SKI RESORT TRAGEDY PROMPTS CHAIRLIFT INVESTIGATION
An engineering consulting firm uses Algor FEA software to design a safer
chairlift grip before busy ski season.
||The Silver Star Mountain Resort, shown here, Whistler Mountain and Lake Louise ski
areas in the British Columbia/Alberta region of Canada employed Pol-X West, Inc., Carson
City, Nevada, to redesign a failed chairlift grip that led to a fatal accident. Les
Okreglak, president and chief engineer of Pol-X West, used FEA software from ALGOR, Inc.
to reduce the design cycle of the new grip to less than six months. Photograph Courtesy of
Silver Star Mountain Resort.
Most skiers and snowboarders rate ski resorts by their average powder base and the
overall challenge and number of slopes. Few likely even consider the safety of chairlifts
at their favorite mountains. Luckily, ski resorts and governing authorities perform
regular maintenance and inspection of chairlifts to ensure passenger safety. In spite of
rigorous standards, sometimes accidents can occur.
On December 23, 1996, Whistler Mountain in British Columbia, Canada was the site of the
worst chairlift accident in the provinces history. Around 3 p.m., while skiers were
riding downhill, four chairs detached from the cable and fell 30 feet. Several other
chairs slid down the cable and collided with chairs in front. Two passengers died and 10
others were hospitalized.
Investigators from British Columbias Ministry of Municipal Affairs, Engineering
and Inspection Branch determined that the accident was caused by the grips that attach the
chairs to the transport cable. The grips relied on gravity to maintain contact with the
Shortly thereafter, during a routine inspection of a lift at another ski resort, safety
officials from the Ministry of Municipal Affairs discovered cracking in a grip similar to
the type of grip involved in the Whistler Mountain accident. This cracking occurred on a
metal insert used to maintain contact with the cable. Such cracking was not found in the
grip involved in the Whistler Mountain accident because that design did not include the
The Ministry of Municipal Affairs determined that both the failed grip and the grip
with a steel insert were unsafe and required that ski resorts discontinue use of all lifts
that employed the grips.
||The failed grip component was part of a detachable-style chairlift that enables
passengers to board and unboard the chairs at a comfortable pace while maintaining a
constant cable speed. Chairs attached to the cable (above) travel at a rate of 1,000 feet
per minute. As the chairs approach the loading station, a detachable grip releases from
the cable and transfers to a track (inset), which decelerates to allow passengers to
unload before accelerating again for reattachment to the cable. Photographs Courtesy of
Silver Star Mountain Resort.
Whistler Mountain, and two other ski areas in the B.C./Alberta region, Silver Star and
Lake Louise, joined together to find a replacement for the flawed grips that would fit the
existing chairlift structure and could be implemented quickly. The chairlifts had to be
operational before the next ski season the following November or the resorts risked a loss
in revenue that could extend through the busy Holiday ski season. They enlisted the
services of Pol-X West, Inc., an engineering consulting firm based in Carson City, Nevada
who used Finite Element Analysis (FEA) software from ALGOR, Inc. to design and test a
reliable alternative to the failed grip.
"With only six months to design, test, manufacture and implement the replacement
grips, we relied heavily on ALGORs FEA software to develop and analyze the new part
design," said Les M. Okreglak, P.E., president and principal engineer for Pol-X West.
"ALGOR software helped us to ensure that the extensive physical testing required by
the B.C. Ministry would be limited to just one prototype."
Okreglak studied both the operation of the existing chairlift systems and the
Ministrys conclusions about the design flaws of the original grips before beginning
The purpose of a detachable-style chairlift is to enable passengers to board and
unboard the chairs at a comfortable pace while maintaining a constant cable speed so
passengers quickly reach the top of the slope. With this system, the cable stops only for
emergencies, such as when a skier falls or misses the chair at the loading or unloading
Each chair is attached to the cable via a detachable grip. According to Okreglak, the cable travels on a pulley system at a rate of 1,000 feet per minute.
As the chair approaches the passenger loading station, the grip releases from the cable
and transfers to a track, which decelerates the chair to allow passengers to load or
unload. The chair assembly then accelerates along the track to the speed of the cable at
which time the grip reattaches to the moving cable.
||The new grip design eliminates dependence on gravity to secure the grip to the
cable, which contributed to the failure of the original design, according to the British
Columbia Ministry of Municipal Affairs. A pair of helical springs exerts the entire
gripping force and prevents slippage. Photograph Courtesy of Silver Star Mountain Resort.
The Ministry of Municipal Affairs identified many factors that could lead to grip
failure and developed a set of criteria for the new design to which Okreglak had to
The grip involved on the Whistler Mountain accident relied heavily on gravity to grasp
the cable without slippage. Investigators discovered that the catastrophic failure of the
gravity-assisted grips occurred when the lift stopped suddenly, causing the cable to
bounce while creating a sudden impact load on the grips. The bouncing movement disrupted
the gravitational force that contributed to the steadfastness of the grips on the cable.
