Mid-rise building-transportation considerations for architects and designers.
By Jeff Walker, thyssenkrupp Elevator
These days, it’s all eyes to the skies, and for good reason. The number of high-rise buildings has tripled since 2000 as urbanization demands are driving the skyscraper boom. While these massive structures and the technology supporting them are stealing all the headlines—you may have heard about a sideways-moving elevator that was unveiled this year—mid-rise buildings remain equally vital in urban and rural environments.
Designing for mid-rise buildings (7 to 20 stories) can present numerous challenges for architects and designers, especially as it relates to the heart of the building, the elevator. There are numerous elevator-related criteria that architects and designers must consider in order to maximize building-transportation efficiency, starting with selecting the correct type of elevator for a project.
Traction elevators are most commonly used for mid-rise applications as they travel much faster than hydraulic elevators and can travel to greater heights than hydraulic elevators. Traction elevators use a hoisting system consisting of multiple belts or steel cables connected between an elevator car and a counterweight. The inclusion of multiple ropes increases suspension safety and reliability. The ropes are wrapped over the machine drive sheave in grooves, where the friction drives the hoisting ropes. Counterweights make elevators more efficient by offsetting the weight of a car and its passengers, so the motor doesn’t have to work as hard.
Gearless traction elevators, which have the drive sheave attached directly to the motor and can travel as far as 2,000 ft., are often more durable than the building itself, use energy more efficiently, and can deliver energy back to the building.
There are two types of traction elevators, machine room (MR) and machine room-less (MRL). Machine room includes larger mechanical components for higher duties, but requires additional space for equipment outside the hoistway. Meanwhile, machine room-less has compact machines located in the hoistway but is limited in duty (i.e. speed and capacity).
When designing a mid-rise building, architects should consider the following:
• population (number of occupants and visitors)
• distribution of people by floor
• typical times of arrival and departure
• critical traffic periods
• energy efficiency, operation cost, and maintenance cost
• alternative dispatching methods, such as conventional dispatching (estimated time of arrival) or destination dispatching (estimated time to destination).
Comparing estimated time of arrival (ETA) versus estimated time to destination (ETD), with ETA, passengers have a shorter wait time but longer passenger ride time, while with ETD, there are longer wait times but shorter passenger ride times. In an ETA situation, the elevator has one piece of information to analyze—the direction. Regardless of the number or demand of people in the lobby or their destination, the elevators work to bring one car down as quickly as possible.
The lone part of the ride that can be shortened is the ETA, but there are some things that can’t be shortened or predicted, such as the uncertainty surrounding how many people are boarding each elevator and where they want to go. This can result in all passengers crowding into one elevator, and a number of multiple floors serviced on each run. Now imagine this happening throughout the entire building. This is common to all ETA systems; the only improvements have been some logic enhancements, such as parking and zoning. But the only part of the ride that can be enhanced is the waiting time until the elevator arrives.
Also, in large elevator banks, when a call is placed in the hall, only one car will arrive to serve that request. The only way a second car will come will be when the first car doors close and another call is registered.
Meanwhile, in an ETD situation, all passengers register their desired destination using a touchscreen kiosk in each lobby, providing more information for the system to analyze. The entire ride of each passenger is known before the elevator arrives, where a variety of things can now occur. The elevator could bring multiple cars down at the same time if a high demand is recognized. Passengers can be grouped in to similar destinations, shortening the ride time. The entire time to the destination can be made more efficient. In some instances, a passenger may wait a little longer to get assigned their appropriate elevator, but ultimately it will make fewer stops enroute to their destination.
A traffic study should be performed when considering elevator options for mid-rise buildings. A traffic study is vital to better assessing how many elevators and groups are needed, as well as speed, capacity, elevator type (MR or MRL), dispatching needs, and handling capacity. For the amount of elevators needed, assess the type of traffic (i.e. office buildings, hospital, residential), the predominant direction of travel, and the intensity of traffic.
All aspects of movement within a building also need to be considered, factoring in lobby size, corridor size, the number of elevators needed, size of elevators needed, and stairways. But what about the space needed for the actual elevator hoistway and platform? Space for rails on either side of the car platform must be accounted for, as well as the potential presence of seismic equipment, which will require additional space. Other considerations include space for counterweight (rear or side mounted), the door space in front of the hoistway, and out-of-plumb tolerance.
