Mid-rise Combustible Construction (NBC and NFC)
Mid-rise Combustible Construction (NBC and NFC) - Transcript
Slide 1
Hi, my name is Sefton Hyde-Clarke. I am a Technical Advisor for Codes Canada.
This presentation is part of a series of presentations on the 2015 editions of Codes Canada and will focus on new provisions related to mid-rise combustible construction.
Slide 2
Before I begin with the technical content of this presentation, I want to speak briefly about the code development system.
It is important to note that the model Codes, which are developed by the Canadian Commission on Building and Fire Codes, must be adopted by provincial and territorial authorities to become law.
This means that Code requirements enacted by legislation within your province or territory might differ from what is presented here. Please check with your local authority.
Slide 3
It's also important to point out that the National Codes are not a federal regulation.
This means it is not NRC or Codes Canada that decides what goes into the Codes, but you!
Codes Canada facilitates an open, transparent, consensus-based process to come up with improvements.
Over 400 committee members volunteer their time to decide on changes to the next Codes.
All committees are balanced between regulators, industry and public interest so that no single category can outvote the other two.
This process typically starts with someone requesting a Code change.
And continues with technical committees developing proposed changes.
It then involves a public review, and finally, approval by the Canadian Commission on Building and Fire Codes.
It's a simple process and it depends on your input.
Please go to the Codes Canada website and find out how you can:
- submit code change requests,
- participate in committees, or
- comment on proposed changes during our public reviews.
Slide 4
And before we start, here are some clarifications on the presentations themselves.
First, This presentation covers only the changes to the 2015 Codes and does not cover how to use or interpret the Codes in general.
There are too many changes to the 2015 Codes to be able to discuss each one individually so I will present only the most significant changes. If you want more details, they can be found in the handbook. As we go, you will find references to the relevant pages in the handbook.
Today, I will stay strictly within the scope of the National Model Codes and won't cover provincial or territorial variations.
Slide 5
New provisions have been added for mid-rise combustible construction which now permit buildings up to 6 storeys in building height to be constructed of combustible construction.
In this presentation we will cover what the new requirements are, how did we get here and what are the key design/construction considerations.
Changes have been made to both the National Building Code (NBC) and National Fire Code (NFC).
Handbook pages 6-8, 40, and 75
Slide 6
Wood is an abundant renewable resource in Canada and with the introduction of mid-rise combustible requirements, this allows a designer the option to choose this method of construction which allows buildings to be constructed faster than if traditional methods were used.
How did we get here?
Prior to the changes to the NBC, mid-rise buildings of combustible construction were being constructed in British Columbia.
Two Code Change Requests (CCRs) were received proposing to raise the limit for combustible construction from 4 to 6 storeys: one from BC limited to residential occupancies and one from the Canadian Wood Council (CWC) for mixed occupancies.
A Code Change Request started the process.
Slide 7
Take a look at the two buildings on the screen. The building on the left is a mid-rise building of combustible construction while the building on the right is a concept for a building of noncombustible construction.
Is it more hazardous to live in the building on the left or the building on the right? When the building is finished, the hazards are the same and a mid-rise building of combustible construction has a similar hazard profile than a similar building constructed of other construction materials and methods.
There are challenges for mid-rise combustible buildings related to when the buildings are under construction and we will take a closer look at provisions that were added to address the risk of fire when these buildings are under construction.
Slide 8
How are buildings of mid-rise combustible construction different? Buildings of mid-rise combustible construction have the same level of safety as buildings constructed of other materials. Sometimes you can't even tell that you are in a building of mid-rise combustible construction.
The image on the left is obviously combustible construction. However, wood has an inherent fire-resistance rating. There have been fire tests that show that some configurations of mass timber provide a fire-resistance rating of several hours (5 hours with some charring only). Think about paper - burning a piece of paper is easy but burning a phonebook is not that easy.
The building on the right is a building of mid-rise combustible construction. In this building there are some noncombustible base building features.
