Designing storage systems to cope with earthquakes

Dr Murray Clarke comments on the performance of storage systems during seismic events, including the recent devastating earthquakes in New Zealand.

[Image right: The ColbyRACK range of storage equipment, which incorporates spine bracing, chemical anchors and special seismic baseplates to reduce damage.]

Designing rack structures to cope with the unpredictable forces generated during an earthquake requires a combination of detailed engineering, sophisticated computer analysis, and a degree of crystal ball gazing – on the part of the engineer – into how the storage system may be utilised over its lifetime.

When structural engineers design a building, they can make reasonable assumptions, guided by design standards, about the likely loadings on the structure during day-to-day use. Warehouse storage systems, on the other hand, could theoretically be empty one day, and full the next. The total load, where loads are located and the height of racks all have a significant impact on the performance of a storage system during a seismic event. 

What happens to storage systems during earthquakes

In non-seismic areas, storage systems are designed to support mostly vertical loads – horizontal loads typically account for only around 0.5% of the vertical loads, making them relatively insignificant.

However, when an earthquake occurs, ground acceleration can increase the horizontal loadings on storage systems by a factor of 10 or more.

The storage system rack frames and beams respond by swaying to and fro, and the total mass within the racks – the stored pallets – begins to move in relation to the ground, creating inertial cross-aisle (transverse) and down-aisle (longitudinal) forces within the rack structure.

The inertial forces acting in the cross-aisle direction will cause the rack to sway, increasing compression in one upright and decreasing the load on the other. The greater the horizontal seismic inertial forces acting on the storage system, the higher the compression and uplift forces will be. These uplift forces result in the racks attempting to pull the uprights’ baseplates free of the ground anchors, which secure them to the floor.

How we design racks to cope with seismic forces

The need to accommodate seismic loads may lead to the use of bigger and stronger uprights and beams in the racking system. Alternatively, some suppliers make use of vertical and horizontal bracing to stabilise the rack in the longitudinal direction. 

At Dematic, we avoid the use of down-aisle bracing whenever possible, preferring instead to opt for the use of “standard” racking uprights and beams, increased in size as necessary to accommodate the seismic loads. 

The use of spine bracing at the rear of rack frames can be problematic since the bracing is prone to damage when putting away and retrieving pallets, possibly reducing its effectiveness under load.

Dematic also employs chemical anchors and special seismic baseplates, designed to behave in a predictable manner in the presence of uplift forces.

Seismic design process

In recent years, Dematic has designed and implemented over a dozen major ColbyRACK pallet storage systems for a diverse range of customers in New Zealand including Progressive, Foodstuffs, Ezibuy, Brand Lines, Provet, Officemax and The Just Group, with all of the installations located in seismically active areas including Auckland, Christchurch and Palmerston North.

Dematic’s storage system designs for seismic areas are proof-checked by specialist local consulting engineers, and follow the four-step PS1-PS4 procedure, which includes independent reviews at all stages from design to installation.

If in doubt, check it out

It has been noted in several reviews of failed racking structures that existing and unreported rack damage may have contributed to rack collapses.

Rack uprights and beams can be easily damaged during forklift operations, and a regular rack inspection and repair process is highly recommended.

If you have any concerns your storage systems are not up to standard, or that their structural integrity may have been compromised, it is recommended an appropriately qualified engineer inspect them as soon as possible.

Obviously a thorough detailed inspection of storage systems is necessary immediately following a significant seismic event.

Also, should the usage of a storage system change significantly from its original design, its suitability for other purposes should also be checked and verified before proceeding.

And finally, caveat emptor

And finally, it is important buyers know what they are paying for when purchasing storage systems. Storage systems may all look similar, but their performance can vary significantly. The flood of cheap, imported and often look-a-like racks means that storage system buyers should insist on detailed manufacturing specifications and any necessary certification to ensure they are actually getting what they pay for, and that it is fit for purpose.

[Author: Dr Murray Clarke, BSc BE PhD MIEAust, is structural design manager at Dematic.]