Structural Assessment, Analysis, and Rehabilitation of Existing Elevator Support Structures

Brent L. Wist, Project Consultant, Simpson Gumpertz & Heger Inc. and Alec S. Zimmer, PE, Associate Principal, Simpson Gumpertz & Heger Inc.

Introduction
Elevators are crucial to the accessibility and serviceability of a building or a structure – they transport people and freight throughout its height. Routine inspection and assessment of elevators is required to identify deficiencies, address code compliance issues, and ensure the life safety of the general public. The American Society of Mechanical Engineers (ASME) 17.1 “Safety Code for Elevators and Escalators” provides requirements for the design of elevators, elevator components, and the elevator support structure, and recommendations for elevator inspection frequency. In the Commonwealth of Massachusetts, 524 Code of Massachusetts Regulations (CMR) 35.00: “Safety Code for Elevators and Escalators” currently provides modifications to ASME 17.1-2013. As a building ages, its owners may elect to study its vertical transportation systems and determine which components require repairs, refurbishment, or replacement. 

Elevator Types 
Traction elevators are one of the most common types of electric elevators. Historically, roped traction elevators were most common in mid-rise and high-rise buildings because of their speed. Roped traction elevators have a pit, a hoistway (including vertical overrun above the highest stop), and an elevator machine room directly above the hoistway (although some machine rooms are horizontally offset from the hoistway). Conventional roped traction elevators are mobilized by a series of cables attached to the top of the elevator car. The cables are driven by an electric motor connected to a drive sheave housed within the elevator machine room. The other end of the cables is connected to a steel counterweight located in the hoistway, adjacent to the elevator car. Vertical guide rails in the hoistway keep the car and counterweight aligned.

In modern low-rise and mid-rise buildings, electric machine room-less elevators are increasingly common. Like their older roped traction elevator predecessors, they have a pit and hoistway with an overrun and car and counterweight guide rails. They may use either wire ropes or belts. The hoisting machines are contained within the hoistway and may bear on the hoistway walls or the elevator car guide rails.

Hydraulic elevators, which are common in low-rise buildings, are typically slower than traction elevators. They have a pit and a hoistway with vertical car guide rails but typically do not have a substantial overrun. Hydraulic elevators are mobilized by a steel piston (or pistons) pressurized by hydraulic oil. In the most common configuration, the elevator car bears on the top of a single steel piston that extends into a casing through a hole in the pit floor. The piston extends up and down vertically to move the car without the need for drive cables or a counterweight. Other common hydraulic elevators include hole-less elevators with telescoping pistons (without a hole through the pit floor) and roped hydraulic elevators. The controllers and hydraulic fluid pumps are outside the hoistway, usually at the lowest floor level.

Elevator Components
The elevator pit is typically square or rectangular in plan and consists of a cast-in-place concrete slab-on-grade or a concrete mat foundation for the floor slab. For pits below-grade, a cast-in-place concrete foundation wall or concrete masonry unit (CMU) wall is often constructed around the perimeter to retain surrounding soil. The pit houses mechanical equipment, elevator buffers (large springs or hydraulic pistons to accommodate impact loads), buffer support steel, and car and counterweight guide rail support steel. Below-grade pits are typically waterproofed, either on the negative side (interior), on the positive side (exterior), or both, to mitigate the infiltration of water. In buildings constructed prior to the 1990s, the existing waterproofing may be a troweled product, such as a cement-based metallic waterproofing coat. (This product is uncommon in new construction today). For existing elevator pits where positive side waterproofing installation is not feasible, owners may elect to use a crystalline waterproofing admixture within concrete repairs. However, a positive side waterproofing membrane remains the most effective form of waterproofing for below-grade elevator pits.

Overview of a typical roped traction below-grade elevator pit.

The hoistway extends vertically from the footprint of the pit up to the underside of the slab or ceiling above. In older buildings, the hoistway walls were typically constructed of cast-in-place concrete, CMU, or terra cotta (clay) tile. Conventional cold-formed steel framing with gypsum board is common in new construction today. Both traction and hydraulic elevators have car guide rails attached to the hoistway walls or the base-building structure (e.g., perimeter spandrel beams or slab edges around the pit opening). Traction elevator hoistways also support counterweight guide rails. Guide rail connections to the hoistway or the base building structure often consist of a steel bracket connection using welding, bolting, or cast-in-place inserts. For multiple traction elevators sharing the same hoistway (no wall between each elevator car), steel or concrete divider beams located between the cars support the car guide rails. Divider beams and car guide rails are typically anchored to the base-building structure at each floor level, though tall stories may require intermediate supports.

