Crane Girder Analysis and Design Methods: A Comparison of BS 5950 and Eurocodes
Crane Girder Design To Bs5950 Pdf
Crane girders are structural beams that support overhead cranes in industrial or storage buildings. They are subjected to complex loading conditions that require careful design and detailing. In this article, we will discuss the main aspects of crane girder design according to BS 5950, which is a British standard for structural steelwork.
Crane Girder Design To Bs5950 Pdf
A crane girder is a beam that carries an overhead travelling crane along its span. The crane consists of a bridge, which spans across the gantry girders, and a crab, which moves along the bridge and lifts or moves loads. The crane girder has to resist vertical loads from the weight of the bridge, crab and load, horizontal loads from the braking or acceleration of the crab or load, and torsional loads from the eccentricity or skewing of the load. The crane girder also has to withstand impact, fatigue and dynamic effects due to the varying and moving nature of the crane loads.
The design of crane girders involves selecting a suitable section type and size, calculating the section properties, checking the strength, stability and serviceability criteria, and detailing the connections, stiffeners and bearings. The design also has to consider the durability, fire resistance and corrosion protection of the crane girders.
BS 5950 is a British standard that provides rules and guidance for the design of structural steelwork. It covers various aspects such as materials, fabrication, erection, loading, analysis, design and detailing. Part 1 of BS 5950 deals with general provisions and buildings, while Part 6 deals with specific provisions for crane supporting structures. In this article, we will focus on Part 6 of BS 5950, which applies to gantry girders supporting overhead travelling cranes.
Crane girder loading
The loading on a crane girder consists of three components: vertical load (V), horizontal load (H) and torsional moment (T). These loads are applied at the top flange level (or above) of the crane girder by the wheels of the bridge or crab. The magnitude and direction of these loads depend on various factors such as:
The capacity and span of the crane
The weight of the bridge, crab and load
The spacing and arrangement of the wheels
The position and movement of the crab and load
The braking and acceleration of the crab or load
The skewing and eccentricity of the load
The vertical load (V) is the sum of the dead load (D), which is the weight of the bridge and crab, and the live load (L), which is the weight of the load. The vertical load can be calculated as:
V = D + L
The horizontal load (H) is the result of the transverse force (Ft), which is due to the braking or acceleration of the crab or load, and the longitudinal force (Fl), which is due to the dragging or pushing of the load. The horizontal load can be calculated as:
H = Ft + Fl
The transverse force (Ft) can be estimated as a percentage of the vertical load, depending on the type and speed of the crane. For example, BS 5950 suggests that Ft can be taken as 10% of V for electrically operated cranes with a speed of up to 90 m/min, and 15% of V for cranes with a speed of more than 90 m/min.
The longitudinal force (Fl) can be estimated as a percentage of the live load, depending on the friction coefficient between the load and the floor. For example, BS 5950 suggests that Fl can be taken as 5% of L for smooth floors, and 10% of L for rough floors.
The torsional moment (T) is the result of the eccentricity (e) or skewing (s) of the load with respect to the shear centre of the crane girder section. The torsional moment can be calculated as:
T = eV + sH
The eccentricity (e) is the horizontal distance between the centre of gravity of the load and the shear centre of the crane girder section. The eccentricity can be caused by uneven distribution or shifting of the load.
The skewing (s) is the angle between the longitudinal axis of the crane girder and the line joining the centres of two wheels on opposite sides of the crane girder. The skewing can be caused by misalignment or rotation of the bridge or crab.
In addition to these static loads, crane girders are also subjected to dynamic effects due to impact, fatigue and vibration. These effects can increase the stress and deflection in crane girders significantly, and should be accounted for by applying appropriate factors to the static loads. For example, BS 5950 suggests that an impact factor of 1.25 can be applied to V, H and T for electrically operated cranes, and an impact factor of 1.5 can be applied for manually operated cranes.
Crane girder section properties
The section properties of a crane girder determine its resistance to bending, shear and torsion. The section properties also affect its stiffness, stability and serviceability. The section properties depend on the type, size and shape of the crane girder section.
The common types of crane girder sections are:
Rolled I-sections: These are standard sections that are readily available and economical. They have high bending resistance but low torsional resistance. They are suitable for light to medium duty cranes with short to medium spans.
Built-up I-sections: These are fabricated sections that consist of a web plate and two flange plates welded together. They have high bending and torsional resistance but require more fabrication and welding. They are suitable for medium to heavy duty cranes with medium to long spans.
Mono-symmetric sections: These are sections that have different top and bottom flanges, such as a channel or a plate welded to a rolled I-section. They have high torsional resistance but low lateral-torsional buckling resistance. They are suitable for cranes with high horizontal loads or torsional moments.
Box sections: These are hollow sections that consist of four plates welded together. They have high bending, shear and torsional resistance but require more fabrication and welding. They are suitable for heavy duty cranes with long spans or special requirements.
The effective section properties of a crane girder depend on various factors such as:
The distribution and location of loads on the crane girder
The interaction between bending, shear and torsion in the crane girder
The effect of shear lag, warping and torsional stiffness in the crane girder