guide the discussion back to one of my previous points. The idea that there is a change in pressure due to a function of time, which could impart a suction on an exterior element.

 

[KootK]

Alright. To start, are you not buying the whole "suction is an illusion" thing? The external pressures certainly do vary with time. How do you see that affecting the situation below?

Air is a gas and, as far as I know, gasses can only push on things, they can't pull on them.

 

[KootK]

I've no way to know if you share this shortcoming but I've struggled mightily myself to understand the true nature of wind. For me, it's because my mind has strongly associated these two phenomena as direct cause and effect:

1) Air rushing past me in a wind and;

2) The sensation of being pushed in the direction of the wind.

I believe that this is to shorten the cause and effect chain in a way that is misleading.

[highlight #FCE94F]If I could somehow force all of the air motion shown in the sketch below to grind to a halt, I believe that I would still feel the pressure exerted by the wind.[/highlight]

This is what my intuition tells me about the cause and effect nature of wind pressure:

FAST MOVING WIND --> MOLECULES MOVING IN A COHERENT DIRECTION WITH THE WIND PUSH ON ME.

This is what I believe is really the cause and effect nature of wind.

FAST MOVING AIR --> GASEOUS PRESSURE ZONES DEVELOP AROUND ME AT DIFFERNT MOLECULAR DENSITIES -->
--> GASEOUS PRESSURE CELLS PUSH AGENST ME DIFFERENTIALLY AS THE MOLECULES IN RANDOM MOTION BOUNCE OF OF ME