Truss, tracks and the escalator structure

Nell's first job for Tamsin at Brackenridge was not a repair. She lifted the upper landing plate of E5, with the unit locked out and both ends barricaded, removed six steps to open a window into the truss, and handed Tamsin a flashlight. "Before you touch a drive or a chain, learn the frame," she said. "Everything in an escalator is held in line by the truss and steered by the tracks. When steps start to rattle, rub the skirt or hit the combs, the cause is often in the structure: a track joint that has opened, a support that has settled, or a frame that was never set level." This lesson covers the truss, its supports, the track system that guides the steps, and the geometry that ties them together.

The truss

The truss is a welded steel frame, usually made of angles or rectangular hollow sections, that runs the full length of the escalator from the upper landing to the lower landing. It is a bridge: it carries its own weight, the weight of every part inside it and the passengers on the steps, and it delivers all of those loads to its supports at the two landings. Each end of the truss rests on a support angle or bearing plate on the building structure, and long units also rest on one or more intermediate supports along the incline. The truss is designed for the passenger load set out in the code and for the building's seismic requirements, and its deflection under load is limited so that the tracks stay in line. Transit guidelines go further, asking for deflection under live load of no more than 1/1000 of the span, because heavy-duty units run loaded for hours.

The truss is fabricated in the factory, often in two or three sections for transport, and the sections are bolted or welded together on site. At Brackenridge, E7's new truss came in an upper section with the drive and a lower section, which is why it needed two separate lifts in ELV-130. Inside, the truss carries the drive machine and controller at the upper end, the tension carriage at the lower end, the track system along its length, the handrail guides and drives on the balustrade brackets, and the drip pans that collect oil and dirt. The outside of the truss is clad by the building's finish: ceiling panels below and side cladding that the escalator contractor does not normally own.

Supports and how the truss moves

A truss is not fixed rigidly at both ends. One end, usually the upper end, is located on its support, and the other end rests on a bearing that lets the truss slide slightly as the building moves, the steel expands and contracts with temperature, and the floors deflect under load. The bearing plates have a minimum overlap with the support angle so that the truss cannot fall off its support if the building sways in an earthquake, and the code's seismic provisions in Part 8 set the overlap and the restraints for each seismic zone. The Pacific Northwest, on both sides of the border, is a high seismic region, so every escalator Northcrest maintains has seismic restraints, and the inspector checks the bearing overlap and the gap at the free end.

Intermediate supports are steel posts or hangers that carry the truss partway along the incline. They are shimmed during installation so that the truss is straight and the tracks are in line, and they are one of the first things Nell checks when steps begin to bump or rub at the same place on every revolution. A support that has settled, been knocked by a cleaning machine or been re-shimmed badly during building work will put a kink in the track. On a mall unit with a short span there may be no intermediate support at all; on the transit station's 36 ft rise units there are two.

The geometry of the incline

Two numbers fix the size of an escalator: the rise, which is the vertical distance between the finished floors at the two landings, and the inclination, which in North America is 30° for almost every unit, although transit designers sometimes choose 27° for long rises to make the ride easier for passengers. From those two numbers, simple trigonometry gives the horizontal run of the incline and its sloping length. The truss is longer than the incline because it extends horizontally beyond each landing to hold the drive machine at the top and the tension carriage at the bottom, and because the steps need flat runs at each landing before they meet the combs. Building designers use the overall length to reserve floor space and the support reactions to design the floors.

import math
rise = 18.0           # ft, finished floor to finished floor
angle = 30.0            # degrees
upper_ext = 8.5       # ft, upper landing extension for drive and flat steps
lower_ext = 7.2       # ft, lower landing extension for tension carriage and flat steps
run = rise / math.tan(math.radians(angle))
incline = rise / math.sin(math.radians(angle))
overall = run + upper_ext + lower_ext
print(f"horizontal run of the incline: {run:.2f} ft")
print(f"sloping length of the incline: {incline:.2f} ft")
print(f"overall truss length:           {overall:.2f} ft")
for name, span in (("one span", overall), ("two spans", overall / 2)):
    print(f"{name}: deflection limit at span/1000 = {span / 1000 * 12:.1f} in")

horizontal run of the incline: 31.18 ft
sloping length of the incline: 36.00 ft
overall truss length:           46.88 ft
one span: deflection limit at span/1000 = 0.6 in
two spans: deflection limit at span/1000 = 0.3 in

For Brackenridge's 18 ft rise, the incline is twice the rise long, because the sine of 30° is one half, and its horizontal run is about 1.73 times the rise. The steps on the incline therefore cover about 36 ft of slope, and the whole truss is about 46 ft long. At the transit station, the longer rise and the gentler 27° slope make the truss more than twice as long.

The track system

Inside the truss, the steps run on tracks. Each step has two pairs of rollers: the chain rollers at the front of the step, carried by the step chain, and the trailing rollers at the back of the step, which run on their own track. The two tracks are set at different heights along the incline so that each step stays level as it climbs, and they come together at the landings so that the steps flatten out into a level walking surface. Between the incline and the landings, the tracks follow transition curves. The upper transition has a larger radius than the lower one because steps arriving at the top from the incline are still rising, and the code links the minimum radii to the rated speed. On the return side, below the passenger side, a second set of tracks carries the steps back upside down.

Tracks are made in sections bolted to the truss, and the joints between sections must be smooth and in line. An open or stepped joint makes the rollers jump, which passengers feel as a regular bump and mechanics hear as a click on every step. Worn tracks, particularly at the transitions where the loads are highest, cause the steps to sit low or tilt, and a step that runs low can catch the comb or ride against the skirt. At the landings, hold-down tracks keep the step rollers from lifting, so that a step cannot rise into the comb. Tamsin's flashlight survey of E5 found a lower transition joint on the right-hand trailing roller track with a visible step of about 0.04 in, which matched a click that the night cleaner had complained about for weeks.

Flat steps and the landings

As the steps flatten out at each landing, they form a short level walking surface before the comb, so that passengers can find their feet before stepping on or off. The code requires at least two and no more than four flat steps at each landing, measured at the comb; transit guidelines ask for three below a rise of about 33 ft and four above it, because transit passengers are often hurrying and carrying bags. The number of flat steps affects the length of the truss and the design of the transitions, so it is fixed when the escalator is ordered. Mall units like Brackenridge's have two; the transit station's have four at the street entrance and three at the platform level.

Above the truss at each end, the building structure forms the landing pit, where the drive or tension carriage sits below the landing plate. The pit must be kept clean and dry: water from cleaning machines, spilled drinks and, at outdoor units, rain and snow all find their way into it. Oil, dirt and paper collect in the drip pans along the bottom of the truss and are a fire load if they are not cleaned out, which is why cleaning the truss is a routine maintenance task in lesson 11.