Selecting a Structure for Your Mid-rise
Humans have been working with multiple storey buildings as far back as the Neolithic era, and likely even before that. At some point, an enterprising ancestor thought that using the roof of their shelter was a great idea, and thus, multi-storey buildings were born! By 8000 BCE, there is even evidence of structures such as the Tower of Jericho including internal staircases to reach the upper area. While these early structures were rarely for human habitation, they laid the foundation for humanity's shift to multiple floor construction.
The past two centuries have seen an explosion in mid-rise construction. As the industrial revolution swept the globe, cities densified, and more and more mid- and high-rise buildings were required to house the booming urban population. This increased densification has continued into the modern day, with mid-rise buildings being built continuously across Ontario, particularly in growing cities across southern Ontario like London, Kitchener, Waterloo, Windsor, Hamilton, and the Greater Toronto Area (GTA).
Mid-rise buildings come with their own unique challenges and solutions compared to high-rise or low-rise buildings. 4- to 8-storey buildings have a huge range of options for construction, and any project of this size would benefit immensely from a structural engineer being involved in the early stages to ensure the selected system is feasible and efficiently laid out.
What goes into the decision?
The biggest impact on a project is the goals of the client. A skilled structural engineer will work with you to identify your goals, such as reducing cost, decreasing construction time, providing a more sustainable design, easier maintenance, or improved aesthetics. While each of these things are common goals, the specific balance and end use of the building is what typically drives the selection of different systems.
Northpoint II, a mid-rise residential structure in north London, ON, supports hollowcore concrete floors on light gauge steel stud bearing walls and concrete shear walls
Another critical component is the architectural layout of the structure. Architects are experienced at determining client or user needs and incorporating them into their designs, and those design decisions often impact the structural system. For example, residential unit sizes can impact the floor system selection since most mid-rise residential buildings have regular demising walls that are natural places for structural walls. Corridor walls and stair shafts are also great to use structurally since they are needed regardless. However, the size of suites can also greatly affect our decisions on the floor system, and each floor system typically pairs best with specific vertical bearing systems.
The other major impact on mid-rise structures is the common use of podium structures, also known as 5-over-1s. These structures usually have very repetitive upper floors, but the lower floors reconfigure significantly to allow for amenity, retail, commercial, or parking space. The reconfiguration often introduces transfer elements, which may be costly. However, a practiced structural engineer can work with an architect or owner to develop the most cost-effective strategy that keeps several storeys supported by often a single element!
Floor Systems
Hollowcore concrete floors are paired with masonry walls to keep cost down and speed up at Southwood Way, an affordable housing building in Woodstock, ON
Gravity and lateral loads in mid-rise buildings are usually quite significant, but still solvable using almost any major floor structure. Some systems, like hollowcore concrete, Hambro, or COMSLAB, are typically chosen when long clear spans are required. Within these systems, the structural engineer balances speed, durability, and weight to find the optimal solution. Hollowcore, for example, is usually the quickest of the three, but Hambro and COMSLAB have advantages in controlling the reinforcing layout to give some strength in the secondary direction or for lateral loading. Cast-in-place systems are also viable, though typically heavier and slower than the other systems for mid-rise structures.
A Hambro floor system is supported by concrete walls at Compass Point in Port Stanley, ON
All of the systems mentioned so far involve a lot of concrete, which can add up to quite a bit of weight. That weight is tied directly to higher seismic loads and larger foundations. That’s where much lighter systems like light framed timber, mass timber, or composite deck on light gauge joist come in. Light-framed timber and joist construction both make use of regular, repeating members to span to the primary vertical supports. They do require additional fire protection compared to concrete as well as some vibration analysis. They can also be constructed easily and have advantages such as each member typically light enough for a single worker to carry.
Mass timber, particularly in the form of cross laminated timber (CLT), is another lightweight system that is becoming increasingly common in the construction industry. Sometimes called ‘plywood on steroids’, CLT is composed of sawn lumber bonded together in alternating layers into much larger planks. These planks can be huge, often up to 10’ wide and 50’ long, enabling them to span multiple supports to reduce construction time and panel thickness. They are commonly paired with glue laminated (glulam) beams to provide exposed timber structures.
Vertical Systems
Not only are there multiple floor systems to choose, but also multiple systems that support the floor. While there are specific subsystems to each material, they can broadly be grouped into cast-in-place concrete, precast concrete, structural steel, masonry, light timber, mass timber, and light gauge steel. Here, again, a talented structural engineer will work with you to choose the best system to suit your goals. For example, an affordable housing project is often focused on reducing cost and building fast, which lends well to vertical systems like masonry or light frame timber. Aesthetics may also impact the decision, since systems like CLT walls or architecturally finished concrete may be left exposed to create specific appearances for end users.
Foundations
Traditional strip footings are suitable for many mid-rise projects, but always check with a geotechnical and structural engineer to confirm!
Most people rarely see the foundations that hold up our buildings, and so it would be easy to assume they are all similar. However, mid-rise buildings alone commonly use systems like discrete concrete footings, piles, caissons, or even raft systems. The selection of appropriate foundation systems varies widely due to how much soils vary, with stiffer and stronger soils often benefiting from discrete concrete footings and weaker soils benefiting from piled or raft systems. This is truly a crossover element in a building; while structural or geotechnical engineers are both knowledgeable about foundations, it takes two of them truly working together to identify constraints and develop solutions.
Putting it all together
While it may seem like there are dozens or even hundreds of potential combinations, only a limited number make sense when you put them together in a single building. For example, it doesn’t make sense to pair concrete bearing walls with light timber joists; the additional detailing and moisture/rot issues alone make it infeasible compared to a pure concrete or pure timber solution. This is what structural engineers are here for: to work with you in selecting the best system for your project and making sure that systems are being chosen and used to their full potential.
This is just a crash course in the many, many options that are available in our industry, and more are coming online and into practice every year. Our team of structural engineers and technologists is dedicated to keeping up with innovations in the industry and mastering existing systems so that we can implement them where they are best served to keep cost and construction time down and durability and quality high.