Tornado Shelter 2

[WARose]

This may be one for the wind tunnel folks.....but I thought I'd ask here first. Got a interesting question from a client here......they have a reinforced concrete tunnel that passes through a berm. In places, it's covered by as much as 120 feet of dirt. The tunnel is about 100 yards long and at mid-length, there is a 60 yard long "T" junction (i.e. an additional length of tunnel). Walking through it, I'd estimate it's about 20'x10'. They use it as a walk through from a parking lot to the plant and also lots of piping is lining the walls.

The question: they've thought about using the "T" portion of the tunnel as a designated tornado/storm shelter.....what they are asking me is: what kind of wind pressures can they expect 50 yards deep into the tunnel (At the junction of the main tunnel and the "T".) They will likely put in some doors as that point.....but they want to know. And I have no clue on this. Any ideas?

 

[r13]

I don't know how far, but once beyond the turbulence zone near the tunnel entrance (similar to pipe flow), there shouldn't be much force on the wall except minor drag force due to roughness of the wall material. If the door is aligned with the wall and closed before the wind hits, I don't expect anything will happen to it.

 

[BridgeSmith]

Would the doors close off the tunnel, one branch of the tunnel, or close off a room outside the wall of the tunnel?

If it closes off the tunnel, you might need to consider the vacuum pressure at the eye of the tornado.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[JAE]

WARose,
A few thoughts that came to me on this:
1. Check the flood elevation to make sure that the tunnel wouldn't be subject to flooding during an event.
2. Be sure to verify what all that piping is. Having gas lines, water lines, etc. are not allowed, or recommended, in shelters - you can perhaps use automatic shutoffs on the lines per ICC 500.
3. Make sure you have the required area for the occupancy served - i.e. typically 5 sf per person - more if there are wheelchairs, etc.
4. Make sure you have the required egress exits from the tunnel. This is based on the occupant load per the 5 sf/person. You might have to add a second egress out the back of the tunnel.
5. Per ICC 500 you might also need an "emergency escape opening" as an additional exit beyond the normal code exits. See ICC 500-14 section 501. If less than 16 people - not required.
6. Needs a fire extinguisher.
7. Requires venting (see ICC 500)
8. New shelters now require toilets....with an existing tunnel you might get it approved without.
9. Emergency lighting
10. The distance from a door of the building served to the door of the shelter should be less than 1000 ft.

As far as wind - I'd somewhat agree with retired13 that there may not be much - but short of a full wind tunnel test I think you'd either:
a) Use the full force per a 250 mph outside door, or,
b) Talk it over with the building official and see if they'd buy off on 50% or something.

 

[WARose]

[blue](retired13)[/blue]

I don't know how far, but once beyond the turbulence zone near the tunnel entrance (similar to pipe flow), there shouldn't be much force on the wall except minor drag force due to roughness of the wall material.

I have been going through my old fluids book (for the first time in quite a while) trying to figure that one out (i.e. the size of the turbulence zone). Glad I have some confirmation for my theory.

[blue](BridgeSmith)[/blue]

Would the doors close off the tunnel, one branch of the tunnel, or close off a room outside the wall of the tunnel?

The plan is to close off that "T" branch. (Which starts about 50 yards deep into the berm.) The question then becomes: how deep to put those doors into the "T" portion to have a tolerable wind pressure.....and what would that pressure be?

 

[WARose]

Good points JAE. Flooding is impossible where the tunnel is. I need to ask on the piping situation. We'd probably need variances on some issues from the local building code people. (Probably based on the logic that the tunnel beats the heck out of being anywhere else in that plant.)

 

[JAE]

A visit to the building official would be a necessity in my view. We did a number of public school surveys and reviews of current/proposed storm shelters in existing buildings.
In one area of a high school we found a perfect place for most all of the occupants....except the building's main gas line was coming in right over the ceiling.

We worked with the mechanical engineer to provide an auto-shut off at the basement wall where the line came into the building.
I don't know how they determined whether the gas already in the pipes inside the space was acceptable. The fear is that a major storm could break a gas line and leak into the shelter space, creating a dangerous condition.

 

[3DDave]

Tornadoes center around low pressure and the most brutal damage is done by the entrained debris. I doubt a door would be helpful as it would just be a plug to be pulled when the low pressure passed by.

I've seen a video of people underneath overpasses (the kind where it's easy to get between the beams, not merely using them as a roof) as a tornado passed by them and without any other interposing doors or walls, just the blocking by the bridge beams, they were fine. I would not be surprised if the high velocity of the air is offset in large part by the decreased density, so the dynamic pressure isn't what a straight line 250 mph is like.

https://www.youtube.com/watch?v=lHBZylcxIvw

At the end they claim that a stronger tornado would have sucked people out; maybe, but there is no suction, there is a pressure delta and so where is the delta that pushes them coming from? Not like they are in a vacuum cleaner hose blocking the flow.

Definitely don't park a car under there and stay in the car. And don't get under there if there's no access to the gap between the girders.

 

[BridgeSmith]

Given its protected location, a practical solution would be to have the walls/doors solid only up to 7 or 8 feet, with chain link or some other wire mesh above that. The pressure would be near zero for that configuration. I'm not sure how that would go over with the AHJ, since it's unlikely to meet the 'letter of the law' for storm shelters. I would think it would meet the intent of protecting people from the effects of a storm or tornado.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[JAE]

There are doors that are FEMA rated for tornado force winds. We use them all the time in school shelters.
For this tunnel - way back 50 feet beyond the entrance, a FEMA door would be definitely fine.
You can also use offset storm rated entrance walls (forcing you to jog/serpentine into the space) to help avoid projectile debris from going into a shelter without a door, but for your deep tunnel I'm not sure that is really necessary.

 

[r13]

Just small notes. The emergency escape door shall be controlled from inside of the shelter (push to open), and the door shall facing the upwind when in open position (wind to push the door shut).

 

[IRstuff]

One of the door's purposes is also to prevent flying debris from scooting down the tunnel and hurting someone and/or blocking the tunnel.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

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