14.2 Elevator Systems: Traction vs. Hydraulic, Core Layout & Sizing

14.2 Elevator Systems: Traction vs. Hydraulic, Core Layout & Sizing

Vertical circulation is the functional spine of multi-story architectural design. Elevator system selection and core configuration dictate building structural efficiency, floor plate leasability, egress paths, vertical shaft fire ratings, and overall spatial geometry. During the schematic design phase, architects must determine the appropriate elevator drive typology (hydraulic, geared traction, gearless traction, or machine-room-less), size hoistways, pits, and overhead clearances, establish bank groupings to optimize passenger traffic flow, and incorporate mandatory life-safety and accessibility codes (ASME A17.1 and IBC Chapter 30).

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Elevator Drive Typologies & Operating Physics

Elevator systems are categorized by their mechanical lifting drive mechanisms, which establish strict physical limits on travel height, operational velocity, energy consumption, and structural machine room requirements.

1. Hydraulic Elevators

Hydraulic elevators raise a cab via an incompressible fluid (petroleum-based mineral oil or biodegradable vegetable hydraulic fluid) pumped into a steel cylinder, driving a steel piston (ram/plunger) upward. Descent is achieved entirely through gravity by opening a controlled bypass valve that returns fluid to a fluid reservoir without motor power.

• Conventional Holed Hydraulic: The hydraulic cylinder extends deep into the ground beneath the hoistway pit inside a drilled well casing. The in-ground borehole depth must equal the total vertical rise of the elevator cab. While structurally stable, drilling boreholes is costly in rocky soils and carries significant environmental risk if petroleum oil leaks into underlying groundwater aquifers.

• Holeless Hydraulic: Uses single-stage or multi-stage telescopic dual pistons mounted above the pit floor on opposite sides of the cab rail. Eliminates subterranean drilling, making it standard for low-rise construction, contaminated soils, or high water tables.

• Roped Hydraulic: Pairs a holeless hydraulic cylinder with steel wire ropes and sheaves configured in a 2:1 mechanical ratio. The piston travels half the distance of the cab, extending vertical travel height up to approximately $75\text{ ft}$ without sub-grade drilling.

• Performance Characteristics:

  • Travel Speed: $100\text{ to }150\text{ feet per minute (fpm)}$ (maximum $200\text{ fpm}$).
  • Travel Height: Practical limit of $50\text{ to }60\text{ ft}$ ($2\text{ to }5\text{ stories}$).
  • Energy Profile: High electrical power draw on ascent ($30\text{–}50\text{ HP}$ pump motor), generating waste heat that requires hydraulic oil coolers; zero motor power consumed on descent.
  • Machine Room Location: Located on the lowest level adjacent to or within $50\text{ ft}$ of the hoistway pit.

2. Geared Traction Elevators

Traction elevators suspend the cab and a counterweight from high-strength steel wire hoisting ropes or polyurethane-coated steel belts passing over a grooved drive sheave. The counterweight typically equals the empty cab weight plus $40\%\text{ to }50\%$ of the rated passenger live load. As a result, the electric drive motor only expends energy to lift the net weight imbalance.

• Mechanical Drive: A high-speed electric motor connects to a mechanical worm-gear reduction box, which turns the drive sheave at lower rotational speed with high torque.

• Performance Characteristics:

  • Travel Speed: $200\text{ to }500\text{ fpm}$.
  • Travel Height: Mid-rise buildings up to $250\text{ ft}$ ($5\text{ to }15\text{ stories}$).
  • Machine Room: Traditionally required an overhead machine room penthouse located directly above the hoistway on the building roof to house the motor, gearbox, deflector sheaves, and controller.

3. Gearless Traction Elevators

Gearless traction systems feature a slow-speed, high-torque direct-drive electric motor (typically permanent-magnet synchronous) with the traction drive sheave mounted directly on the motor shaft, completely eliminating the mechanical gearbox.

• Performance: Gearless traction can serve substantial height and speed, but the selected equipment, traffic zoning and structural conditions impose real limits. It is not literally frictionless or silent; coordinate drive support, guide rails, power, heat, vibration and maintenance access.

4. Machine-Room-Less (MRL) Traction Elevators

The advent of compact, lightweight permanent-magnet synchronous motors (PMSM) revolutionized elevator planning by creating the Machine-Room-Less (MRL) elevator.

