Should I consider it and what are the differences between conventional and seismic racking systems?
Pallet racking is abundant in almost every warehouse and distribution center around the globe.
They are often installed as structures exceeding 10m in height and containing tons in pallet loads, suspended high above the floor to allow for staff to go about their busy daily work schedule.
While racks that are correctly designed and installed pose little risk where static loading factors are concerned, what happens when you factor dynamic loads such as forklift impacts or earthquakes? Below we will look at some aspects of the seismic pallet rack design and help you decide whether to consider it or not.
Forces exerted on the system
In addition to static forces present on a racking system, occasionally it may also be subject to dynamic forces resulting from either an impact or an earthquake.
In terms of forklift impacts, these can be very high, of short duration, localized, and could lead to a risk of rack collapse. This risk can be easily avoided and almost eliminated by installing high quality protectors.
In the instance of an earthquake however, the forces are somewhat different; they can be a lot more prolonged and affect the entire system as a whole. Depending on the duration the earthquake persists, the entire racking row will move or oscillate and possibly continue to do so for a few seconds after the quake dissipates.
If the design is not seismic therefore, larger forces in one part of the system can be transferred to a weaker part and overload it, causing it to fail and risking an entire collapse. Indeed system design under an earthquake must be flexible yet strong enough to allow some movement under stress, but this movement has to be harmonious and equally distributed.
Standard pallet racks under an earthquake scenario will be subject to a variety of forces that can be magnified throughout the earthquake duration.
Some factors that could cause a standard racking system to fail:
The total load on the rack at the point in time
if the rack is fully loaded to the max static capacity, the dynamic forces may far exceed the static ones.
Center of gravity of the total loads
If the higher levels are loaded and the lower ones empty, the system will exert more movement and may lead to much higher forces and failure.
Direction of ground movement during the earthquake
upward / downward / horizontal forces exert different forces on the system and can expose weak points in places not imagined under a static load.
Magnitude of earthquake
the higher the magnitude the higher the movement and hence forces exerted
Quality of rack installation and continued rack inspection
pallet racking needs to be installed by qualified installers and aligned properly, both vertically and horizontally so that no side forces are present that can be magnified further in the event of an earthquake. Regular rack inspection should also be carried out to ensure that any misalignments created over time for whatever reason are eliminated.
Should I consider seismic pallet racking when building a new warehouse?
The short answer is, it depends. In some regions of the world it is mandatory to follow seismic regulations no matter what the configuration, height or load the system is subject to. In other regions it is not a regulation so the customer has to make a decision based on risk and budget. Seismic racks are usually twice the cost in material and much higher in installation costs / time. Are you installing 4m high racks with a couple of beam levels, or are you looking at 10m racks with 6 beam levels, where the bay load is much greater and risk of collapse is much higher? The higher the system the more you should be considering seismic racking.
Devastating earthquakes are one of those events that occur once in a lifetime, or may never be witnessed in one’s lifetime. However, what is certain is that in a seismic rated zone, such an event will occur, and when it does you will probably wish you had seismic designed racks. If the operations of a company are critical with a lot of staff doing storage, retrieval and picking operations (e.g. pharmaceutical or food distribution), not having seismic racks can be a catastrophic decision if the black swan event materializes.
How do I ensure my proposed system is of seismic design?
This is something that only the manufacturer of the pallet rack system can provide. No distributor or third party is in a position to certify suitable seismic proofing of any rack, and certainly not by adding a few cross braces in a standard system.
Features of seismic design at a glance
1. Considerably larger post profile
where a typical post profile for the majority of normal configurations is about 90mm and 2mm thick, a seismic design may take it to up to 150mm wide and 2.6mm thick (depending on supplier design)
2. Special seismic foot plate
An element often overlooked, the footplate is one of the most important parts as that is the point where the forces of the ground intersect the system. Therefore it is normal to see a much larger foot plate both in terms of foot area but also in thickness and more points of fixing to the floor.
