25 January 2005
It is important to note that many, if not most, DoD structures will fall in
the VLLOP and LLOP categories. The required Tie Forces are generally easy to
provide in reinforced concrete and structural steel construction; simple spreadsheet
calculations will be sufficient and little additional design effort will be required.
Developing the required Tie Force capacities in masonry, wood frame and cold-formed
steel structures may require more effort, as many standard connections and member
configurations are not sized to carry these tensile loads. Design and analysis effort will
be required on the part of the engineer to develop connection designs that can provide
the Tie Force capacities for these types of structures. Some guidance is given in
Appendices E and F for connections in wood frame and masonry structures; additional
information on connection design is available from vendors and design code bodies.
Finally, for the purposes of this UFC, it is recognized that the structural
configurations typically employed for both wood frame and cold-formed steel are very
similar, as cold-formed steel construction typically consists of thin wall steel studs and
horizontal members, sheathed with gypsum board or wood-product panels. Thus the
requirements for, and approach to, progressive collapse design for these two materials
are essentially identical.
The Tie Force requirements within this UFC are very similar to those provided
in the British Building Standards for reinforced concrete, structural steel, and masonry
and have only been marginally modified to make clearer some of the connection and
The British Tie Force requirements were developed in response to the Ronan
Point accident in 1968. The steel and reinforced concrete requirements are different in
form and magnitude and appear to have been developed by separate code writing
bodies (as with ACI and ASCE in the United States). The masonry requirements are
almost identical to reinforced concrete, with the exception of the additional requirements
for developing full vertical ties (Table 7-1). Attempts to uncover the processes and logic
by which these requirements were developed were partially successful and, in
discussions with British engineers, it has been noted that engineering judgment was
used for some of the requirements. The results of the background research are
presented in the following sections.
It is noted that the British Tie Force requirements are adopted almost
verbatim in this UFC and it has been assumed that they are directly applicable to US
construction, i.e., that there is sufficient similarity between current British and US
construction practices that the Tie Force requirements can be applied to US
construction. Additional research and analysis are needed to determine if a new or
modified set of Tie Force requirements should be developed. However, the Tie Force
requirements presented in this UFC have been effective for the British over the last 3
decades and are the most prescriptive procedure available for Indirect Design. In lieu of
additional research and analysis, they are deemed to be sufficient for DoD construction.