Slide 9
With everyone sitting at the table, the two Code Change Requests were discussed and concerns were raised by everyone involved. Concerns were addressed through an objective-based solution.
The objectives covered included:
- limit damage due to structural failure,
- limit fire to fire compartment of origin,
- safety of egress path,
- protection during construction, and
- protection of adjacent properties.
Slide 10
There were concerns related to different areas of expertise such as fire or structural safety and there were also concerns related to firefighter safety and requirements were added to address those concerns.
What we will see with the new requirements is that many of them relate to firefighter safety. For these new mid-rise buildings the firefighter strategy is both an internal and external attack. This was an important consideration as many of the protection features that we will discuss relate back to the need for an external attack.
Slide 11
The proposed changes from BC and the CWC were considered and it was decided to introduce two additional options for building size and construction relative to occupancy. One for Group C major occupancies (residential) and one for Group D major occupancies (office). Two additional building blocks were introduced in the NBC with specific limitations and the National Fire Code (NFC) provisions were added too.
We also looked at mixed occupancies, fire protection, structural design, seismic design and building envelope. Special care was taken in protecting the building during construction. The overall objective here was to reduce the likelihood of a congregation and minimize the potential impact to neighboring buildings.
I'll start by describing the building on the outside and work my way in. It's best to think about the building as an entire system rather than trying to evaluate each building component individually and this includes the changes to the National Fire Code.
The building material, as defined in the code, is combustible construction, but it's not limited to engineered wood, for example it includes light frame wood construction.
To determine a building's height and area limits we consider the building's overall fuel load. Fuel loads in a building are controlled and compartmentalized by limiting the overall height and area of that building. The more protection features you add the higher and larger building you're allowed. This concept is used extensively throughout the building blocks of the National Building Code.
Slide 12
I will now explain the analogy between building area and fire compartment area using Lego blocks.
If you have a building that consists of 12 fire compartments represented by 12 Lego blocks, the fuel load is the same no matter how you group the blocks. In essence, the risk in a 1 storey building with 12 fire compartments is equivalent to the risk in a 6-storey building with 12 fire compartments.
Slide 13
Building Area
The first step was to define the physical limits of a 6-storey building of combustible construction.
After that, additional/existing protection measures that would provide an equivalent level of safety to both occupants and buildings with the current construction practice were analyzed.
For a residential occupancy, the maximum building area would be 1,500 m² with an aggregate area of 9,000 m².
The aggregate area is calculated by adding the area of each story and represents the overall fuel load of the building. In the example with the Lego blocks the aggregate area is the area of all 12 Lego blocks
The image shows the scale of the proposed 6-storey combustible construction building in brown compared to a 6-storey noncombustible construction buildings in grey. Note that there is no area limits for noncombustible buildings not more than 2 storeys in building height.
Slide 14
Here is another way of showing the differences between the proposed 6-storey combustible building footprint area with what is currently permitted for a 4-storey combustible building and a 6-storey noncombustible building.
What you're looking at is a representation of the building footprint in a plan view. Before the introduction of the new 6-storey mid-rise building, the previous height and area limit for a combustible building is shown in red. You'll notice that the building area at 4 storeys is only slightly larger than the new 6 storey counterpart shown in blue.
Furthermore, the mid-rise building represents a 75% reduction in building area compared to a 6-storey noncombustible building.
A designer may choose to use the mid-rise combustible construction provisions to build a 4-storey building, which would have a building footprint of 2,250 m². However, the designer would be bounded by other requirements, which apply to mid-rise buildings.
Slide 15
Similar to residential occupancies the same principles were used to address major occupancies for business and personal services. The image shows the new height and area for these major occupancies. The relationship between the new mid-rise buildings and their non-combustible counterparts are the same for businesses.
The maximum floor area would be 3,000 m² with an aggregate area of 18,000 m².
The image shows the scale of the proposed 6-storey combustible construction building on the bottom (in brown) compared to a 6-storey noncombustible construction building on the top (in grey).
