188.8.131.52 Traction Elevators
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Elevators are the most common type of transportation system to move people or cargo from one floor to another in buildings. Since the first passenger elevator was installed in New York in 1857 by Elisha Otis, elevators have become a ubiquitous mode of vertical transportation in buildings and they are found in practically all modern buildings around the world. An elevator is a machine that either pulls or pushes a car (or cab) located in a shaft (also referred to as hoistway). There are two main types of elevators which vary depending on the mechanical means in which the car is lifted or lowered. In hydraulic elevators a pressurized fluid in a piston is used to lift and lower the car whereas in traction (sometimes referred to as roped) elevators the car is pulled by steel ropes moved by a pulley system.
Traction elevators have made high rise construction possible. These systems are continually evolving to achieve faster speeds, smoother and quieter operations. Currently there are geared traction elevators, gearless traction elevators, and "machine-roomless" elevators that use flat belts instead of steel cable. Traction elevators are complex mechanical systems that have many moving parts and electronic components and failure of any of the components could disable the functionality of the entire system. Maintaining limited functionality of some elevators following an earthquake is critical for continued operations of essential facilities or the evacuation of hospital patients.
The primary components of the traction elevator system are the elevator cab (car), counterweight, cab guide rails, counterweight guide rails, guide rail support brackets, electric motor, electrical control panel, ropes, sheaves, safety braking mechanisms, door mechanisms, and a shock absorber at the bottom of the shaft. The system may be tied to a seismic switch or a smoke detector which would facilitate safe shutdown in the event of an earthquake or fire. Any of these components could be damaged if not properly restrained.
- Typical Causes of Damage
- Seismic Mitigation Considerations
Building Code Provisions
Seismic loads for elevators are determined using Minimum Design Loads for Buildings and Other Structures (ASCE/SEI 7-10) Chapter 13. Elevators are classified as mechanical components, and are sensitive to both story drift and acceleration. Elevators designed in accordance with the seismic requirements of Safety Code for Elevators and Escalators (ASME A17.1) are deemed to meet the requirements of ASCE/SEI 7-10, provided the forces and displacements used in the design meet the provisions of ASCE/SEI 7-10.
- Elevators, hoistway structural systems, elevator equipment supports and attachments, and controller supports and attachments must be designed to meet the force and displacement requirements of ASCE/SEI 7-10.
- Bracing exemptions for small diameter piping and piping supported on hangers less than 12 inches long do not apply to elevator piping.
- Elevators that operate at speeds greater than 150 feet per minute must be provided with seismic switches that are triggered by earthquake shaking. The seismic switch will prevent a potentially damaged elevator from being used.
Retrofit Standard Provisions
Seismic Rehabilitation of Existing Buildings (ASCE/SEI 41-06) classifies components of elevators as acceleration sensitive. Shafts and hoistway rails that rise through several floors are both acceleration and deformation sensitive.
- Elevators are subject to the requirements of ASCE/SEI 41-06 when the performance level is Immediate Occupancy, or in regions of high and moderate seismicity when the performance level is Life Safety.
- The evaluation must consider the anchorage of the equipment and controllers, as well as the construction of the elevator shafts.
- Acceptance criteria may be based on prescriptive procedures, or the elevator components may be analyzed for the force and displacement demands.
Typical Causes of Damage
- The most common cause of damage in elevators is counterweight derailment. Counterweights are the heaviest component in an elevator and typically weigh the same as the cab filled at 50% capacity. Elevators are often programmed to spend more time in the ground floor and therefore when earthquakes occur the counterweight is likely to be located in the upper part of the building where the largest acceleration demands occur. This means that often counterweights are subjected to forces that are larger than those experienced by the cab (car). Counterweight derailment is a very serious type of damage because once it is out of the guide rails the heavy counterweight may violently swing in the hoistway and collide against other components causing the damage to guide rails and rail brackets. In some cases collisions with the cab occur if the elevator is operated with its counterweight derailed. In other cases, severe pounding of a derailed counterweight against the guide rails or the hoistway can cause damage to the frame of the counterweight and individual block which are typically made of cast iron or steel plates may become loose and fall into the shaft or onto the roof of the cab.
