By Tibor Kokai, Ph.D., P.Eng., Yolles
he thriving real estate market in cities worldwide has made the development of tall residential and mixed-use buildings financially attractive. In Canada, condominium towers between 40 and 82 storeys...
he thriving real estate market in cities worldwide has made the development of tall residential and mixed-use buildings financially attractive. In Canada, condominium towers between 40 and 82 storeys high are becoming more common. When properly executed, these high-density investments bring profits for developers and they benefit the construction industry through spin-off jobs.
The design of tall residential structures, however, carries a particular set of engineering challenges for the structural engineer. Compared to the usual mid-rise and small apartment buildings, very tall buildings have much more stringent requirements for controlling movement and response to earthquake and wind. Even compared to tall office buildings, they have stricter criteria for horizontal accelerations during building sway. Also, to help ensure the project is a viable development, the engineer must consider not only structural performance, but also overall cost, architectural space, constructability and speed of construction.
Structural supports, sway and sleeping residents — design criteria
Design criteria governing all buildings in Canada are detailed primarily by the National Building Code of Canada, along with codes and guidelines from other government agencies and associations such as the Canadian Standards Association and the American Concrete Institute.
The two most important design loads in the design of tall structures are the gravity and lateral loads, one acting vertically, and the other horizontally. Gravity loads include the self weight of all installed structural, architectural, mechanical and electrical components, and the so-called live loads of items such as furniture, partitions and occupants. Lateral loads include the wind and earthquake loads and the soil pressures.
Both the gravity load design and the structural floor systems for tall residential buildings are practically the same as those used for low or mid-rise ones. The difference is in the supporting structure (columns and walls), which have to be more robust for the tall structures since they carry more floors, thus more weight and larger lateral loads.
Designing for lateral loads is usually a greater challenge for tall buildings than for mid or low-rise structures since the response to wind and earthquake excitation is more pronounced. The taller the building the more it sways. To keep the sway within the limits defined by the building codes, tall buildings require a stiffer lateral load resisting system (shear walls and moment frames). Consequently, the overall building cost per unit area is more for tall buildings than for low or mid-rise apartment buildings with the same floor system.
One especially important criterion for tall residential building design is the assessment of the horizontal accelerations of the occupied floor levels. In any tall building excessive horizontal accelerations or torsional velocities may cause the occupants to experience motion sickness. However, the difference is that people feel motion and acceleration more when they are asleep. For residential buildings, therefore, there is a stricter limit on the allowed accelerations. Approximately 1.9% of gravity is the current acceptable upper limit for horizontal accelerations for all residential buildings due to winds (calculated for the maximum wind occurring every 10 years). For office towers the limit is 3% of gravity.
The numerical assessment of tall structures may require non-linear finite element analysis or a linear iterative approach when assessing horizontal movements and accelerations in wind or earthquake. Models of tall buildings are generally tested in wind tunnels to assess the appropriate wind loads and accelerations. For some very tall or slender towers, where the structure cannot provide adequate stiffness, the accelerations are controlled by adding active or passive damping systems, as is the case with the 52-storey, 1 King Street West Tower in Toronto, where a tuned mass damping system is used. Not only is there an added cost of providing the damping equipment, but also these systems have to be located in the penthouse, in prime space that would otherwise reap handsome rewards for the developer.
The sway limit of buildings is defined in codes through the so-called inter-storey drift limit. This limit specifies the maximum movement any floor can undergo for any type of building, and it is defined as the floor-to-floor height divided by 200 to 1,000. The code recommended value is approximately 500, but the value used for a particular building depends on the cladding system, type of partitions used and their interface detail. Obviously the taller the building, the harder it is to maintain the sway limits. When sways occur that are larger than those recommended, the interface between the partitions and cladding, and between partitions and the floor structure above, will undergo deformations they cannot tolerate.
Floors and buildability
There is no real difference in the design and construction of the floor systems for high-rise as opposed to smaller buildings. However the floor system can have a serious implication for the speed of construction and the building height and, thus, affect the cost. For a tall building, the preferred floor systems should be lighter — to reduce the weight; shallower — to reduce the overall height; and easier to construct — to increase the speed of construction.
Usually it takes a three to four day cycle to construct a typical floor, so a full day’s difference in a construction cycle for a 50-storey tower can have a significant impact on overall cost. Minimizing the floor-to-floor height of the typical floor will yield the most cost-effective building — but not necessarily the most functional or aesthetical one.
In Canada, reinforced concrete flat plate is the most commonly used floor structure for both a typical mid- or high-rise residential tower spanning approximately 7.0 metres or less. This structure not only safely carries the loads to the supporting walls and columns, but also accommodates the myriad of electrical and data conduits and exhaust ducts from the kitchen and bathrooms, while ensuring the proper fire and sound rating between the floors. In a high-rise tower, fine-tuning the design of a typical floor is very important, since every mistake or superfluous reinforcing bar is amplified by the number of typical floors.
For larger spans, reinforced concrete or structural steel beam and slab systems may be more feasible. However, these systems are usually deeper or thicker than the flat plate, so they require an even taller building.
All floor systems are required to meet the vertical deflection criteria defined by building codes, as well as the criterion determined by the type of cladding system supported by the edge of the floor structure. The deflection criteria can vary from span/300 to span/600. Floor systems that do not meet the design criteria can affect the performance of the cladding system and finishes, leading to problems for the occupants.