In addition, the second flawed design featured a steel insert that experienced cyclical
loading when speeds of the chair and cable differed at the instant when the grip
reattached to the cable. This cyclical loading caused cracks to develop at the sharp
corner of the insert. The cracking increased due to impact loading created when the cable
made sudden stops to accommodate passengers who had difficulty boarding the lift at the
loading station, according to Okreglak.
"The impact load placed additional strain on the weakened metal inserts of the
grips, which, in combination with a disruption in the gravitational force, could result in
grip failure as well," Okreglak said.
Okreglak and his firm were challenged to design a grip that would eliminate the
dependence on gravity to secure the grip to the cable thereby removing an additional
source of grip slippage resistance. In addition, he needed to eliminate use of the metal
insert that was prone to strain. The firm first created a basic 2-D drawing using AutoCAD
to create a basic plan of their design that could later be used in manufacturing. They
designed the clamp as one unit that included two symmetrical portions, including one
mobile jaw and one static jaw that rely on a pair of parallel helical springs to exert the
clamping force. The clamp was designed to surround 270°of the 1-7/8" diameter wire
cable so that it could pass the pulleys at both ends of the cable system and release
easily from the cable at the loading and unloading platforms.
|Okreglak used SolidWorks to create 3-D solid models of the fixed and mobile jaws
of the new grip design and then transferred the files via IGES file format to ALGOR for
The firm then created 3-D models for the fixed and mobile jaws using SolidWorks, which
then transferred via IGES files to ALGOR Finite Element Analysis software for linear
stress analysis. Okreglak used ALGORs automatic meshing tools to create a surface
mesh, then enhanced the mesh around small holes in the fixed jaw model using Merlin
Meshing Technology. He then used ALGORs Hexagen, an automatic solid mesh engine, to
create hybrid meshes of both brick and tetrahedral solid elements.
Okreglak analyzed the fixed and mobile portions of the grip independently regarding the
compressive force of the springs. In addition, each portion was evaluated concerning the
load of the cable against the point where it meets the grip. Since the cold temperatures
of the mountains would affect the performance of the metal components, specifically A148
casting steel, Okreglak factored material properties at -50°F conditions. Boundary
conditions were applied at the points where the jaws attached to cable and where the top
portion of the mobile jaw would join with the hanger portion of the chair.
||In the first of two ALGOR linear static stress analyses, Okreglak analyzed the
stresses on the components under normal operation when the grip is attached to the cable.
Okreglak accounted for the effect -50° F temperatures would
have on the performance of casting steel used for the jaws in the setup of the analyses.
Linear static stress analyses were performed under two specific conditions: when the
jaws were closed and attached to the cable, and when the jaws were open for transition to
the loading track, where the force of the springs was the greatest. Okreglak used
ALGORs built-in visualization tools to view the stress results using a von Mises
"Our analysis confirmed that the design was well within the allowable limit of 1/3
of the material yield point. We were then able to build a prototype based on ALGORs
analysis," said Okreglak.
||Okerglaks second stress analysis studied the jaws when they were open for
transition to the loading track, where the force of the springs was the greatest. Stresses
for both results were within the allowable range of one-third the material yield point.
Pol-X West built the prototype using A148 casting steel and subjected it to the
required physical tests regarding strain, fatigue and slippage. The strain gauge
recordings were performed in a field test that included an instrumented grip that was
assembled on the original chairlift mechanism.
"The strain gauge analysis matched closely to ALGORs FEA results,"
Okreglak said. "This physical test made me feel confident in the performance of the
software and the design."
The fatigue analyses consisted of in-plant trials at Bacom Donaldson, a third-party
engineering consulting firm based in Vancouver, British Columbia.
The grip was attached to a cycling machine that opened and closed the grip 500,000
times. The second trial included attaching the grip on an assembled hanger and chair with
a load equivalent to four passengers. This test put the grip through 5,000,000 endurance
cycles. This final test, which checked the grips slippage force, was performed in
the plant with the unit attached to a 1-7/8" diameter wire cable installed on a test
stand. The trial tested the grip in normal operation, as well as under conditions
simulating the grips performance if it lost the use of one or more springs. In
addition, the grip was tested for use with cable that is 6% smaller to 10% larger than the
standard 1-7/8" diameter.
||Stringent physical prototype testing was mandated by the Ministry of Municipal
Affairs. Here the grips plucking force is tested in an in-plant trial.
"The grip performed as expected in all the tests and complied with the Ministry of
Municipal Affairs guidelines. With the use of ALGORs FEA software, we were
able to confidently design a single prototype that tested well, which enabled us to move
quickly to the manufacturing and implementation phases of the project," Okreglak
The scrutiny of the product did not end with the final testing of the prototype. Since
the component was critical to the safety of the entire lift, the manufacturing process was
also held to rigid standards imposed by the Ministry of Municipal Affairs. Each of the
1,000 manufactured grips was x-rayed for material flaws while still in the casting molds.
Bacom Donaldson, who specializes in metallurgy, inspected every finished product to ensure
"This grip is solid. With the tough design criteria, rigorous testing in
manufacturing and scrutiny by a regulatory body, we were presented a situation that
ensured a very safe product while utilizing the existing lift equipment and minimizing the
resorts downtime." Okreglak concluded. "The lifts installed with the new
grips have since operated successfully, without incident."