- Counterweight and car guide rails often are bent as a result of large inertia forces acting on the counterweight and car that push the guiderails during the earthquake. Damage typically occurs when the counterweight or car are between floors where the rails are not attached to the structure and therefore are subjected to large bending deformations. Guide rails also get damaged as a result of pounding against the counterweight or the car. Bent rails often need to be replaced, which may require several days.
- Guide rails are attached to the structure by steel brackets. As shown in Figure 220.127.116.11-1 the rails are attached to the brackets by clamps that hold the flanges of the rail to the bracket. Both brackets and clamps are damaged as a result of the forces acting on the counterweight or car. The most common type of damage consists of bending of the brackets.
- Lateral motion of counterweight and of the car relative to the guide rails is constrained by a guide rolling system typically consisting of three guide wheels (or guide shoes). Guiding systems are comprised of several parts. Details vary from manufacturer to manufacturer, on the load capacity of the elevator, the traveling speed and on whether the guide is for the car or the counterweight. An example of a spring adjustable guide rolling system is shown in Figure 18.104.22.168-15. The axle bolts, pins, and supports of these rolling systems are particularly vulnerable during earthquakes where very large forces are produced in these small mechanical components as a result of inertia forces acting on the counterweight and car.
- Landing switches and limit switches are sensing elements that allow an adequate positioning of the car at each floor level. These switches consist of brackets extending into the hoistway. Damage to these elements consists of being bent by lateral motion of the car or the counterweight. They can also be damaged from steel ropes swinging in the shaft and colliding with the switches or by falling counterweight block.
- At the top of the elevator is a room that houses the elevator machines. The main components housed in the elevator machine room are the electrical motors which are usually directly attached to the shaft. The elevator machines must be properly anchored to the base otherwise a sliding may occur during earthquakes.
Damage in Prior Earthquakes
Earthquake Date Magnitude Number of Derailed
Nisqually February 28, 2001 6.8 224 Van, Turkey October 23, 2011 7.2 218 Darfield, New Zealand September 3, 2011 7.1 30 Whittier Narrows October 1, 1987 5.9 91 Loma Prieta October 17, 1989 6.9 249 Northridge January 17, 1994 6.7 690
- According to survey responses collected by the Division of Occupational Safety and Health Elevator, Tramway, and Ride unit, following the 1994 Northridge earthquake, the following issues were observed:
- Water damage-sprinkler pipes pulled apart flooding machine rooms and pits causing water on top of cars, damage to door operators and door detectors, and soaking car processors and car station. Several oil buffers had to be rebuilt.
- Falling debris in hoistways, falling plaster, loose concrete blocks, broken glass resulted in damaged door interlocks and misalignment of hatch switches and bent fascias.
- Building settlements bent elevator guide rails; structural damage, loose and bent divider beams shifted guide rails and bending support brackets.
- 3,528 cabled elevators were removed from service by a seismic protective device. 710 devices did not operate as intended. Some devices were located in the elevator pits which were flooded.
- 968 electric cabled elevators sustained earthquake damage.
- 2 minor injuries were reported, one of which was sustained by a fireman trying to open hoistway doors.
- 39 elevators required rescue.
- 57 instances were reported where "unauthorized persons" reset earthquake devices. Fortunately, only 5 elevators sustained additional damages.
- 688 counterweights came out of guide rails. 8-1b rails were inadequate even with additional intermediate guide rail bracket retrofit. Several counterweight frames were twisted and bent with a few dislodged subweights. Some counterweight roller guide shoe mountings disintegrated.
- Following the 2010 Chile Earthquake, it was reported that 50%-80% of hospital elevators were damaged. In a number of locations, patients had to be carried down many flights of stairs, often cluttered with fallen debris, in order to evacuate them to safety. The most common damage was due to unrestrained movement of the counterweights, resulting in damage to the guide rails. Movement of unrestrained mechanical equipment was another problem. One security camera at a military hospital in Santiago, Chile captured a sequence through the open door of an empty elevator cab where the counterweights derailed and then the subweights came crashing down on top of the cab. (Source: Bill Holmes, March 2010).
- In addition to property damage, passengers may become trapped in the cab following an earthquake and may need to await extraction by qualified elevator technicians. Failure of elevators are particularly critical to high-rise buildings that may become inoperable without elevators.