Squares, circles, triangles and lateral load resistance
Selecting the lateral load resisting system for a high-rise residential tower means minimizing the amount of material, construction time and costs. It also calls for reducing the impact of the structure to create as much freedom as possible for the architectural design. With taller buildings the variety of materials available for the structure is not as great as it is for smaller buildings and is usually limited to concrete and steel since these materials have the appropriate strength and ductility.
Finding the right structural layout for a tall building is a combined effort, starting with the architect’s vision, then modified by the wind tunnel and structural consultants. For various building heights there are characteristic typical floor plates, which also define the possible lateral load resisting systems.
The “spine scheme” or rectangular layout shown
in Figure 1 is the most common layout for typical low- and mid-rise condominiums. In this scheme, the central corridor separates the units, and regularly spaced shear walls are perpendicular to it. The system is inherently stiff for the building heights used, thus acceleration and sway issues do not arise. The arrangement of shear walls geometrically defines the floor support system, therefore a flat plate (one-way slab) system is generally used. This type of construction is more common in buildings up to 30-35 floors.
For “point” towers typically 40-65 stories high, a variety of lateral load resisting systems are possible, as shown in Figures 2, 3 and 4. The plan geometry for these towers can vary widely, but due to aerodynamical issues their footprints are closer to a square, circular/ellipsoidal, triangular or rectangular shape and their many variations. There are no real height limits for point towers, as long as the appropriate acceleration and sway criteria are met.
The typical structural system used for towers between 40 and 65 floors is the so-called coupled shear walls system illustrated in Figure 2. In this system, the walls are linked or coupled with overhead beams to make the wall assembly stiffer. The structural system is approximately 30% stiffer than the same un-coupled wall system would be. By introducing coupling beams, the number of shear walls, or the amount of material used in shear walls, can be reduced by approximately 30%, enabling more architectural freedom and improved unit layouts.
Within the same building height range, a variety of structural systems are possible. Figure 4 shows a central core with outrigger walls located at the mid-height and the top of the building. The two-storey outrigger walls connect the core to the perimeter columns and thus stiffen the system — similar to the way that a boat’s masts are strengthened by stays. The absence of shear walls outside the central core permits greater architectural freedom than the system shown in Figure 2.
The above structural systems can fully accommodate the kind of architectural criteria common to low- or mid-rise condominiums, with floor-to-floor height glazing, balconies and relatively cost-effective cladding systems.
In buildings over 65 floors, however, structural systems require increased stiffness to meet serviceability criteria. Figure 3 shows a perimeter frame, or perimeter tube, system which has columns connected with edge beams around its entire perimeter — this system is stiffer than the shear wall system. If augmented with outrigger walls connecting the inner core to the perimeter tube, the system can support towers of up to 100-120 floors. It does have architectural limitations, however, as balconies are not possible if the perimeter beams project upwards and the perimeter/edge beams limit the view. Also, constructing a typical floor using a perimeter tube system is slower than building a flat plate shear wall system. BCE Place in Toronto is an example of a perimeter tube system, and Yolles is currently designing two proposed residential towers in the city using a perimeter tube system.
The design process for lateral load resisting systems of point towers is an iterative and very complex process. A fine balance has to be achieved between the architectural interior design requirements and the wall locations, column spacing and room dimensions. Typically, the architectural space is compromised by structural requirements outlined by the design criteria. Usually the ideal locations for walls and columns do not match the location of the demising walls. As a result, every proposed layout is fully analyzed by the structural engineer during the unit layout design process to ensure that all strength and serviceability criteria are also being met.
Constructability, geography and the building envelope
While the serviceability and strength of a structure are calculated using defined mathematical equations, constructability issues are more subjective and dependent upon geographic region. Technologies may exist in certain regions and not in others, affecting both the design and the construction cost. While a flat plate shear wall system (Figure 1) is generally used on mid-rise condominiums in Toronto, for example, it is not necessarily the most cost-effective structural system in other large cities in Canada or in many other parts of the world.
Figure 2 shows a shear wall system where the coupling of walls successfully reduced the number of shear walls but added complexity to the contractor’s work. Nevertheless, the system was deemed more cost effective per unit area than the system shown in Figure 1.
In general, structural systems are selected based on their cost per unit area, as well as the associated construction time. As a result, in Canada, structures are generally built to ensure minimum overall cost rather than to minimize the use of materials.
The building envelope, or cladding system, is generally designed by architects and envelope specialists, but there is a critical interaction between the envelope and the structural system. The deformation limit of the cladding system must relate to the vertical movement of the floor edge (supporting the cladding) due to gravity loads, and the lateral movement of the floor edge due to wind loads. If this limit is exceeded, the cladding’s performance will be compromised. The magnitude of the movements at the critical zone at the cladding-floor edge interface will usually dictate the type and thickness of the floor system and the amount and thickness of the shear walls.
When designing tall residential towers, the structural engineer is sometimes faced with parameters that on the surface at least, might seem to be in conflict. The issues include cost, architectural space, structural efficiency, durability, sustainability, and speed and method of construction. Addressing these issues requires technical expertise, experience, and collaboration. Most valuable, however, is transferring ideas and lessons learned from one type of structure or building sector to another.
Tibor Kokai Ph. D., P.Eng. is a partner with Yolles, structural engineers of Toronto.