Technical
Illustration of fatigue design of
a crane runway beam
26 NSC
January 19
28
As indicated in the technical article1 in the September 2018 issue of New Steel Construction
Richard Henderson of the SCI discusses the fatigue design of crane runway beams with an
illustrative design example.
Crane Loading
The loads on crane runway beams are determined in accordance
with BS EN 1991-32. This code sets out the groups of loads and
dynamic factors to be considered as a single characteristic crane
action. The relevant partial factors are set out in Table A.1 in Annex
A of the code. At ultimate limit state for the design of the crane
and its supporting structures, the characteristic crane action being
considered is combined with simultaneously occurring actions (eg
wind load) in accordance with BS EN 1990. The final ultimate design
loads from the crane end carriage which are supported by the
runway beam can thus be determined.
The groups of loads are identified in Table 2.2 of BS EN 1991-3
and include the actions listed in the table below. Several of the
loads have a dynamic factor associated with them which depend
on the class and function of the crane.
Item Description of load Dynamic factor
1 Self-weight of crane φ1 or φ4
2 Hoist load φ2, φ3 or φ4
3 Acceleration of crane bridge φ5
4 Skewing of crane bridge -
5 Acceleration or braking of crab or
hoist block
-
6 In-service wind -
7 Test load φ6
8 Buffer force φ7
9 Tilting force -
Unfavourable crane actions have a γQ value of 1.35, not the usual
value of 1.5. Fatigue assessment is regarded as a serviceability limit
state with a partial factor of 1.0.
Fatigue Assessment
BS EN 1991-3 provides a simplified approach to designing crane
runway beams (gantry girders) for fatigue loads to comply with
incomplete information during the design stage, when full details
of the crane may not be available. The crane fatigue loads are given
in terms of fatigue damage equivalent loads Qe that are taken as
constant for all crane positions. The fatigue load may be specified
as follows:
Qe = φfat λiQmax,i
where, as stated by the code, Qmax,i is the maximum value of the
characteristic vertical wheel load, i and λi = λ1,i λ2,i is the damage
equivalent factor to make allowance for the relevant standardized
fatigue load spectrum and absolute number of load cycles in
relation to N = 2.0 × 106 cycles. This concept was discussed in
reference 1.
The damage equivalent dynamic impact factor φfat for normal
conditions may be taken as:
and fat,2 =
1 + 1
2
fat,1 =
1 + 2
2
The factors φfat,1 and φfat,2 apply to the self-weight of the crane
and the hoist load respectively.
In BS EN 1991-3, Annex B Table B.1 gives recommendations for
loading classes S in accordance with the type of crane and Table
2.12 gives a single value of λ for each of normal and shear stresses
according to the crane classification. Overhead travelling cranes
are in either S-class S6 or S7 so that, having selected an S class, the
corresponding λ value is determined. (The classes Si correspond
to a stress history parameter s defined in BS EN 13001-13 but the
details are not required for this example).
The method for carrying out the fatigue assessment is set out in
section 9 of BS EN 1993-64. Once the fatigue loads are determined,
the stress ranges (denoted ΔσE,2 ) for the critical details of the crane
runway beam can be calculated. These are the damage equivalent
stress ranges related to 2 million cycles. The fatigue stress range is
multiplied by the partial factor for fatigue loads γFf stated in BS EN
1993-6 section 9.2 which is equal to 1.0. The critical details must be
categorized according to Tables 8.1 to 8.10 in BS EN 1993-1-9 and
the detail category number noted. The category number (denoted
ΔσC ) is the reference value of the fatigue strength at 2 million
cycles. The partial factor for fatigue strength is γMf and is given as
1.1 in the National Annex to BS EN 1993-1-9 for a safe-life fatigue
assessment. The fatigue check involves showing that, for direct
stresses:
Ff E,2
1.0
/C Mf
A similar check is required for fluctuating shear stresses:
Ff E,2
1.0
/C Mf
If both direct and shear stresses are present, a further check is
required.
Example
Consider an EO travelling crane of S-class 6 and hoisting class HC3
supported on 8.0m span runway beams in steel grade S355 which
have laterally restrained compression flanges at 2.0 m centres.
The crane is wholly inside a building and so there are no other
simultaneously occurring actions. The relevant weights of the
crane, the proportion of the weight applied to the end carriage in
the worst case and the resulting maximum loads are:
/Portal_frames#Crane_actions
/Fatigue_design_of_bridges#The_mechanism_of_fatigue