3. Special frame braces and patterns
where the majority of frames are constructed using W or Z patters, seismic bracing are often thicker and follow an X pattern giving it a more rigid structure.
4. Larger beam profile
Similar to the above, a larger beam profile is often used for seismic design (e.g. 3ton beam under normal configuration may require a beam rated at 3.6 tons) to achieve more stiffness. In addition to this, the usual safety pins may often be replaced with bolts making the structure even more rigid.
5. Special floor fixings
In seismic design it is often normal to see much larger and longer bolts in combination with special chemicals (like Hilti injectable mortar) which adds more seismic proof properties.
6. Special cross / vertical braces
If the structure needs to sustain more than the basic materials above are rated to handle, extra vertical and lateral braces may be required to increase the system stiffness.
7. Seismic towers
As above, the supplier may choose to increase capacity further by adding an extra tower on selected bays of the configuration.
Can my manufacturer carry out seismic design calculations?
Not all manufacturers are able to carry out seismic design. Seismic design is very specialized, and experienced structural engineers in materials sciences are required for this task.
An F.E.M. design certificate alone does not mean the system is certified under seismic loads. Also certain manufacturers may not be able to design for regions outside their territory. Manufacturers in China for example, where thousands of tons pallet racking are manufactured, cannot necessarily design to European norms, the regulation required in the EU. Different regions have different norms so be sure that your selected supplier can design to the regulation of your country or region and get them to certify this in writing along with proof of their calculation, so that you can get a third party engineer to validate it if you so wish.
An additional check is to ask them for a list of seismic references already commissioned in your territory or designed to the same norms that you require.
What data do I provide to the manufacturer in order to get a seismic design?
Usually manufacturers will ask for the following specific data in order to provide you with a design:
Regulation to follow: e.g. Eurocode 8 for Europe, UBC1997 for the US etc.
Ground acceleration factor (Amax): this is the coefficient of ground acceleration for your region. Usually your civil engineer can provide you with this information
Soil type, denoted in letters A to E: this describes the soil of the area that the warehouse will be built on and it is categorized broadly on the strength of the soil, e.g. clay, granite etc.
Note that the usual information, such as pallet details and beam levels will also be needed.
Can I modify existing racks from non seismic to seismic design?
If you have an existing system and you wish to modify it to be seismic, this is not possible. As you have seen above, seismic design includes a variety of elements and the essence of it is its unique configuration/material to specific client requirements. Adding extra cross braces, which are seen in many installations has absolutely no effect on the system’s ability to perform under an earthquake but merely just adds some stiffness to the bays.
Can I change the configuration at a later date if my requirements change?
This is a task that needs to be confirmed by the supplier. For example, changing the position of the first beam, or adding an extra level can alter the entire system capacity so its best to get written approval beforehand.
Conclusion
Seismic design is not a trivial affair and certainly it is not a case of adding a few cross braces to increase its strength. It is a totally custom and fit for purpose configuration based on the unique and specific customer requirements along with the seismic profile and regulations of the location the system is to be installed in. This means that you cannot take one project and apply it to another. Every project has to be designed and simulated from scratch to deduce the appropriate system configuration. Correct seismic calculation by the supplier should also be run on a computer simulation program under max load to ensure that the structure can withstand one of these rare black swan events that nature can throw at us.
Whether to implement such a system or not is really up to the client, unless the local regulation demands it. It is indeed considerably more expensive and so the risk factors have to be carefully evaluated. Some manufacturers, such as SSI Schaefer will refuse to quote non seismic systems in a seismic region as the risk to their business is simply too great.
Simply put, if you can afford it, don’t think twice about it.
On a last note, no pallet rack design is classified as seismic if the building and especially the concrete slab is not constructed to similar standards. Remember that in the event of an earthquake, upwards as well as downwards forces will be applied to the floor bolts so if you are considering a seismic rack design be sure to factor in a compatible floor by consulting your building contractor or civil engineer.