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RE: Low LOP Progressive Collapse Horizontal Ties

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Eric,
I am not particularly fond of some of the methodology proposed in the examples either. The examples were predicated on some computer modeling work by ARA about a year ago or so. The provisions were developed by the US Army Corps of Engineers Protective Design Center in Omaha. Contact Ed Conrath about these provisions.

The concept is to provide a continuity of load path for the tension developed as the structure deforms and forms a catenary after a large section of wall is removed by a blast. It is intended to accommodate the normal methods of platform wood frame construction. There are other (probably better) methods that can be used to provide that continuty.

The bottom of the walls is not addressed because there is no good way to resist the out of plane forces. The only thing that can be done is to assume that the blast takes out the wall entirely, then the remaining wall has to resist the structure in its damaged state.

Bomb blast pressures at the range considered can not be resisted by conventional construction. But the pressures drop at a cube of the distance. Therefor the damage to conventional construction is highly localized because of the standoff provided by site work. That is why we can use conventional construction with some modifications to accommodate progressive collapse. But trying to resist the forces from a bomb blast (hardening) is futile.

Regards,
Harold Sprague


From: "Eric Tompos" <etompos(--nospam--at)ntainc.com>
Reply-To: <seaint(--nospam--at)seaint.org>
To: <seaint(--nospam--at)seaint.org>
Subject: Low LOP Progressive Collapse Horizontal Ties
Date: Thu, 26 Jan 2006 16:20:35 -0500

Can someone please clarify the requirements for 'horizontal ties to external
walls and columns' in UFC 4-023-03 (Progressive Collapse).  The examples in
the Appendix don't seem to match the requirements.

Section 7-2.6 requires horizontal ties at "each external column and, if the
peripheral tie is NOT located within the wall."  It goes on to require
horizontal ties "at each floor and roof level"

Section 7-2.4 requires internal ties "must be anchored to peripheral ties"

The result, in a bearing wall if the peripheral tie is in the wall, only
internal ties are required (no horizontal ties), as shown in in the example
in Appendix F.  However, the example only addresses the top of an exterior
wall.  At the bottom of the wall there is no peripheral tie in the wall, so
based on Section 7-2.6 a horizontal tie is required at the floor level.  In
all of the design examples I have seen the bottom of the wall is not
addressed.  Am I misreading this requirement?  Is a horizontal tie required
at both the top and bottom of each exterior bearing wall?

And, if it is to be "anchored or tied horizontally" how is this being
achieved with a strap, as shown in the examples, when a strap would provide
no resistance in the horizontal direction?

Thanks in advance,
Eric Tompos, PE

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