Advisory Desk / Codes and Standards
NSC 29
May 19
AD 430:
Wind load on unclad frames
The purpose of this note is to correct errors
in BRE Special Digest SD5 which lead to the
prediction of significantly higher wind loads on
unclad frames than were calculated using the
report which SD5 superseded.
BRE published Special Digest SD5 in July 2004.
The document was produced principally because
at the time, the current guidance for determining
wind loads on frames, lattice structures and
individual members was based on the BS code
of practice CP3 Chapter V: Part 2 which had been
withdrawn in October 2001. SD5 is based on
BS 6399-2 and includes guidance on determining
loads on individual members and lattice
structures. It also includes a section on unclad
building frames which is based on and intended
to supersede BRE report BR173, Design guide for
wind loads on unclad framed building structures
during construction.
BR173 considers a series of identical parallel
frames of overall width W at spacing S. The
parameter S is used to select the appropriate
normal force coefficient CD according to the ratio
W/S and the total solidity ratio denoted φ. In a
given direction, φ is presented in BR173 as the
sum of the horizontal and vertical solidity ratios:
φ = φv + φh. In the direction perpendicular to the
secondary beams, the horizontal solidity ratio
used is the equivalent solidity ratio which allows
for all the secondary beams in a bay denoted
φ = φv + φh* (see item iii in the design example in
BR173 para. 4.2.2). In SD5, the total solidity ratio
is erroneously given as φ = φv + φh + φh,s ie the
equivalent horizontal solidity ratio φh,s is added
to, instead of substituted for the horizontal
solidity ratio φh. The total solidity ratio in this
direction should be given in SD5 as φ = φv + φh,s .
The spacing of the secondary beams is used in
the determination of the equivalent solidity ratio
for secondary beams. In BR173, this parameter is
also denoted S and is likely to be different from
the frame spacing but unfortunately, SD5 does
not differentiate between the two parameters.
In SD5, the relevant equation is no. 11:
φh,s = (φ1 + φ2)φh where φ2 = (n – 1)(S/d – 7.5)/25.
According to BR173, in the expression for φ2 the
parameter S is the secondary beam spacing not
the frame spacing. The equivalent expression in
BR173 is equation (4): f2 = (Nbb – 1)(S/b – 7.5)/25.
The secondary beam spacing should be used in
the determination of φ2.
The approach to determining the wind
load on unclad structures (lattice structures,
frames and individual members) in SD5
(corrected as indicated) can also be used with
BS EN 1991-1-4 and its UK National annex as the
design pressures have identical target reliability
to BS 6399-2.
Contact: Richard Henderson
Tel: 01344 636555
Email: advisory@steel-sci.com
New and revised codes & standards
From BSI Updates April 2019
NEW WORK STARTED
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