• Spatial Integration: The compact gearless drive motor is mounted directly inside the hoistway—anchored either to the structural guide rails or to an overhead structural beam at the ceiling of the shaft. The digital controller and disconnect switch are housed in a shallow metal cabinet ($8\text{ to }12\text{ inches}$ deep) integrated into the wall adjacent to the top landing entrance frame.

• Architectural Advantages: Eliminates the bulky rooftop penthouse, reducing structural dead weight, eliminating penetrations through roof weatherproofing membranes, and preserving building height under restrictive municipal zoning envelopes.

• Performance Characteristics:

  • Travel Speed: $150\text{ to }500\text{ fpm}$.
  • Travel Height: Up to $250\text{ to }300\text{ ft}$ ($20\text{ to }25\text{ stories}$).
  • Industry Baseline: MRL traction has rapidly become the dominant commercial standard, replacing geared traction and encroaching heavily on low-rise hydraulic markets.

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Elevator Typology Comparison Matrix

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Core Planning, Sizing & Spatial Clearances

Elevator cores represent permanent structural shear elements that cannot be easily relocated or expanded once constructed. Accurate schematic dimensioning is essential.

Passenger Cabs vs. Service / Freight Cabs

• Passenger Elevators: Standard commercial capacities range from $2,500\text{ lbs}$ (small office/multi-family) to $3,500\text{–}4,000\text{ lbs}$ (corporate office/hotel). Interior cab clear heights are typically $8\text{ ft }0\text{ in.}$ with $7\text{ ft }0\text{ in.}$ door heights.

• Service / Freight Elevators: Rated at $4,000\text{ to }5,000+\text{ lbs}$, featuring taller interior cab heights ($9\text{ to }12\text{ ft}$) to accommodate furniture, drywall pallets, and tall equipment. Service cars require heavy-duty floor finishes (diamond plate aluminum or heavy vinyl), protective wall pads, and separate service vestibules connected directly to the building loading dock.

Where IBC 3002.4 applies, demonstrate that at least one elevator accommodates the specified 24-by-84-inch stretcher in the required configuration. A 3,500-pound capacity label alone does not prove dimensional fit. Verify cab, door, maneuvering geometry and hoistway with the manufacturer.

Pit Depth & Overhead Clearance Dimensions

• Pit Depth: The recessed structural depression at the base of the hoistway below the lowest finished landing. It accommodates mechanical car and counterweight buffers (spring buffers for speeds $\le 200\text{ fpm}$; oil buffers for speeds $>200\text{ fpm}$), safety governors, compensating rope sheaves, and structural safety clearances. Pit depths range from $4\text{ ft }0\text{ in.}$ for low-speed hydraulic/MRL units up to $8\text{ to }10\text{ ft}$ for high-speed traction cars.

• Overhead Clearance: The vertical clear distance from the top finished landing to the lowest underside of the hoistway ceiling structure. It provides mandatory run-by safety clearance for maintenance technicians standing atop the cab roof when the counterweight is fully compressed on its buffer. Clearances range from $14\text{ ft }0\text{ in.}$ for low-speed units up to $20\text{ to }25+\text{ ft}$ for high-speed traction cars.

Elevator Lobby Sizing & Bank Configurations

Elevator lobby width and bank arrangement are program and traffic-design decisions coordinated with accessible circulation and egress. Test queue depth, opposing flows, furniture, security and wayfinding using the selected equipment. Common planning widths and four-car/eight-car groupings are not universal code minima or absolute functional limits.

Elevator lobby width and bank arrangement are program and traffic-design decisions coordinated with accessible circulation and egress. Test queue depth, opposing flows, furniture, security and wayfinding using the selected equipment. Common planning widths and four-car/eight-car groupings are not universal code minima or absolute functional limits.

Elevator lobby width and bank arrangement are program and traffic-design decisions coordinated with accessible circulation and egress. Test queue depth, opposing flows, furniture, security and wayfinding using the selected equipment. Common planning widths and four-car/eight-car groupings are not universal code minima or absolute functional limits.

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Elevator Traffic Analysis & Waiting Intervals

Elevator performance in commercial buildings is governed by two fundamental traffic parameters evaluated during the morning peak arrival period (up-peak traffic):

1. Handling Capacity (HC)

Handling capacity measures the percentage of the total building population that the elevator system can transport in a 5-minute peak period:

• Commercial Office Buildings: Requires an up-peak handling capacity of $12\%\text{ to }15\%$ of the building population.

• Multi-Family Residential: Requires a 5-minute handling capacity of $5\%\text{ to }7\%$.