In recent shake table testing of a full scale 5 story building performed at University of California San Diego (see Section 6.1.3), a fully operational machine-roomless traction elevator was subjected to severe earthquake motions. The elevator remained operational for all but the largest tested earthquake motion although in a normal installation, the elevator would have tripped off line by seismic sensors. For the largest motion (inter-story drifts exceeding 3%) the gypsum wall board partition distortions caused the elevator shaft doors to jam thus making the elevator in-operable. Elevators have interlocking controls that when the shaft doors are not operational, the elevator is not operational.
Figure 22.214.171.124-7 Derailed car roller assembly of the guide rail at the Santiago airport in the 2010 Chile Earthquake (Photo courtesy of Gilberto Mosqueda, SUNY Buffalo). The three rollers are supposed to travel along three sides of the stem of the T-shaped steel guide rail.
Figure 126.96.36.199-8 Derailed car roller assembly of the guide rail at the Santiago airport in the 2010 Chile Earthquake (Photo courtesy of Gilberto Mosqueda, SUNY Buffalo). The three rollers are supposed to travel along three sides of the stem of the T-shaped steel guide rail.
Figure 188.8.131.52-9 Derailed car roller assembly of the guide rail at the Santiago airport in the 2010 Chile Earthquake (Photo courtesy of Gilberto Mosqueda, SUNY Buffalo). The three rollers are supposed to travel along three sides of the stem of the T-shaped steel guide rail.
Figure 184.108.40.206-11 Unrestrained movement of the counterweights damaged the counterweight assembly and bent the counterweight guide rails; over half the subweights dropped onto the top of the cab and damaged the cab framing in the 2010 Chile Earthquake (Photo courtesy of Rodrigo Retamales, Rubén Boroschek & Associates).
Figure 220.127.116.11-12 Anchor bolt failure of the elevator generator set due to inadequate edge distance for the bolts at the Los Angeles Regional Public Hospital in the 2010 Chile Earthquake (Photo courtesy of Bill Holmes, Rutherford & Chekene).
- All of the components of the traction system need to be restrained, anchored, or detailed to resist lateral forces in all directions and accommodate seismic movement. The system must be designed to accommodate the anticipated inter-story drift over the height of the elevator travel. Guide rails and door frames must all be detailed to accommodate lateral deformations. All of the mechanical and electrical equipment, sensors, and guides need to be properly anchored or restrained.
- All elevators should be inspected by qualified elevator technicians following an earthquake. Elevators should have a seismic switch or safety features that allow for safe shutdown in an earthquake.
- Elevator safety is governed by the prescriptive requirements in Handbook on Safety Code for Elevators and Escalators (A17.1/CSA B44) a document that is continually evolving to reflect new elevator technologies. In addition, Performance-Based Safety Code for Elevators and Escalators (A17.7/B44.7) is the next step in the evolution of elevator safety codes in the United States and Canada. Local or state jurisdictions may have other elevator requirements.
- As an example of state jurisdictions, the California Office of Statewide Health Planning and Development (OSHPD) provides special requirements for elevators in California hospitals, such as Title 24, Section 3007. These include requirements for a seismic switch connected to the essential electrical system, a visible or audible alarm, the ability to function at a “go slow” speed of not more than 150 feet per minute until the elevator can be inspected, and an additional sensor that will disable the elevator if the governor tail sheave is dislodged. For these systems, the seismic anchorage, guards and switches all need to be inspected annually.
- The internet provides information regarding hydraulic elevators. A few websites are linked below:
- How Elevators Work describes the workings of hydraulic elevators and provides links to other resources
- Jobsite safety in the elevator industry is discussed on Disaster Planning Links
- The Elevator Escalator Safety Foundation and major elevator suppliers such as The Otis Elevator Company and Schindler Elevators provide additional resources.
- The National Elevator Industry, Inc. has other resources including a discussion of the Performance Based Code for Elevator Safety
Figure 18.104.22.168-13 Small “machine-roomless” traction elevator with flat belts at the Academy of Sciences, San Francisco, California (Photos courtesy of Eduardo Fierro, BFP Engineers). Clockwise: Overview of elevator at roof; view of guide rails and sheaves; top of cab showing belts; view down shaft at belts, counterweight, guide rails and guide rail brackets.
Figure 22.214.171.124-15 Spring loaded elevator guide with polyurethane rollers. Details of elevator guides vary from manufacturer to manufacturer, elevator load capacity, elevator speed, and whether used for guiding the car or counterweight.