UFC 4-023-03
25 January 2005
B-3.4
Wood Frame Tie Forces.
The British do not provide explicit Tie Force requirements for wood frame
construction. However, the British provide some guidance for timber construction in
BRE 2003, which states that: "The regulations suggest high tie forces to take account of
the typical wide spacing of post and beam structures using steel or concrete materials.
While these tie forces could be designed to spread through the timber frame elements it
is not considered a practical option, although not impossible." BRE 2003 goes on to
state that the Alternate Path method "...provides the appropriate route for platform
timber frame structures" and guidance on the length of the removed members is
provided. Additional details on the AP method for wood are provided later in this
appendix and in Appendix E.
In this UFC, Tie Force requirements for wood frame (and cold-formed steel)
are specified in a similar manner to those for reinforced concrete and masonry, since all
four types of structures rely on load-bearing walls, posts, and columns and share a
number of similar connection and floor configurations. The differences are in the values
for the upper limit of Ft (the "Basic Strength"), the constants in the equation defining Ft
as a function of the number of stories, and the scaling values for the internal tie strength
in Section 7-2.4.2. These are based on a similar analysis as shown in Section B-3.1.1.1
and Figure B-1. In this case, typical values for American light frame wood construction
were used: Dead Load = 0.72 kN/m2 (15 psf), Live Load = 2.39 kN/m2 (50 psf), and
span A = 4.6 m (15 ft), resulting in an upper limit of Ft = 21.9 in SI units and 4.92 in
English units. Note that the constants in the equation for the Basic Strength as a
function of the number of stories have been scaled from that for concrete and masonry,
to give the correct upper limit.
Full scale testing by Building Research Establishment Ltd. (BRE) and TRADA
in the U.K. showed exceptional performance of a six-storey wood frame building. The
overall testing of wood frame construction was called Timber Frame 2000. The testing
included an evaluation of the building's resistance to disproportionate collapse as
required by U.K. building regulations. The testing for disproportionate collapse included
the removal of a section of load-bearing wall in the corner of a perimeter wall and the
removal of a section of interior load-bearing wall. In both cases, the deflections were
limited and the building showed excellent resistance to disproportionate collapse.
Although differences may exist between typical wood frame construction in the U.K. and
the U.S., this testing is still the most relevant data available on the performance of wood
frame structures with respect to disproportionate collapse. For this reason, the
experience gained in the U.K. is used heavily in this UFC. Design and best practice
guidance developed based on the results of this test entitled, "Multi-storey timber frame
practices guide indicates the bridging approach is the "appropriate route for platform
timber frame structures" and provides guidance on the length of notional member to be
removed.
B-12
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