Approved Code of Practice for Cranes
Appendix E: Stability requirements for the design of tower, portal and high pedestal cranes under seismic loading
E1 Design
(1) Application
Every crane to which this code applies shall be designed to be stable under seismic loadings for the following load combinations:
1.0 L1 + 0.665 L2 + E
0.9 L1 + E
Where:
L1 = dead loads due to dead weight
L2 = live loads including hook load and shall be taken as that which causes the maximum tipping moment
E = earthquake loads calculated in accordance with Appendix D of this approved code of practice.
(2) Procedure
(a) Divide the crane masses into a convenient number of sub masses and establish the centre of gravity of each of these.
(b) Calculate the total moment due to dead weight of the sub masses including the effects of the deflections due to these dead weights.
Note: Remember not to neglect the twisting moment at the top of the tower in consequence of the out-of-balance moments due to the masses of the jib, counter jib, counterweight, load (if applicable), ropes, pendants, etc.
(c) Calculate the total overturning moment due to the individual seismic forces acting at the centre of gravity of each of the sub masses.
(d) Calculate the deflections of each of the sub masses due to seismic loadings and compute the total moment due to ten times these deflections (i.e. ten times the P-delta moment due to seismic loads).
(e) Add (b), (c) and (d) to obtain the total overturning moment tending to tip the crane.
(f) Using the moment obtained in (e) against the righting moment due to self-weight and stabilising ballast at the crane base, determine whether the crane is stable.
(g) A satisfactory degree of design stability under seismic loadings is achieved when the downward force due to the total mass of the crane and its stabilising ballast exceeds the uplifting force by 20 per cent.