Introduction
At Forensic Services Asia the forensic investigation of rack collapses is largely carried out by our mechanical engineers. We have investigated rack collapses in Thailand, Malaysia and Indonesia.

Efficient utilisation of storage space is increasingly effected by the installation of storage racking systems. Goods are packed onto standard pallets that are lifted into modularised slots many tiers high. Although the designs of these system are superficially similar, there is significant differences in the fine detail that give rise to structural problems that either exist at the outset or develop in service. The designs are now so fine-tuned that if a section fails, the collapse can carry through to involve the whole of the installation. We have seen rack collapses that involve as many as 1100 pallet positions.

Racks can be considered in two main categories as follows.

Selective Racking
This configuration places all loads on an aisle and is the most commonly used in industry as it provides efficient use of space, yet it allows immediate access to everything in storage. The rack consists of vertical columns (frames) which support horizontal load-supporting members (beams). The front and rear column sections are tied together by horizontal braces.

This design easily allows diagonal bracing both longitudinally and transversely. Additional stability can be gained by connecting the front and rear frames to each other or at the end to the building structure. Alternatively, diagonal strength can be obtained from the horizontal members because of their width and distributed connection with the columns.

Drive-in/thru Racks
This design has bays three or more loads deep. To allow a forklift or other device access to the loads, instead of the pallets straddling load beams front-to-rear, parallel to the aisle; they straddle load beams (support rails) left-to-right, perpendicular to the aisle. These systems offer a highly efficient method of storing large quantities of similar loads by allowing the forklift driver to enter the rack system and place the load on structural rails.

A drive-thru rack is merely a drive-in rack that is accessible from both sides of the rack.

Of course, since the lift truck must enter the structure, no beams are allowed to span left to right in the truck’s path. This means that the only beams allowed parallel to the aisle may be those placed overhead to tie the vertical members, either frames or single columns, together. There will be a lack of stability parallel to the aisle, particularly with drive-thru racks, unless the rack is either connected to the building at the end(s) or the drive through capability is compromised to some extent with diagonal bracing on, for example, every fifth aisle.

Maintenance
Even the most robust design will suffer damage with usage. There is usually column damage at corners where forklifts turn. Beams can be dislodged when loads are moved. Beams normally slot into the columns and clips used to prevent dislodgment. However some clip designs are ineffective and with very high racks it can be difficult to even spot damage from the ground. Lightweight diagonal bracing can be perfectly adequate in terms of engineering design, but very susceptible to damage.

Usage
Finally, failures can occur when usage is changed, resulting in imposed stress that the original designer did not allow for.