2. Interval / Average Waiting Time (AWT)

The interval is the average elapsed time between successive elevator cars departing the main ground lobby floor during peak periods. The resulting passenger waiting time determines tenant satisfaction:

• Excellent (Prime Class-A Office): $<25\text{ seconds}$

• Good (Standard Commercial Office): $25\text{ to }30\text{ seconds}$

• Fair / Acceptable: $30\text{ to }40\text{ seconds}$

• Residential Multi-Family: $45\text{ to }60\text{ seconds}$

Round Trip Time (RTT)

The Round Trip Time (RTT) is the average time (in seconds) required for an elevator to depart the ground lobby, accelerate, make calculated probabilistic intermediate passenger stops, discharge occupants, return express to the ground floor, and reopen its doors. The total number of elevators ($N$) required to serve a building zone is calculated as:

$N = \frac{RTT}{\text{Target Interval}}$

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Life Safety, Firefighter's Service & Smoke Control

Under ASME A17.1 / CSA B44 and IBC Section 3003, all passenger elevators must provide dedicated two-phase emergency operations for life-safety response:

Phase I Emergency Recall

Phase I recall is designed to immediately remove elevators from public access during a fire:

• Activation: Automatically initiated by smoke detectors located in elevator lobbies, hoistways, or elevator machine/control rooms, or manually by a keyed three-position switch (RESET, OFF, ON) at the designated primary recall landing (typically the ground lobby).

• System Response: All cars in the bank immediately cancel registered car calls, close their doors, and travel non-stop to the primary recall floor. If the fire alarm was initiated by a smoke detector in the ground lobby itself, the cars automatically divert to a pre-designated alternate recall floor (typically the second floor or adjacent grade exit).

• Arrival: Cars park with doors fully open to allow occupants to exit safely into the lobby. Hall call buttons and in-car passenger buttons are completely disabled.

Phase II Firefighter's In-Car Operation

Phase II operation allows emergency responders to take direct, manual tactical control of individual elevator cars:

• Activation: Firefighters insert a master key into a locked control cabinet inside the cab and turn the switch to ON.

• Manual Control: Hall call buttons remain disabled. The car responds only to in-car floor selections made by the firefighter. Crucially, the doors do not open automatically upon arrival at a floor; the firefighter must continuously press and hold the DOOR OPEN button. If smoke or fire is observed outside through the door gap, releasing the button immediately causes the doors to power shut, protecting the crew.

Hoistway Smoke Protection & Pressurization (IBC Section 3006)

Vertical elevator hoistways naturally act as building chimneys, accelerating the upward spread of toxic smoke and hot gases via stack effect during a fire. To prevent hoistways from contaminating upper floors, the International Building Code enforces strict smoke containment rules for buildings where elevator hoistways penetrate more than three stories:

1. Enclosed Elevator Lobbies (IBC 3006.2): Fire-rated smoke barrier partitions (typically 1-hour fire-resistance rated) with self-closing, gasketed smoke doors enclosing the elevator landing doors at each floor.

Hoistway pressurization can be an allowed smoke-protection alternative when every condition is met. It does not universally remove all lobby requirements, particularly for special elevator categories. Verify pressure, door operation, smoke-control design and the applicable Section 3006/909 provisions.

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Worked Example: Equipment Envelope and Traffic

Assume a top landing 94.5 feet above grade, a 115-foot zoning cap, four cars serving 1,000 people, 13 effective passengers per loaded trip and a 115-second round trip. These are supplied project inputs. Manufacturer A’s MRL arrangement needs 16.5 feet from top landing to underside of the hoistway roof and a 2-foot roof construction allowance. Its top is $94.5+16.5+2=113$ feet, below the stated cap. Manufacturer B’s alternative requires an additional 16-foot overhead room in this exercise, producing 129 feet and failing the same cap. Other equipment configurations require their own dimensions rather than a universal MRL-versus-geared height rule.

Five-minute handling is $4(13)(300)/115=135.65$ passengers, or 13.57% of the 1,000 served. Departure interval is $115/4=28.75$ seconds. These are preliminary up-peak checks; they are not identical to average waiting time or proof of acceptable traffic for every use. Confirm the traffic model, car loading, dispatching, accessibility, stretcher fit, pit, overhead, power, fire/smoke operation and maintenance routes before selecting the system.

Escalators and moving walks can carry substantial flows but do not replace required accessible vertical routes or exits. Coordinate their openings, landing space, protection and structural support. Use the applicable elevator and building codes and current manufacturer layouts.