A Program Management Approach: Design Challenges (Part 3)

This section will cover considerations related to?(5) Elevators, (6) Stack Effect, and (7) Security & Threat / Risk Assessment.

You may refer to the previous article that covers (3) structure (4) building movement HERE

(5) Elevators?

Elevators are the primary means of vertical circulation in super tall buildings, and the elevator scheme will arguably have the most significant impact on the massing of the building, the core layout, and the overall floor efficiency.? Designing the vertical transportation system for super-tall buildings requires considerable research and knowledge of elevator manufacturers in order to specify the performance requirements for the elevators in a Class A type building where passenger wait times and travel times are less than 30 seconds.????

Occupant load.? One parameter that needs to be established by the owner and architect is the building’s occupant load.? This, along with specified wait and travel times, determines the elevator needs.? Densities from as low as eight square meters per person to as high as twelve square meters per person are typically selected for Class A office space, and, in a super tall tower, different densities may be assigned to different building zones.?

Double Deck Cabs & Destination Dispatch. In order to transport the large population of occupants during peak periods and to minimize the space the elevator shafts take up, high-speed double-deck elevators traveling upwards of 10 meters per second are commonly specified. Utilizing modern programming algorithms has allowed for double-deck lifts to become an efficient means of moving passengers while minimizing core shaft spaces. While some premium systems allow for variable floor heights, most double-deck systems are conceived with the notion of standardized floor-to-floor heights, thus adding a constraint to the building design.??

Destination dispatch is another strategy that can reduce wait times and can also be used on double-deck systems. In a destination dispatch system, passengers select their destination floor in the elevator lobby and a queuing program identifies the elevator with the shortest wait time and travel time and instructs the passenger to take that elevator. This is opposed to a conventional system where the passengers select either up or down in the elevator lobby and then select their destination floor after they get into the elevator. Destination Dispatch is effective during ingress when passengers going to various floors are grouped together to minimize stops. For egress, such as lunch or the end of the day, where nearly all passengers have the same destination, this system does not provide much improvement. The figure below illustrates the destination dispatch concept.?

Destination dispatch illustration?

Piston Effect. As air is pushed by the elevator cabs through the shafts, pressure ahead increases, and noise can become an issue. This is called the piston effect and relief is provided by venting of the shaft at the top and bottom and providing aerodynamic shaped shrouding on the lift cabs to cut the air.??

Building Movement. Lifts are designed to match building sway tolerances and sensors are in place to slow or stop lifts when limits are exceeded. Additionally, cables may resonate with the building frequency in supertall towers during high winds or other lateral building movements. This is managed by adding a supplementary damping mechanism to the cables or by programming the lifts to “home” to floors to shorten the cable lengths.?

Varying Floor to Floor Heights. In addition to the advance of double-deck elevator cars, some manufacturers have developed double-deck cars where the cabs can separate slightly, allowing for different floor heights in the building serviced by the same elevator. At least one manufacturer is providing dual-cabs that both run in the same elevator shaft, operating as the double-deck at the lobby and then independently in the shaft (carefully choreographed and programmed by sophisticated control processors).??

Full Height Elevators. While most elevators in supertall towers work in zones, there are always some elevators that run the full height of the building, including firemen’s lifts, observation deck lifts, and service lifts. As buildings grow from super tall to mega-tall (above 600m), and the weight and other limitations of steel cable become a limiting factor, manufacturers have begun developing alternate materials and manufacturing methods. One advance currently on the market is carbon fiber elevator cables which weigh less than steel cables and, therefore, consume less power. Carbon fiber also does not stretch or elongate as steel cables do when spanning hundreds of meters. Of course, the cable with the least weight and stretch would be the “cable that does not exist,” and there are elevator manufacturers working on systems that replace cables with linear motors to move the cab. However, this innovation is not expected to be on the market for another half a decade.?

(6) Stack Effect?

The stack effect is caused by temperature and pressure differences between the outside of the building and inside of the building shafts.? Depending on the climate, the stack effect can be a function of differential air density including rising hot air to a cool exterior (cold climate requiring interior heating) or falling cool air to a hot exterior (reverse stack effect) as illustrated in the graphic below.????

