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RE: Glazing Blast Loads

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The ASTM's and UFC provide a simplified method to use linear elastic design.
The actual blast pressures you get from the charges and standoff distances in
the UFC 04-010-01 are much much higher than the equivalent 3 second pressures
the ASTM gives you.  The equivalent loads in the ASTM empirically provide
laminated glass that is thick enough for the blast loads based on
observations where the glazing goes into membrane type behaviour and doesn't
become a projectile into the room.  The glazing ASTM is for glass exhibiting
linear behaviour. In reality the framing goes plastic and the connections
need to be strong as the framing .

The way the framing would have to be checked without the simplified methods
is with a nonlinear dynamic analysis with numerical methods.  Doing this you
can account for how member vibrates and the duration of blast pressures. You
check to see the ductility and end rotations are within response limits
assuming the member goes plastic during loading.  It requires specialized
software that is not available to the public and normally requires a couple
of weeks of training to use correctly.

The UFC wants you to use the minimum standoff distance because based on the
"level of protection" the standoff distance could be reduced for future
construction to the minimum standoff distance.  This provision means that for
a lower level of protection the blast loads required for design can actually
be higher since the standoff distances are less.

The UFC simplified method provides conservative designs.  




-----Original Message-----
From: William Haynes [mailto:gtg740p(--nospam--at)gmail.com] 
Sent: Friday, November 14, 2008 3:54 PM
To: seaint(--nospam--at)seaint.org
Subject: Glazing Blast Loads

For blast loads on glazing per UFC 4-010-01, what is the load that the
connections and glass frames have to take?

In this Structure Mag article it says:

"Framing members and connections are designed for a service load equal to two
times the resistance of the glazing"

http://www.structuremag.org/article.aspx?articleID=760

1. There is a huge difference in designing for 2 times the resistance of the
glazing as compared to what ASTM F2248 says:

 "This practice assumes the framing system supporting the blast resistant
glazing shall attach mechanically to the structural framing system with
fasteners that will resist forces generated by a uniform load acting on the
blast resistant glazing that has a magnitude AT LEAST 2 TIMES THE MAGNITUDE
OF THE 3-SECOND EQUIVALENT DESIGN LOAD AS DETERMINED HEREIN." ("HEREIN" lies
loads that are based on standoff distances and amount of explosive, not on
the resistance of the glazing system)

2. Also, the UFC says on page B-15: 

"The actual design load is dictated by the glass type and thickness
determined by ASTM E1300" ( I take this to mean the connections should equal
at least 1.0 times the glazing resistance?) 

and 

"The connection design load will be determined in accordance with ASTM F 2248
based on the applicable explosive weight at the actual standoff distance at
which the window is sited, but not greater than the conventional construction
standoff distance."

http://www.wbdg.org/ccb/DOD/UFC/ufc_4_010_01.pdf


3. ASTM E1300 has nothing about connection design that I can find.

So where is 2 times the resistance coming from? 

W. Haynes




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