One remarkable aspect of the 7.4-magnitude earthquake that devastated parts of southern Turkey and Syria in February this year was that while the Port of Iskenderun was badly damaged by fire and container stacks collapsed like Jenga bricks, the cranes stayed bolt upright. Iskenderun is located about 100 km from the epicentre of the earthquake, and a hospital in the town did collapse.

It is not known whether the STS cranes in the port are fitted with seismic defences, but the point arising, according to leading US-based structural engineering and crane consulting firm Casper, Phillips & Associates, Inc (CP&A), is that the forces acting on the cranes were in the longitudinal direction and not, crucially, in the lateral, or trolley traverse, direction.

Mike Zhang, vice president of CP&A, explained that if an earthquake is within ±30 degrees of parallel to the long travel direction, the cranes could withstand even  greater forces than the 7.4-scale of the Turkey earthquake. The circumstances in Iskenderun were very different to the Kobe earthquake of 1995, where the port cranes  were destroyed due to liquefaction of the terminal. Here, the crane’s landside and waterside legs spread apart, causing yielding and buckling, leading to collapse.

One recent development from CP&A is its BASIS seismic protection system for STS cranes. As previously reported, this is installed on two Dalian HHI container cranes supplied to another port in Turkey, Asyaport, around two years ago (see WorldCargo News, September 2021, p26).

Explaining the development of BASIS, Zhang said: “We ran multiple major seismic records including the Kobe earthquake in our Non-Linear Time History Analysis [NLTHA]. We ran NLTHA simultaneously in three directions – two orthogonal horizontal directions and one vertical direction – for each seismic record.

“According to our research, the seismic energy transferred to the crane in the gantry direction is limited by the maximum sliding friction between crane wheel and rail, or due to the flexible stiffness of the crane in the gantry direction.” 

If a major earthquake hits the STS crane in the direction of trolley travel, and an effective seismic protection system is not installed, the crane may suffer severe structural damage. In the trolley direction, the crane is much stiffer and can gain seismic energy from ground motion during a major earthquake event. There is a BASIS design for isolation in the gantry travel direction, but CP&A normally recommends installing the system only in the trolley direction.

Helpful slippage

Amplifying this point, CP&A’s Richie Phillips added: “When we did a study for Asyaport, the crane wheels actually slip on the rail, which limits the earthquake loads from that direction. The wheels slipping isn’t that much different to a friction damper, if you think about it, The main advantage of the friction damper is that the designer can specify the slip force.”

Phillips continued: “We estimated that the BASIS system would lower the loads [base shear] by just 5%-10% in the gantry direction, so it really wasn’t worth the cost. Some cranes may still benefit from a damper in the gantry travel direction. For example, a crane may have a rail clamp brake, or perhaps a shiploader with an attached roller system may benefit from a gantry direction damper.”

So far, Asyaport is the only firm customer for BASIS, but there is an ongoing dialogue with several leading terminal operators, and feasibility studies and conceptual designs have been drawn up for retrofits in a leading port.

Customer-led

As previously reported, the other STS cranes in Asyaport were built by the Mitsubishi-Anupam joint venture and, for seismic protection, are fitted with a Mitsubishi hydraulic isolation system acting on the sill beams. However, crane operators complained that the friction dampers were slipping during normal operations.

The BASIS damper does not slip under normal operations, or when the trolley is brought to an emergency stop, so the structure maintains the same dynamic and static stiffness as a crane without the seismic device. If the port is also in a hurricane zone and the tie-downs have to be engaged, the seismic system can be locked to prevent the dampers from slipping.

As Phillips stated, not all cranes require robust seismic design. CP&A has, for example, written several STS crane specifications for Georgia Ports Authority in the US, whose cranes are in a high wind (hurricane) zone, but do not require any special seismic systems.

The problem is that many cranes that need seismic protection do not have it. This is becoming more of an issue as cranes get larger. Historically, STS cranes have been designed for 0.2G static horizontal acceleration in major seismic areas. However, cranes built to handle large ships can be subjected to more than 0.6G lateral acceleration in the trolley direction, even during a moderate earthquake.

The current industry standard for seismic design is to prevent cranes from collapsing during large earthquakes, but Phillips views this perspective as too narrow. The crane may be severely  damaged and out of service for months, unable to help with vital disaster relief. Zhang said that BASIS can handle up to 540mm spread between landside and waterside legs, and this claim has been peer-reviewed and tested.

BASIS pin

CP&A has also developed a special BASIS stowage pin assembly that can accommodate transversal crane movement during an earthquake while still preventing sliding during a hurricane.

The job of a stowage pin is to prevent wheel slippage and potential ‘runaway’ events. However, because it is typically attached to the ill beam, when engaged, it also locks trolley travel direction. This means that in an earthquake, considerable energy will be transferred to the upper structure if the earthquake hits the crane in or near trolley travel direction. 

Read this item in full
This complete item is approximately 980 words in length, and appeared in the October 2023 issue of WorldCargo News, on page 21. To access this issue download the PDF here