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Fatigue design
Illustration of fatigue design of
a crane runway beam
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:
fat,1 =
4 NSC
and fat,2 =
1 + 1
2
Technical Digest 2019
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:
Item Load (kN) Proportion
of load
Load on end
carriage (kN)
End carriage and bridge (Qc) 164 50% 82
Crab (Qc) 36 90% 33
Payload (Qh) 300 90% 270
For the purpose of this example, consider load group 1 from Table 2.2 of
BS EN 1991-3:
φ1 Qc + φ2 Qh + φ5 (HL + HT)
where HL and HT are caused by acceleration or deceleration of the crane
bridge and for simplicity will not be considered further. From Table 2.4 of
BS EN 1991-3, the upper-bound value of φ1 = 1.1 and the value of φ2 is given by:
φ2 = φ(2,min) + β2 vh