×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Tornadic Shelter Design Loads

Tornadic Shelter Design Loads

Tornadic Shelter Design Loads

(OP)
I am currently researching design loads to withstand a tornadic event for an upcoming project.  I have found the FEMA 361 document that contains a bulk of the information I have been trying to locate.

However, one issue I am having difficulty in finding information deals with impact load due to a collapsing structure.  My structure is a two story building, but the first level will only be designed as the shelter in an effort to save money.  This assumption would indicate that the upper level is not designed to withstand the extreme tornadic loads, thus giving the potential for collapse of the second level onto the hardened structure below.

Does anyone have any recommendations or references on how to account for the impact loading created by the collapsing structure above?  I am sure this involves impulse-momentum theory to a certain extent, but was hoping to find something simple so I do not have to relearn kinetics.

RE: Tornadic Shelter Design Loads

My guess is that if the first floor can survive a tornado - then a relatively light second floor falling on it won't do much damage.

Design the second floor to blow away and you won't have to worry!!  Probably what will happen anyway....

Toto - we aren't in Kansas anymore!!

RE: Tornadic Shelter Design Loads

(OP)
My initial thought was to design a light second floor, but the owner requests CMU or concrete walls with a brick veneer.  While the roof is light (bar joists and metal deck), I'm concerned with the possibility of the CMU/concrete wall collapsing onto the floor.  This would be ~100 psf plus whatever effect for impact.  Would an impact factor of 2 be reasonable?

RE: Tornadic Shelter Design Loads

Why not design the second floor CMU to survive a 210 mph wind force?  Simple span between the floor and the joists/roof with pressure at 100 psf (+/-).

RE: Tornadic Shelter Design Loads

(OP)
One objective is the cost...the idea behind hardening the first level only was to lighten the roof structure and prevent the use of ballistic resistant windows at the second level.  While hardening the entire structure would lighten the second level structure (reduce floor live load from the tornadic prescribed 100 psf to office live loading), it would require a much heavier roof structure to combat the tornadic prescribed live load of 100 psf in addition to the roof wind pressures of (+100 psf/-200 psf).  For missile resistance, the roof also needs to be a 6" minimum thick slab.

RE: Tornadic Shelter Design Loads

Let the roof fail, just design the walls/joists for horizontal loads from tornado winds.  This negates the walls becoming impact loads on the first level ceiling.

RE: Tornadic Shelter Design Loads

(OP)
If the roof fails, then I would lose the diaphragm action that supports the CMU walls laterally (out-of-plane).  Then the walls would effectively become 16' high cantilevered parapet walls that would have to resist ~175 psf wind pressure.  This is something to consider, but would add significant cost to the wall reinforcing.

RE: Tornadic Shelter Design Loads

Since you are costructing the home in CMU, I would think just designing the entire structure as partially enclosed for the wind load, and move the harded concrete roof to the top of the 2nd floor (or steel trusses and roofing) would not be much difference in cost.

RE: Tornadic Shelter Design Loads

Design a light roof system with the minimum possible connections to cmu wall.  Reinforce the second floor masonry wall with horizontal and vertical reinforcing (not every cell necessarily).  Increase the vertical reinforcing as you move from the roof to second floor so that when it fails it will just be a mangled mess but not actually laying on the second floor.  Not sure how to put numbers to this just an idea.

The real problem sounds like the owner/architect has placed too many constraints on you to be able to arrive at a reasonable solution.  If the owner justs wants to see cmu on the interior maybe you could use light framing with a thin adhered interior that resembles cmu.

Good luck

J  

RE: Tornadic Shelter Design Loads

Why not just follow the FEMA standards for safe areas. You are concerned with a tornado and not just a hurricane. Look to FEMA 320. Your client may feel this is a good option as opposed to total protection from the very, very rarely hit of an F5.

The impact of the collapsing upper floor and still being in the same county during a storm is a very nebulous situation.

Is this a residence or a commercial structure? If you are attempting to design the first floor and everything that can happen to the second floor, complicated by the usual overly conservative, simplying design assumptions, you can easily create a monster. - A tornado covers a very narrow path and F3 to F5 storms are not that common on a probability scale for a single location.

Mass and reinforced conrete masonry/concrete are obviously your best options.

The FEMA suggestions (life safety - not property) can easily be adapted to below ground (basement) or to first floor situations. Frequently, a large walk-in closet (8x10) can serve the purposes and an adjoining bath can easily/economically be an added safe area.

Trying to design for an F5, especially with the simplifying assumptions is just a folly, since the economics and constructability pose huge problem.

I saw a home near Long Beach/Biloxi, Mississipi that withstood the 29' direct hit of storm surge from Katrina (lesser winds, but storm surge). It looked just fine 4 months later. It was on over 1200' ($10,000/ft) of shorefront, so that owner could afford to pay for the design and constructon. - I hope you have that type of client.

RE: Tornadic Shelter Design Loads

(OP)
The project is for a utility company.  The floor to be hardenend contains their control room and adjacent electrical/HVAC rooms to keep the control room running.  Even without the hardened structure request, they are requesting this to be classified as an essential facility, which would dictate an importance factor of 1.15 under normal wind loading (90 mph - 3 second gust).  The upper/"disposable" level contains management offices, conference rooms, etc.



 

RE: Tornadic Shelter Design Loads

Section 6.3 of FEMA 361 is entitled, "Large Falling Debris". I beleive it has the information you are looking for.

RE: Tornadic Shelter Design Loads

hopefully there'll be a meeting going on when the tornado hits, and we'll get rid of some "disposable" management too.