The elevator machine room, exclusive to conventional traction elevators, houses the electric motor, drive sheave, and other support components. Steel sheave beams (or machine beams) span across the hoistway opening and support the electric motor and drive sheave. For machine rooms with cast-in-place concrete slabs, the sheave beams may be cast in the slab or bear on the surface of the slab. The elevator equipment manufacturer typically provides the sheave beams. The electric motor and drive sheave are usually supported on a built-up steel platform located on the sheave beams.

Overview of a typical roped traction elevator machine room.

Preliminary Site Visit
An existing elevator rehabilitation or replacement project typically begins with a visit to the site by the project team. The project team often consists of the building’s owner, an architect, a structural engineer, an elevator consultant, and an elevator technician. The visit typically begins at the pit area and progresses upwards through the hoistway, before reaching the elevator machine room. The structural engineer’s role during the site visit is to identify and record information including, but not limited to, the following:

• Plan dimensions of the elevator pit, including the location of buffers, buffer support steel, or other miscellaneous equipment.
• Distress or deterioration of the elevator pit or waterproofing materials. Hammer sounding of the pit floor and walls may be required to identify delaminated concrete or debonded negative-side waterproofing.
• Elevations, plan locations, and geometry of car and counterweight guide rail connections, divider beams and respective connections, and door sills.
• Plan dimensions, dead-end hitch locations, sheave beam locations, perimeter framing geometry, and the elevator machine room’s slab thickness.
• Distress or deterioration of the elevator machine room structure including the sheave beams, the machine room floor slab, and the surrounding building and roof structure. 
• Condition and configuration of overhead lifting beams that can be used to rig elevator equipment into position.
• Confirm the anchorage or restraint of all non-structural equipment.

Standing water, corroded support steel, and deteriorated pit waterproofing.

Analysis
The structural engineer's scope is to confirm that the base building structure (including the divider beams) is adequate to support the guide rail connections, the elevator machinery, and the piston loads, buffer loads, and guide rail loads imposed on the pit. The design of elevator components such as guide rails, buffers, buffer support steel, and sheave beams is usually the responsibility of the elevator consultant, although the structural engineer may need to evaluate the buffer support steel.

The elevator consultant is responsible for providing the guide rail reactions (both running and seismic), buffer impact loads (both car and counterweight), and loads on the sheave beams (both static and dynamic) to the structural engineer. 

During the beginning stages of analysis, it is imperative to determine the seismic design category (SDC) of the building using the code applicable to that jurisdiction. Although strength and stiffness both need to meet the code requirements, stiffness typically governs the design of guide rail attachments, divider beams, and sheave beam supports. The table below summarizes common design deflection requirements related to elevator components under the responsibility of the structural engineer:

Deflection requirements for typical elevator support components

Repair and Retrofit Design
Repair and retrofit of existing elevators requires close coordination within the project team. The structural engineer must understand the extent of elevator refurbishment anticipated by the project team. If the elevator is to undergo significant modifications such as new buffers, new support steel, new guide rails, new motor and sheaves, or an increase in operating speed or payload, significant structural modifications may be required to comply with code requirements. For example, if a new, larger-capacity car is installed within an existing hoistway, the buffer impact loads will increase, and the pit slab may need to be locally strengthened. Small refurbishments and equipment replacements (e.g., renovation of the car interior) may require no structural modifications to the existing system, aside from the repair of any deteriorated structure. The most common types of repair and retrofit required for existing elevators include the following:

Pit

• Negative (interior) waterproofing repairs or replacement.
• Concrete repairs to the floor slab or the perimeter foundation walls.
• Refurbishment or replacement of structural support steel.
• Installation of a temporary submersible pump to mitigate water infiltration (installation of a conventional sump pit is costly and disruptive).

Hoistway

• Stiffening or strengthening of existing guide rail connections and divider beams.
• Localized repairs to concrete divider beams or cast-in-place concrete hoistway walls.

Elevator Machine Room

• Localized concrete slab repairs.
• Anchorage of non-structural elevator equipment to prevent toppling during a seismic event.