Implications.? Potential stack effect impacts include 1) difficulty opening and closing doors; 2) whistling airflow through doors and cracks; 3) elevator door operability and whistling; 4) difficulty balancing or control mechanical systems; 5) increased energy costs due to uncontrolled airflow from outdoors (in cold climates) or loss of conditioned air to the outdoors (in hot climates), and 6) ingress of odors from outdoors.?

Mitigation & Management.? Identifying and mitigating stack effect impacts is an inexact science and variables include local climate, tower configuration, wind conditions, length and openings in shafts, and applied controls.? Computation flow dynamics (CFD) can be used during the design to model and analyze stack effect impacts. This should be done early in the design process after initial core configurations have been established and before detailed lift lobbies and access controls are completed.?? Minimizing or eliminating direct leaks from the exterior to the interior is a major control methodology such as providing closed lift lobbies, revolving doors, and tight cladding design.? Other methodologies include pressure balancing such as shaft exhaust, floor MEP balancing, or direct ventilation.

(7) Security & Threat / Risk Assessment?

Super-tall buildings are high-profile targets that produce international media attention when threatened or compromised.? Accordingly, they are common targets for would-be perpetrators looking to impact either the tenants, the owner, to garner attention, or to make a political statement.? Due to the long service life of these towers, the design must anticipate not only today’s needs but those of the foreseeable future.? These are challenging decisions because, unlike other design disciplines where there are well-defined code requirements, decisions for security and blast protection are at the option of the owner.? An owner may opt to provide no blast protection, which is typical for most buildings, or they may opt for the protection of the structure against catastrophic failure only, or they may opt to provide as much protection to the buildings occupants as can reasonably be afforded.????

Threat & Risk Assessment Report.? Should an owner decide to implement enhanced security and blast protection, they should retain a specialist security consultant to provide a Threat and Risk Assessment (TARA) Report to identify potential security threats, develop strategies to deter or mitigate those threats, and allow the owner to make informed decisions regarding what to implement.? The TARA should be started in the early design stage because different strategies to mitigate risk can have different costs and be less architecturally imposing.

Design Considerations.? Proper planning of screening points and access points to prevent potential threats from getting too close to the building is a cost-effective security measure.? For example, bollards or a plinth incorporated into the landscaping can serve to keep potential threats at a distance is more cost-effective than fortifying the building structure itself against an impact by a vehicle loaded with explosive material.? Once this is done, the building itself should be looked at for fortification.????

In terms of design, the most significant threat is normally that of explosions or blast effects.? Glass is the most fragile building material and, in the event of an explosion, glass fragments historically account for more injuries and fatalities than the pressure of the blast itself.? Therefore, the design of the glass where it could be potentially exposed to a bomb threat needs to be considered.? The typical remedy is to use laminated glass on floors of a building that are most likely to be exposed to a bomb threat.??

Building Structural Considerations.? One of the most basic levels of security consideration is that the tower is able to withstand an attack without collapsing.? To make a structure more blast-resistant, the design community has developed a “progressive collapse design” which provides the structure the ability to withstand losing anyone primary member without failing.?? In addition to checking structural member performance, designers need to check the other components and their performance as well.? For example, floor slabs designed for gravity loading that are suddenly placed in reverse load condition due to the upward pressure of a blast may lose their structural integrity and collapse downward once the blast wave subsides if they are not designed to withstand such an event.??

Mechanical & Electrical Considerations.? Another security consideration is fortification of MEP systems, particularly those required for emergency use.? This is normally accomplished through redundancy and separation between the primary and redundant equipment.?? This can also be aided by fortification of vulnerable MEP areas.??

Security & Monitoring.? Conventional systems such as security access control and CCTV systems should be designed to provide monitoring and surveillance of the building perimeter, entry, exits, fire egress, and critical system rooms and plant rooms. Access Controls provides door control with proximity card readers, biometric readers, door monitors, and magnetic door locks to provide a secure zone. CCTV provides coverage video surveillance with motion and infrared sensing to monitor secure spaces.? Security systems can lock down areas providing secure isolation to any possible threat to the building.??

Buildings are meant to house people and super tall towers are meant to house thousands of people who need to access the tower in a reasonably stress-free and timely manner.? Balancing the need to allow convenient vehicular and pedestrian access with the need to keep the building constantly secured against an attack is a difficult balancing act.? Getting it right involves the collaboration of the design team, the owner, and all project stakeholders to create a system that runs smoothly while maintaining safety.?