Don’t forget to factor in overhead size. When an elevator is stopped at the top landing, additional space is required to accommodate the mechanism on top of the elevator car that operates the door, the elevator lifting structure or crosshead, the car enclosure ventilation system, sufficient space for elevator maintenance and inspection personnel, overhead structures, as well as counterweight run-by and buffer stroke.
Shifting gears to the lowest floor of the building, space will also be needed to accommodate the platform frame, the safety plank that supports the elevator safety device, buffers that stop the elevator if it travels at an operating speed past the lowest floor, refuge space, an elevator-stop switch, access ladders, as well as sprinklers and a sump pit, where necessary.
For those leaning toward a traction elevator with a machine room, the size of that room must be determined. While equipment can vary, standard options typically include a hoisting machine and elevator controller and drive, governor for safety application, a main electrical power disconnect switch for each elevator, lighting and ventilation, and access for maintenance personnel.
Another key consideration is venting, which is an absolute necessity for buildings over four stories, accordiong to building codes that require venting of the hoistway to the atmosphere. In mid- and high-rise buildings, open vents can induce vertical air movement throughout the building, leading to a stack effect, which can create enough pressure to restrict elevator door operation at certain floors, particularly during winter months. Make sure to consider location and size of the vent openings as it relates to hoistway and machine room equipment.
Electrical energy is needed to run a traction elevator, however that energy can be reduced with the assistance of the counterweight. Regenerative drive technology is important as it generates power back into the building system. Consider automatic fan and light shutdown and LED lights as these components can reduce a cab’s energy consumption by 90%. Emergency power systems are also vital; they run traction elevators in the event of a power loss, which is critical in buildings such as hospitals.
Know code requirements
International Building Code 2012 & 2015 require Fire Service Access Elevators (FSAE) for buildings with an occupied floor of more than 120 ft. above the lowest level of fire department vehicle access. No fewer than two or all elevators shall be FSAE. As part of being up to code, the FSAE elevator must be able to accommodate an ambulance stretcher.
What are some of the other code requirements for FSAE? Hoistway lighting must be the entire height of the hoistway; protected lobby (greater than or equal to 150 sq. ft.) is required for all floors except the level of exit discharge, or the predetermined floor where passengers get off the elevator in case of an issue; and wiring and cables located outside the hoistway need to have a 2-hr. fire resistance or be protected by construction having a fire resistance of at least 2 hr. Also, standby power needs to be online within 60 sec. of power loss with a minimum of 2 hr. endurance to power, and standby power must support elevator equipment, ventilation, and cooling equipment for elevator machinery and control spaces, as well as elevator car lighting.
Finally, architects and designers need to be up to speed on the latest innovations and technology. For years, the mid-rise elevator market has been stagnant—until now. This year, a mid-rise elevator is being introduced that offers speed, travel, and capacity combinations that improve operational efficiency and the experience for the passenger.
The elevator uses belts instead of steel cables as belts can have a longer life, and their smaller sheave sizes translates to smaller overhead space. Also, reduced hoistway size means more leasable or usable space. The mid-rise elevator also provides building owners with an option they are unfamiliar with—they will no longer have to sacrifice size for speed; this elevator can have as much as a 5,000-lb. capacity and still go 500 ft./min.
Regenerative drives are standard, which will feed unused energy back into the building rather than sending that energy to the machine room just to be dumped as heat. Predictive-maintenance technology is also standard, which will result in maximum uptime and decreased service time.
A series of elevator enhancers can also be included with the mid-rise elevator to further improve passenger wait and travel times in a safer, more personalized environment. These enhancers can allow building managers to group and assign passengers to elevators, move people more rapidly to their destination, adjust passenger flow in real time, and personalize touchscreens and kiosks like never before.
Jeff Walker is vice president of technology with thyssenkrupp Elevator, Alpharetta, GA (thyssenkrupp.com). With the company for 33 years in various capacities, he has a B. S. in Mechanical Engineering from the Univ. of Memphis and is currently the chairman of the Mechanical Design Committee with ASME A17.1/CSA B44 Safety Code for Elevators and Escalators.
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