RE: Tornadic Shelter Design Loads

btw, when is a "tornadic event" not a tornado ?

 

RE: Tornadic Shelter Design Loads

I of 1.15 at 90 mph is no where near the forces a tornado can produce!!

Look up the F factors for tornadoes - but 150 mph is quite common if I remember correctly.  I think an F5 goes into the range of almost 300 mph!!

RE: Tornadic Shelter Design Loads

Isnt tornado alley clasified as a special wind region?

RE: Tornadic Shelter Design Loads

Just because a structure is in "tornado alley" does not mean every structure has to be designed for a tornado. - That difference between 100 mph and 250 mph is not realiatic and prevents most construction and is the reason codes are life safety oriented instead of "stand forever, no matter what happens".

The FEMA suggestions (FEMA 320) I quoted earlier are regarding life safety are are intended to provide a safe haven that is immunue to the penetration od projectiles.

Tornados are a very brief weather condition that affects a very, very small area. The general philospohy is that an EF5 tornado with 250 mph winds will almost detroy any normally usable structure and that there must be a "hardened" safety area the is close and conventient enough to save lives.

A tornado is not at all like a hurricane - much much stronger, much faster, no advanced warning and unexpected.

Falling debris is minor of little consequence if you can evacuate. The higher level of facility protection is a management decision predicated on the value and importance of the facilit, while life-safety is not a management decision. A straw can pentrate a body (if it survives) much easier than the 2x4 that it does. - You do not see wood frame structures (even with Kevlar layers) for a good reason. The FEMA tests at Texas Tech have shown the folly of the concept.

Dick

RE: Tornadic Shelter Design Loads

I am also having to look at designing a storm shelter in a daycare facility.  

TTUengr51, did you have any luck finding how to estimate the impact loads on the roof?  FEMA 361 gives a couple of roof sections and describes how they perform against penetration, but I don't find anything the tells you what loads you apply to the support structure of that roof from the impact load.

RE: Tornadic Shelter Design Loads

From someone who lives and works in Kansas (near Greensburg). The main concern is wind-borne debris.  Keep in mind that tornadoes have the tendency to pick large objects and deposit them far from their original locations.  If your walls are CMU then it is very likely that the wall will separate into individual blocks before impact...depending on how you reinforce the wall.  If you use the mentioned FEMA guidelines for storm shelters you should be ok.   

RE: Tornadic Shelter Design Loads

The storm shelter that I am looking at is inside of a building, below the regular roof structure, and directly below the 5800 lb. air handling unit on the roof.  In an extreme event, the roof and unit will collapse onto the roof of the shelter.  It is possible that the unit will be picked up and dropped on the shelter also.  The unit impact is what I am concerned about.  The FEMA tests look at deck penetration, but I am thinking about the beam load at the wall and the wall capacity to handle it.
 

RE: Tornadic Shelter Design Loads

3 ft thick walls?

 

RE: Tornadic Shelter Design Loads

The ends of joists welded to embed plates in top bond beam and three rows of bridging in the same axis direction of the deck can brace the top of the CMU wall against cantilever type moments, (rather than simple span).  The joists with a positive connection to air handling unit will negate any impact from falling HVAC.

RE: Tornadic Shelter Design Loads

If you are designing for a day care center, the main criteria is life safety. Tornadoes are very short duration, but much more violent that hurricanes or other storms. If you have a lightweight structure (steel frame, wood), you should expect total destruction for a EF 3 or better (worse?).

As a minimum, you should assume the 6000# unit drops on the top of the shelfter in addition to other building debris. If the shelter is below grade, the people may be lucky and have a big enough wind (EF 3 to 5) to remove everything, but there are no guarantees with life safety.

Depending on the room size, even the FEMA 320 suggestions may be adequate, so reinforced block or reinforced concrete (8" thick and steel at 8" o.c.) based on a 12'-2x4 shot at 150 mph should take care of the debris danger. The structural design will depend on the building/cell configuration and dimensions.

The doors and floor/wall/roof anchorage are critical. The doors should resist any pentration and should open into the safe area to eliminate people being trapped with debris.

RE: Tornadic Shelter Design Loads

Way out of my league but having been an observer several years ago on the hardening of some control rooms and electrical facilities by using a ASCE blast resistant design.
After these structures were built and series of storms the question of hurricanes and tornadoes came up as far as the existing blast resistant design would perform.
The blast resistant designed buildings have reinforced concrete walls and roofs. I've been trying to find out what the roof design load was as in the event of an explosion there would be a lot of flying debris.
Evaluation of the current design revealed that the buildings would stand a cat 5 hurricane and an excellant chance at an F5 tornado.
Still recalling from memory, the roof on these buildings was a relatively simple design that caused no discussion in the design meetings about cost.

The point of my post is would this type design fit your second floor design not as the roof but floor?


 

RE: Tornadic Shelter Design Loads

(OP)
To recap my situation and research...

I'm utilizing a composite steel framed floor (shelter roof) with an 8" concrete slab on 2" composite decking, which gives me the recommended uniform thickness of 6" as described by the FEMA 361 missile impact testing.  

To design the floor/shelter roof, I am using a impact load of around 220 psf, which accounts for the roof weight (20 psf) and wall weight (8" CMU w/ 4" brick veneer ~ 90 psf) multiplied by an impact factor of 2.  I determined this factor by using some impulse momentum principles.  In addition to the impact load, I have additional roof live load of 100 psf as designated by FEMA 361.  

 

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources