High-Rise Concrete Construction
Concrete is a near ubiquitous building material. In Ontario for instance, almost every building uses concrete in some fashion, whether in the form of foundations, floors, or even just floor toppings. It is used in structures from 1-storey low-rises all the way up to skyscrapers. High-rises in particular regularly use concrete, whether that’s a 10-storey building with concrete shear walls and hollowcore floors or a 30-storey tower using concrete cores, floors, and columns. Concrete high-rises are commonly used to house and shelter us in the form of residential apartments, retirement apartments, and long-term care homes.
co-elevate was the structural engineer on the team behind high-rise concrete structures such as the Village of Erin Meadows in Mississauga, the Village of Tansley Woods in Burlington, and Roseland Apartments in London
What makes up a concrete high-rise?
Floors in concrete high-rise structures are typically cast-in-place, post-tensioned, or precast concrete slabs. Cast-in-place and post-tensioned slabs may be supported by beams below the slabs or directly by columns. Precast hollowcore floors supported by walls and beams are common in shorter high-rise buildings, though they are increasingly being used in structures taller than 20 storeys. Total precast structures are also regularly being built beyond 20 storeys range, though they are beyond the scope of this post as we could write an entire post on total precast! Other popular systems include the Hambro system by Canam and composite deck systems.
The vertical load bearing systems in high-rise concrete buildings consist of a mix of walls and columns. Columns pass load down in compression, which can become quite large as you approach 30 storeys tall. Even a 10-storey building can see regularly column loads in excess of 8,000 kN , or somewhere around 200 full grown elephants! In high-rises, columns are often relatively small and lower strength at the top of the building, but they get larger and increase in strength as that load adds up floor by floor down the building; it’s not unheard of to see 40MPa, 50MPa, or even 60MPa mixes at the bottom of some high-rises!
Walls can be independent walls or assemblies of walls, such as the concrete cores around elevators and stairwells. Both subtypes of walls are used to resist gravity loads through compression and lateral loads (like wind and earthquakes) through shear and bending. High-rises usually reduce their wall sections and reinforcing as you go up the building to save concrete and steel, though some walls, like those around elevator shafts, may stay the same size for the full building height to ease installation of sensitive equipment.
Concrete columns and walls often work together to resist loads in high-rise buildings. In this case, the hollowcore floors are supported by Deltabeams, which are supported in turn by walls and columns.
All of this load then gets dispersed by the foundations into the soil below to minimize settlement. Foundation systems are their own specialty and vary drastically depending on size, soil conditions, and more, but often fall into conventional strip & pad footings, piled footings, or raft footings for high-rise structures.
Foundations are where structural engineers and geotechnical engineers overlap. Conventional strip footings and pile systems both serve to support major buildings across Ontario and the world.
Concrete challenges
Concrete is known for many things, but ‘light’ is not one of them. A concrete building weighs more than comparable steel and timber buildings, though this isn’t usually an issue within the building structure itself. The challenges arise when you reach podium levels that realign floor plans and the foundation level where the weight has to disperse into the soil. Fortunately, structural engineers have tools to provide load transfers to support the weight, though some of those transfer elements can begin to look almost like small bridges inside a building!
Sustainability is another common challenge with concrete structures. The production of cement is quite carbon intensive, which is why the industry has been pushing for more supplementary cementitious materials (SCMs) like fly ash, slag, and silica fume to reduce the cement needed in the mix while simultaneously using waste materials. Recycling old concrete as aggregate is another common way that concrete suppliers try to reduce their impact. Stone and sand are common resources, but they are still finite, and reusing what would have been discarded concrete also allows us to reduce the impact on quarries.
The number of trades involved in concrete work can also be quite significant. Ironworkers, carpenters, pump operators, concrete finishers… the list goes on, but there are many people who need to communicate and work together to build concrete. Thankfully, reinforced concrete has been around for some time, and the trades involved know the intricate coordination well to provide strong foundations and structures.
It’s all in the details
How a concrete building goes together can make a big difference, and that’s decided by the detailing of the structure. Adding too much reinforcing in the wrong spot can actually lead to brittle failures, while adding too little reinforcing can lead to excessive cracking, movement, and premature failure. Structural engineers and reinforcing detailers work together to ensure that reinforcing is placed in the right amount and in the right spot, such as the reinforcing in beams, the built-up cages at the ends of shear walls, or splicing dowels to pass load between floors.
The elements or details may also adjust as you move up in a building. Loads typically reduce as you reach higher levels, so the walls and columns can usually reduce in size, concrete strength, or amount of reinforcing. Skilled engineers account for that and will step down wall sizes or reduce reinforcing while carefully ensuring the result is still buildable. This can be as simple as stopping every other bar once you reach certain levels or reducing from 12” (305mm) to 10” (254mm) to 8” (203mm) walls as you go up while accounting for eccentricities that result in the wall thickness change.
Why choose concrete?
If concrete is heavy and requires detailing, why does it get used so often? The main reasons are because it is durable, resistant to fire, and well-suited to the loads of high-rise structures. Concrete is durable and resistant to fire, both aspects that are critical in long lasting buildings. Fire resistance and being non-combustible are particularly important for high-rise buildings; the extra travel distance for occupants means they need extra time to safely exit the building in the event of a fire.
The loads found in high-rises are readily resisted by concrete columns and walls of reasonable size. Structural engineers can make almost any material work within almost any structure, but other materials like timber and masonry impose architectural constraints because of their required spacings, though they are still capable of reaching 10 storeys. Steel is actually stronger than concrete, but that comes at a cost, at least in our market today. Concrete is very well understood under seismic loading as well, and the flexibility of detailing allows us to create structural ‘fuses’ within the structure where we know the concrete will deform in a safe and ductile manner when an earthquake hits.
Like every material, concrete has it’s place in the structural engineer’s toolkit. High-rise buildings in the 10- to 30-storey range are particularly suited to the use of concrete, though there are alternatives available and local market conditions influence the decision to use it. Skilled structural engineers make use of concrete where it makes sense, while accounting for its challenges in their designs.