Reaching to the Sky: The Fundamentals of High-Rise Design
Over the past 20 years, there was a global acceleration of high-rise construction, including in the United States. Today, high-rise development is once again on the minds of developers and municipal leaders. As cities continue to pursue policies stressing density, developers are responding by pushing structural systems to greater heights.
In promoting density, city leaders are trying to increase developmental efficiency, and what can be more efficient than building up to create space for more uses without requiring additional land?
As one of America’s premier engineering firms, WGI, Inc. has decades of experience designing buildings of all shapes and sizes, including high-rises. In this paper, we will discuss the current high-rise trends and what developers need to know about their development.
We will look at what constitutes a high-rise building and how the categories of high-rises differ, provide an overview of the Code Sections that guide design, compare structural systems and their feasibility at different heights, discuss structural shape efficiencies and building aerodynamic shape efficiencies, examine the importance of a Wind-Tunnel Test, and the limitations of a code-only based approach.
WHAT IS A HIGH-RISE BUILDING?
A high-rise is any building that is greater than 75-feet tall, according to the International Building Code (IBC). This typically means a building of five floors or more. However, there are different levels of high rises, with anything over 420 feet considered a Super High-Rise.
A building of 492 feet (150 meters) is considered Tall, or a Skyscraper; a building of 984 feet (300 meters) is considered Supertall; and a building of 1968.5 feet (600 meters) is considered Megatall. Looking at the largest buildings in Texas cities, Houston’s JP Morgan Chase Building is Supertall at 305 meters. Dallas’ Bank of America Plaza (281 meters), The Independent in Austin (212 meters), and San Antonio’s Marriott Rivercenter (166 meters) are tall.
DESIGN APPROACHES BY HEIGHT
While certain rules, such as aspect ratio, apply to all high-rises, all high-rises are not the same. As a result, subcategories of buildings have their own design rules. As structural engineers, we account for the strength of materials and serviceability (the vertical/lateral displacement of a member or a group of members) in our designs.
In addition, building accelerations need to be considered when structures exceed a certain height. When dealing with high-rise design, building behavior characteristics must be evaluated; however, one may have a greater impact than the other depending on the height of the structure.
While not considered high-rises, short structures (less than 25 feet) have a Height-to-Length (H/D) ratio <1. This lower H/D ratio provides greater strength, and building movement is easily controlled. Strength typically dictates the layout and design of the lateral force-resisting system since lateral displacements are typically small.
With short structures, the architect has more control in the column/wall/brace layout, and the engineer has less control in the form and function of the building.
Typical building functions of short structures include:
- Mixed-Use Development
- Office Buildings
- Multifamily Developments
- Health Care
For intermediate structures (or squat structures), lateral displacement begins to play a bigger role in the design of the lateral force-resisting system. Intermediate structures have a height-to-length ratio of 1<H/D<3. As buildings get taller, they experience more movement so it’s incumbent upon designing engineers to consider both building strength and building movement. Architects must obtain more input from engineers regarding bracing locations earlier in the design process. This provides for more efficient building behavior.
Typical building functions for intermediate structures include:
- Mixed-Use Development
- Office Buildings (Individual Lease)
- Hospitality (Mid-Tier)
- Health Care
Finally, tall structures (or cantilever structures) have a H/D>3. Building movement, which includes both lateral movement and accelerations, will have a significant impact on design because of the building’s height. Typically, more effort is required in taller buildings to control this movement. Stiffness controls building movement; the stiffer the design, the less it will move. The engineer needs to be involved at the earliest stages of the project, guiding the owner to the most beneficial outcome for the building. Due to increased building accelerations, users are more likely to feel movement at the upper levels of the structure; therefore, the engineer must pay greater attention to user comfort in the building’s design.
WIND DESIGN SHAPE EFFICIENCY
Wind load is one of the most challenging aspects of designing a high-rise building. Wind impacts its structural integrity by causing stress on the building itself. The wind will impact the comfort of users if it causes excessive sway and accelerations. It is also important to consider the wind effects from surrounding structures. For these reasons, the building’s shape will considerably influence the building’s ability to shed wind.
The building’s shape will impact efficiency. Typically, round buildings create the least amount of wind drag, with triangular designs being slightly less efficient, and square designs the least efficient. An inefficient design can create vortex shedding, which is an oscillating flow of air as it passes the building at certain velocities. Depending on the size and shape of the building, vortex shedding can exacerbate building movement and acceleration, which can potentially affect the building’s occupants.
In addition to the building’s shape, there are other strategies that can provide shape complexity—to “confuse the wind”—and minimize surfaces for the wind to interact with or grab. These strategies include adding stair-stepping corners, through-building openings, and rotating or twisting building forms along its height.
How do you know which strategies to use? Wind-tunnel testing needs to be part of the design process. The testing should be site-specific, based on the building’s dynamic properties. In addition to demonstrating any accelerations that will be created upon the building by anticipated wind levels, the testing also determines occupant comfort. Testing should include component and cladding pressures, and it should also address directionality and its effect for various recurrence intervals (for instance, 10-year wind vs. 50-year wind events). As it pertains to cladding pressures, there is a good probability that the results will be lower from the code-approved wind-tunnel testing than those determined by the alternate methods in the governing building code. This potential reduction in wind pressure can have a significant impact on the reduction of the costs related to cladding and cladding connections.
Wind-tunnel testing should also examine how supplementary active and passive damping could reduce building accelerations. As a rule, concrete is more effective than steel as a damping material because of its mass. Supplementary damping approaches including passive viscoelastic dampers, tuned slosh dampers, tuned mass dampers, tuned liquid column dampers, and simple pendulum dampers.
The primary consideration in high-rise design is to minimize the impact of lateral load — most commonly wind load, seismic load, hydrostatic, and earth pressure. Buildings developed in areas that are prone to earthquakes must prioritize seismic load. These differ from wind loads because they are typically instantaneous, rather than continuous. Finally, hydrostatic and earth loads can exert significant pressure against below-ground structures such as basement walls and retaining walls.
As it relates to overall building efficiency, the building should have an aspect ratio of 1:6, meaning every 6 feet of height requires 1 foot of width (although an aspect ratio of 1:10 is permissible in certain cases). Ultimately, the bottom line is that the higher you build, the wider you need to build, to mitigate the impact of wind and other lateral loads.
COMPLICATED BUILDINGS REQUIRE SPECIAL EXPERTISE
America is in the midst of a high-rise development boom. However, designing and developing high-rise buildings is a complicated process requiring precise engineering and testing to assure that the design takes into consideration all the factors that can impact the building and its performance. WGI has the experience and expertise you need to design a high-rise building meeting your unique needs.
Let the experts at WGI help you design a building that will fit perfectly onto your site and provide exceptional performance.
For more information about this report or to have a conversation with one of our experts, please contact us:
Mike Oler, PE
Victor Cordova, PE
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