Would your racking survive a 7.1 Richter Scale Earthquake?

by Case Studies on 2011-04-06

On Saturday 4th September 2010 at 0430 a significant earthquake measuring 7.1 on the Richter scale hit the Garden city of Christchurch in New Zealand. It caused significant damage of billions of dollars to the city’s infrastructure as well as the lives of the Kiwis who had to endure the devastation and trauma of the event.

Christchurch is a beautiful city with some outstanding architecture which suffered damaged as well as private dwellings and Distribution centres in total over 60,000 insurance claims.

When the SSI Schaefer team arrived in Christchurch and inspected pallet racking in several facilities to understand the magnitude of this catastrophe several observations were made:

1. The team believed considerations were not made in regards to structural ductility leading to brittle failures;

2. Considerations of actual ground acceleration under earthquake loads were not fully considered in designs;

3. There was evidence of progressive decay of the frame capacity under the earthquake load, in the down aisle   direction with bending  and plate tearing at beam/column joints leading to excessive lateral deflections due to the fact racking did not have sufficient down aisle-rack capacity. This was noted with:

a. Insufficient spine bracing.

b. Not enough levels fixed with spine bracing.

c. Failed welds at beam end-plate weld arrangements which were only designed for static rack loads and not the additional forces  required for earthquake design.

d. Tearing of the rack uprights that occurred at the beam connection due to thin walled upright sections being used.

e. There was also evidence of progressive decay of the frame capacity under the earthquake load in the cross aisle direction due to under designed Uprights/frame bracing.

f. Many rack column/base plate/fixings appeared to be under designed allowing for distorting/bending laterally and reducing the axial capacity of the column section at the connection.

g. The repair status of some racking was beyond belief with welded stub sections and inadequate splicing of uprights and very little maintenance.

The SSI Schaefer team would point out that a Schaefer Cold Store with two Satellite Systems was still fully operational after the earthquake because it was designed to meet all building and seismic activity codes.

UNDERSTANDING SEISMIC ZONES
In order to understand the Seismic Zoning method, we must first understand what effective peak ground acceleration means and how it is measured against gravity.

First let’s discuss what is meant by ground acceleration. In order to measure the acceleration of an earthquake, it must be measured against Gravity (or 1.0g). Gravity is the rate at which an object falls when dropped from being at rest in a vacuum. It is quite a high rate of acceleration. It is approximately the same as a car traveling 100 meters from rest in just 4.5 seconds.

There are studies that show that much of the damage done in earthquakes is, perhaps, due rather to the velocity of the back and forth movements of the earth, rather than to the ground acceleration. However, the mean and peak ground accelerations do have much to do with the intensity of damage a building may have to withstand. Consequently, engineers and designers rely a great deal on the measure of the peak ground acceleration, as compared to gravity, to determine how strong an earthquake force a new building may have to withstand.

There are instruments called accelerographs that measure ground acceleration against the value of gravity. (acceleration in g/10) These values are gathered from around the world to create a seismic-risk map, which is used by engineers and builders when designing earthquake resistant structures for different parts of the country.

SEISMIC MAPPING
The map portrays the level of ground shaking that can be reasonably expected to occur at a site in the next 50 years. It is based on knowledge of the rates of earthquake occurrence and the activity of faults in the region. The map is one of several types used to portray earthquake hazard.

The values shown on the map are "peak ground acceleration (PGA) in percent of g with 10% probability of exceedance in 50 years". Therefore, the map represents ground motions that can be reasonably expected in a 50 year period.

The force on a building during an earthquake is proportional to ground acceleration. Such forces are prescribed by the UBC. During an earthquake the ground acceleration varies with time. The acceleration values shown on the map are the peak or maximum values expected during the earthquake. ‘g’ is a common value of acceleration equal to 9.8 m/sec/sec (the acceleration due to gravity at the surface of the earth). 30% of g is the acceleration one would experience in a car that takes 9 seconds to brake from 60 miles per hour to a complete stop.

The ‘10% probability of exceedance in 50 years’ refers to the fact that earthquakes are somewhat random in occurrence. One cannot predict exactly whether an earthquake of a given size will or will not occur in the next 50 years. The map takes the random nature of earthquakes into account. It was constructed so that there is a 10% chance (1 chance in 10) that the ground acceleration values shown on the map will be exceeded in a 50 year time period.

Another important thing is factored into plotting of the seismic-risk map is attenuation. Attenuation is, basically, how far earthquakes waves are felt, and what is the duration of the earthquakes. This is very different in various parts of the nation.

Next, the values on the seismic-risk map are figured this way: If you live in seismic zone 4 (Christchurch zoning), you have a one in ten chance that an earthquake with active peak acceleration level of 0.4 g (4/10 the acceleration of gravity) will occur within the next fifty years. Likewise, if you live in zone 1, you have a one in ten chance that an earthquake with an active peak acceleration level of 0.1g (1/10 the acceleration of gravity) will occur within the next fifty years.

A seismic zone map is based on a statistical compilation of the number and the magnitude of past earthquakes. Therefore, it is an indication of where the next earthquake is most likely to occur, how often and the magnitude. There is no direct correlation between seismic zone and Richter scale, except past experience shows that the worst earthquakes occur in the higher seismic zones.
New Zealand is classified under either Zone 2 or 4 based on the historical number of past earthquakes.    

SSI Schaefer strictly follow and base the design of Racking system in New Zealand on FEM 10.02.02 and NZS 1170.5 - 2004 (seismic code) and or Uniform Building Code 1997 edition – Vol.2 Division IV “Earthquake Design”.

They consider based on above codes
  •    The pallet load and seismic action are applied on cross aisle and down aisle direction.
  •    The action forces are based on the standards above.
  •    The frame profile and bracing requirements are determined by forces acting on cross aisle direction.
  •    The beam profile and spine/plan bracing is determined by forces acting on down aisle direction and rack configuration used
  •    Ductility
            
The Rack Components are fully checked in the static calculation as below:
  1.    Base plate size and thickness as per requirement (vertical force and shear force).
  2.    Floor fixing as per requirement (vertical force and shear force).
  3.    Post profile selection based on compression force and bending
  4.    Frame bracing check based on tension capacity, bearing and etc.
  5.    Beam design.
  6.    Spine and plan bracing component checking.
  7.    Point load acting on floor by racking post and spine bracing   

SSI Schaefer have built in many earthquake zones such as the Satellite and Drive In systems in the Philippines, three tier R3000 Shelving systems in California, Mobile Racking Systems in Chile.

Alan Clark
Managing Director
SSI Schaefer PH: 1800 SCHAEFER(724233) or Email:  info@schaeferssi.com.au 
www.ssi-schaefer.com.au