Fatigue of bracing in buildings
BS5950 states that buildings subject to fluctuating wind loads do not need to be checked for
fatigue but EC3 contains no such statement. Richard Henderson of the SCI considers the issues
and illustrates a fatigue check of wind bracing in a conventional building.
Clause 2.4.3 Fatigue in BS5950-1:2000, a code specifically for
the design of steelwork in buildings, states “Fatigue need not
be considered unless a structure or element is subjected to
numerous significant fluctuations of stress. Stress changes due
to normal fluctuations in wind load need not be considered”.
The ANSI/AISC 360-16 Specification for structural steel buildings
Chapter B clause 11 states “… Fatigue need not be considered …
for the effects of wind loading on typical lateral force-resisting
systems …”. BS EN 1993-1-1 and BS EN 1993-1-9 (Part 1-9) include
no such clause but BS EN 1993-1-1 forms the foundation for
a series of codes for the design of bridges, towers and other
structures. Bridges are routinely checked for fatigue. Other
structures such as chimneys and masts may be subject to windinduced
oscillations and need to be checked for fatigue.
The connections at the ends of wind bracing are often made
using gusset plates, fillet welded to end plates and beam flanges.
Tubular tension/compression bracing members may have bolted
spade-end connections fillet welded to end plates.
Fatigue Strength Curves
An introduction to fatigue design was published in NSC magazine
last year. Part 1-9 clause 7.1 gives the fatigue strength for nominal
stress ranges for a range of details, identified in Tables 8.1 to
8.10. The fatigue strength is defined by a (logΔσR) – (logN) curve
for each detail category as shown in Figure 1. For a constant
amplitude nominal stress range, the curve gives the number of
cycles to failure or endurance. The curve number is the detail
category and is the constant amplitude nominal stress range that
will result in failure after 2 million cycles. The curves change in
slope at N = 5 million cycles. For nominal stress ranges lower than
a certain value known as the cut-off limit ΔσL , fatigue damage is
considered not to occur. The curves are based on the results of
tests on large-scale specimens collected over several decades.
Fatigue damage can be calculated for a given detail using the
relevant fatigue curve from Part 1-9 to determine the number
of cycles to failure Ni for a given stress range i and using Miner’s
summation for fatigue damage Σni/Ni where ni is the number
of occurrences of this stress range over the life of the structure.
The fatigue damage should be less than or equal to 1.0 for the
detail to be acceptable (see Part 1-9, Annex A clause A5). Some
fillet welded details are in the lowest classes of detail identified in
Part 1-9 Table 8.5: either detail category 36* or 40.
BS EN 1991-1-4 Annex B includes a graph of the number of
times in 50 years that a wind gust load equals or exceeds a given
proportion of the once in 50 year gust load, expressed as a
percentage (see Figure 2). This curve is introduced in Annex B for
use in the procedure for determining the structural factor cscd in
wind load calculations. BS EN 1991-1-4 gives no guidance on the
use of the curve for fatigue calculations due to gust loads.
Figure 2: Number of gust loads Ng during a 50 year period
The relationship between the quantities is given as:
= 0.7 ( log(N)) 2 + 17.4 log(N) + 100
This graph provides the spectrum of stress ranges to which a
detail is subjected. An unconsidered examination of the graph
suggests that a load equal to 15% of the once in 50 year load
(ΔS/Sk = 15%) occurs about 5 million times during the 50-year
design life of the building. If the design load results in a stress
equal to yield, a serviceability stress range of 36 MPa (about
355 × (0.15/1.5)) occurs enough times to cause a fatigue failure
in a class 36 joint, for which a constant amplitude stress range of
36 MPa causes failure after 2 million cycles.
Figure 1: Fatigue strength curves 26