If you work in the construction industry or talk to Structural Engineers long enough, you will hear the term “lateral stability” used quite often. The lateral stability of a building is possibly its most important and critical characteristic, but does lateral stability in buildings mean?
In simple terms, the lateral stability of a building is its ability to remain horizontally stable when a lateral force is applied to it. The lateral force may be applied either through wind loading, earthquake shaking or a collision.
This article will explore how Structural Engineers ensure that the buildings they design remain stable when mother nature shows her wrath. There are a number of lateral stability systems Structural Engineers can choose from and each one is best suited for a particular building height, location and construction material.
What is a Lateral Stability System in Buildings
In order to ensure that a building is stable, Structural Engineers needs to define a clear load path for the horizontal load to travel from the point at which it is applied, to the buildings foundations.
The lateral stability system is a composition of various structural support elements which transmit the horizontal load from wind, earthquake or collision from the point at which the load is applied to the buildings foundations.
The next sections of this article will explore afferent types of lateral stability systems, we will go from short low-rise structures all the way to super tall skyscrapers.
|Stability System||Type of Building||Appropriate Height (Approx.)|
|Wall Bracing||Domestic Houses||9m (29.5 ft)|
|Portal Frame||Warehouse and Industrial Builings||9–15m (29.5 – 49.2 ft)|
|Shearwall||Commercial, Apartments, Hospitals, Hotels||15–40m (49.2 – 131 ft)|
|Life/Stair Core||Low to medium height buildings||15–220m (49.2 – 722 ft)|
|Coupled Core and Outrigger||Tall Buildings/Very Tall Buildings||200–400m (656 – 1312 ft)|
|Tube System||Very Tall/Super Tall buildings||250–600+m (820 – 1968 ft)|
Wall Bracing Stability System
If you live in a stud framed house comprising conventional domestic type construction, there is a good chance that your homes lateral stability system comprises a series of braced walls.
The bracing within your walls is hidden beneath the plaster (drywall) away from view. A Structural Engineer specifies the braced walls location. The braced walls are often evenly spread across your home to evenly distribute the lateral wind-loading which is applied to the building’s façade. A braced wall withstands the lateral load which is applied parallel to its length. Therefore an adequate amount of braced walls need to be orientated in both the north-south and east-west wind directions.
Where possible, Structural Engineers will locate braced walls at the buildings corners as these are the most effective locations…
Braced walls can take several forms, the most common two being either a series of metal strap cross braced bays or plywood bracing bays.
For structures that have a lager plan area and have no opportunity to add internal walls for bracing, a braced wall option may not be feasible. Moving up the hierarchy of lateral stability systems, lets take a look at the next stability system on the list…
Portal Frame Lateral Stability System
A portal frame is a structural framing arrangement comprising columns and rafters with fully fixed connections that allows the frame to withstand lateral wind loading through bending interaction between the connected columns and rafters.
The GIF below is a very exaggerated deflected shape of a simple steel portal frame deflecting under wind, notice how the rafters bend as it resists the lateral movement of the frame.
The key to a portal frames success is the critical location where the column and after meets. This is where the “magic happens”, and by magic I mean the point of largest stress due to bending as the fame sways laterally. At this location, Structural Engineers will often specify a “haunch” it is a deeper section of rafter which reinforces against this peak stress.
You will often find this type of lateral stability system in warehouses and industrial factories. These types of buildings need large open spaces relatively free from columns and walls which makes them ideal for a portal frame lateral stability system.
Next time you undertake the great Australian tradition of visiting your local Bunnings warehouse on a sunny weekend, take the opportunity to look up at the roof structure. Chances are stability to your local Bunnings store is provide by a series of steel portal frames.
With taller and heavier buildings a steel portal frame arrangement becomes an unviable option. This is where we need to start to turn up the dial on stiffness and start to look at more effective lateral stability arrangements…
Shear Wall Lateral Stability System
For mid to low rise commercial buildings constructed of reinforced concrete or composite steel, a shear wall stability system is an ideal choice.
The structural behaviour of a shear wall in a commercial building is very similar to a braced wall in your house (discussed earlier). However with the introduction of reinforcement and concrete the shear and bending capacity of a shear wall is magnitudes higher than a braced wall in your house. Shear walls can also be constructed from reinforced blockwork.
Special care needs to be taken by the Structural Engineer when designing a concrete shear wall to detail it with adequate ductility requirements to satisfy the Seismic design code of the region.
