Appendix E: Seismic requirements for the design of tower, portal and high pedestal cranes

E1 Design

The computation of seismic load combinations are detailed as follows:

[1] Notation

C_d Seismic design coefficient for the appropriate seismic zone and fundamental period of the crane from Figure E1.

Note: If a crane is to be used in different seismic zones, then it must be designed for the highest intensity zone in which it will operate.

Figure E1: Seismic design coefficient (rigid and intermediate subsoils)

E Earthquake loads or their related internal moments.

Note: Lateral forces on the suspended load may be neglected in determining E, viz. Wt = L1.

L1 Dead loads due to dead weight.

L2 Live loads including the hook load.

S Structural type factor:

Note: S is to be determined separately for each direction under consideration:

1. Diagonal bracing members capable of plastic deformation in tension only,
   S = 2.5 or by special study.
2. Diagonal bracing capable of plastic deformation in both tension and compression,
   S = 1.6 or by special study,
3. Structures where the yielding mechanism under the action of lateral seismic forces is plastic hinge rotation,
   S = 1.0.

T Fundamental period of vibration of the structure in the direction under consideration.

V Total horizontal seismic force or shear at the base in the direction under consideration.

Note: The structure shall be designed to withstand a total horizontal seismic force, V = CdS (Wt) in each direction under consideration.

Wt Total reduced gravity load above the level of lateral ground restraint.

[2] Load and load combinations

1.0 L1 + 0.65 L2 + E
0.9 L1 + E

[3] Moments

1. P-delta moments are the sum of the products of the vertical weights on the crane and the corresponding horizontal seismic deflections.
2. In calculating P-delta moments, the following conditions shall apply:
   1. The deflections shall be assumed to be four times those calculated due to the combination of seismic force and P-delta moments.
   2. For all members, the loads or stresses resulting from P-delta moments calculated on this basis shall be no greater than 0.2 times the corresponding strength of the member.
3. Torsional moments need not be calculated for tower cranes.

[4] Explanatory notes

1. Structural type (S) factor:

Is intended to reflect the potential seismic performances of different structural systems, taking primarily into account the ability of the structural type concerned to dissipate energy in a number of deformation cycles into the inelastic range.

Frames that utilise diagonal members acting as ties, which are capable of plastic deformation in tension only develop load displacement hysteresis loops of a very pinched nature, upon cyclic loading beyond yield level, because of the inability of the diagonals to sustain a significant compressive load. These pinched hysteresis loops result in a much lower level of energy dissipation than that provided by diagonals capable of plastic deformation in both compression and tension, where more stable hysteresis loops are formed. This results, in turn, in a higher S factor. In design of diagonally braced structures, care is necessary to avoid undesirable effects such as lateral buckling of diagonal struts or columns or chord member hinge mechanisms.

Where the yielding mechanism is of a flexural nature, involving plastic hinge rotation, buckling of compression flanges must be avoided.

2. P-delta moments:

A P-delta moment is the bending moment that is developed when the point of application of a vertical gravity load is moved sideways by horizontal seismic deflections. The value of the P-delta moment is the product of the vertical load P and the corresponding horizontal movement of its point of application.

In flexible frames responding into the inelastic range, delta may reach large values, and P-delta moments can make up a large portion of the loading on the structure. The ultimate objective of these P-delta provisions is to provide an adequate margin of safety against the possibility of residual inelastic deflections tending to accumulate in one direction over a series of successive cycles of response until the total deflection becomes great enough to cause collapse. The maximum credible value for the accumulation of residual deflections has been assessed as 10 times the deflection caused by the design loading. The basis used in the code has been to limit the loads resulting from the Pdelta moment in this situation to no more than half the strength of the members affected. Hence, for deflections assumed to be four times those given by the design loading, the loads resulting from the corresponding P-delta moments require to be no greater than 0.2 times the respective strengths of the loaded members.

3. Seismic design coefficient (C_d):

Shall be taken from Figure E1 for the highest seismic level/zone in which the crane will operate. It is expected that the coefficient zone factor = 1.2 will apply for all except a few cranes designed for a specific function and permanent location in a zone of lower seismic level. The values given allow for the following:

1. Lower design seismic forces than would be applied to buildings of the same period, for the following reasons:
   • Most cranes spend only a portion of their life in an erected condition, and they also have a shorter total life than buildings. This has the effect of reducing the level of earthquake intensity that has a given probability of occurrence during the erected life of a crane below the level that has the same probability of occurrence during the life of a building.
   • The risk to life resulting from collapse of a crane would be less than that resulting from collapse of an occupied building.
2. A material factor of 0.8, assuming steel construction, which is incorporated in the figure.

[5] Design method

Structures will experience response accelerations greater than the values given by the design forces. This means that structures of cranes will have to be designed to withstand a series of cycles of response involving deflections substantially greater than yield deflection. The magnitude of the deflections to be provided for should be taken as a minimum of 5 divided by S.

Relative member strengths should be proportioned so that the inelastic yielding takes place in members that can develop a high level of ductility. In proportioning the relative strengths, account should be taken of the margin by which the actual strength of any member can exceed the specified minimum strength.

The foregoing outlines the minimum acceptable seismic design requirements. Higher values may be specified as required.

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