As our hypothetical buildings gets even taller, a pure shear wall stability system can become inefficient and costly. Luckily, taller buildings require concrete shafts to house lifts as well as fire escape stairs. They also require other “back of house elements” for to support the mechanical, hydraulic and electrical requirements for the building. There needs to be a place to store all of these elements, and it just so happens to be a perfect structural element to provide stability for taller structures…
Lift/Stair Core Stability System
The previously mentioned elements; stairs, lift shafts and plant rooms are usually located adjacent to one another to make the floor plate as efficient as possible. They also usually need to be fire separated from adjacent rooms on the floor. This makes it ideal to house these elements within concrete walls.
The arrangement of concrete walls which house the lift/stair/plant at each floor is called the “core” and sometimes referred to as a lift/stair “box”.
The core of a building acts very much like a steel RHS (Rectangular Hollow Section) cantilevering from its foundation and reaching for the sky. The box shape of a buildings core gives it exceptional torsional stiffness as well as lateral stiffness in both orthogonal directions.
To the right in the image below is an snapshot from an analysis model of a 24- storey office tower I am currently working on in Brisbane. The slabs, beams and columns have been filtered off from view to reveal the buildings core, the primary lateral stability element for this building.
Because the wall arrangement of a core is usually repeated over many floors within a tall building, the core can be efficiently constructed using a jump-form system.
A jump-form system (or slip form system) is a series of pre-fabricated formwork shutters connected together to form a single structure. When the concrete walls are poured and cured, the jump form system uses the newly constructed walls and its in-built hydraulic jacks to lift itself to the next level to construct the walls above. This process is repeated until the entire core box is completed to the top of the building.
Constructing the jump form system is quite a task in itself. However for high-rise buildings, the time spent in setting up the form-work system is more than offset by the automated process of forming up the walls of the core multiple times in the tower.
The vast majority of tall buildings in Melbourne and Australia use a core box as the primary element within their lateral stability system. A long-standing example of this in Melbourne is the Rialto Towers which was constructed in 1982 and stands at 250m (820 ft) .
Rialto Towers stretches about 250m into the sky. It is an example of how far you can push the limits on a building which has a stability system comprised primarily from its lift/stair core box.
When a building gets taller and more slender, the core requires assistance from other structural elements to give it the stiffness it needs to brave what mother nature can throw at it…
Coupled Core and Outrigger Stability System
When the core box alone isn’t enough to give a very tall building the stiffness it needs, the next stability system in the hierarchy is a core coupled with an outrigger system.
An outrigger system comprises three separate elements:
- Core Box
- Outrigger Walls
- Mega Columns
Outrigger walls are generally connected to the core box. At the other end they are connected to the mega columns. The connection of the core box to the mega columns via the outrigger walls increases the effective depth of the stability system. Using the cantilevered beam analogy again, the deeper the stability system (or beam) the stiffer it is.
The following images shows two stability systems, one with just a core arrangement, the other with a core supplemented with an outrigger system. This is a 240+m (787 ft) apartment building I was involved with in the Melbourne CBD.
To further understand how an outrigger system works on a building, imagine how a mobile crane operates. When the mobile crane is set-up, its stabilising legs are spread out at each corner of the crane. Once the stabilising legs are in place, this allows the crane to lift much heavier loads and at a much longer reach before it topples over. The same principle applies to an outrigger system on a high-rise building.
An iconic building which uses an outrigger system as pat of its stability framing is the Eureka Tower in Melbourne. The Eureka tower stretches to the clouds at 297m (874 ft).
There is still more opportunity to squeeze stiffness out of a high-rise tower however. When there is no other place to go, how do Structural Engineers make a skyscraper super stiff and super strong? They make the entire building envelope a cantilevered beam…
Tube Lateral Stability System
Maximum stiffness and rigidity can be achieved from a building when the Structural Engineer specifies the entire perimeter as a box beam (or tube). This is called a tube stability system. Oftentimes the buildings core also provides stiffness to the structure as well, this is often called a “tube within a tube system”.
Perhaps one of the most extreme examples of this is 111 West 57th Street in the United States (better known now as the Steinway Tower). This is probably one of my favourite buildings as a Structural Engineer. One look at the floor plan and it quickly becomes apparent that the entire building footprint resembles that of a core element.
Due to its very slender aspect ratio, the entire width of the building contributes to its effective depth and therefore its stiffness.
To put things into perspective of just how slender the Steinway Tower is, here is an image during the excavation of its foundations, you can see the width of the building at its based compared to the excavation equipment performing the dig…
I hope you enjoyed this article, please feel free to leave a comment below regarding future topics you would like to see covered on Sheer Force Engineering.
If you enjoy exploring different building systems, check out THIS link where I go through different types of basement wall construction methodologies.