Slide 16
Again you can see the scale of a 6-storey combustible building in brown compared to a 6-story noncombustible building in gray. Looking at the plan view again the mid-rise building footprint shown in blue is a 58% reduction in area compared to the noncombustible building shown in green. You'll also notice that the footprint area of a 4-story combustible building is slightly larger, but in fact the mid-rise building has a larger aggregate area.
Slide 17
Building Height
Remember we talked about firefighter concerns? Building height was one of their concerns and their ability to use their hose streams if the building was too high.
The maximum height measured from the floor of the first storey to the uppermost floor level was limited to 18 m. This height limit was chosen specifically to limit buildings that have mezzanines. The code allows you to add a mezzanine to each story which increases the overall building height without increasing the number of storeys.
This way the building stands taller than the number of storeys as defined in the Code.
For mid-rise buildings however, because the 18 m height limit is measured to the uppermost floor level which includes mezzanines, that prevents the floor levels from becoming out of reach to firefighters. The idea is not to have a 6-story mid-rise building with a mezzanine that makes it look like a 7-story building from the outside.
Another concern raised by firefighters was their ability to reach the roof assembly with the fire hose. If the roof is too high off the ground it's impossible to suppress a fire with an effective hose stream.
Slide 18
Building Height
To address this concern, a 25-m limit measured to the highest point on the roof was introduced. Above 25 m, the roof assembly has to be either noncombustible or fire-retardant-treated wood with a flame-spread rating less than 25.
You also need a Class A roof covering classification. Under 25 meters and the roof assembly is permitted to be combustible construction with a roof covering classification of either A, B or C.
Flexibility was built into the code to allow non-contiguous roof assemblies at different elevations to be evaluated separately as shown in the image.
The upper roof can be noncombustible while the lower portion of the roof can be combustible. The intent is that the provisions are modular to accommodate different roofing designs.
Most of these buildings will likely have flat roofs, but the intent was to leave the option for sloped roofs to allow for design in areas where roofs need to be sloped.
For example, some parts of Canada actually require slope grooves to accommodate large snow accumulation.
Slide 19
Exterior Cladding
Another challenge identified by firefighters was how can we protect the exterior of the building from vertical fire spread while allowing architects to explore creative designs pleasing to the eye. We are trying to prevent fire spread from within the building spreading up the exterior wall.
Exterior fire spread was identified as a critical concern for these buildings and so:
- a maximum of 10% of the surface of the building face is permitted to be combustible cladding, or
- the cladding can be combustible or noncombustible (without limits) if the material is subjected to testing in conformance with the CAN/ULC-S134 standard and satisfies the performance criteria stated in the NBC.
The designers will still need to comply with the construction requirements of the exposing building face (limiting distance requirements).
Slide 20
Now let's talk about facing streets. Recognizing that access for firefighters to the building face is critical to stage an effective external attack, provisions are introduced that allow adequate access by streets along the perimeter of the building.
The diagram represents three buildings separated by firewall construction so that they are considered 3 separate buildings.
To allow access to firefighters to the building perimeters that would be acceptable for them in response to a fire emergency, mid-rise combustible construction buildings (containing Group C and D major occupancies) require that not less than 25% of the building perimeter be within 15 m of a street.
What does this mean? Take this picture as an example. You'll see that the building in the middle is only served by a street along one of its short edges, which is obviously less than 25% of the overall building perimeter; therefore it does not conform to the 25% perimeter rule, while the other two buildings on either side do. The middle building would not be permitted under the new mid-rise requirements.
This new provision has significant impact on infill projects in urban areas where street access on the side of a building is not always possible.
Slide 21
Another potentially challenging situation occurs when the elevation of the access route is too far below the ground floor of the building. Remember that the objective here is to allow for an external attack, which means firefighters need to reach the upper floors and roof assembly with an effective hose stream.
Yet another height limit is introduced - this time measuring 20 metres from the access route to the uppermost floor level. Keep in mind the 18 metre height limit still applies between the ground floor and the uppermost floor level. This limits the potential construction of a six-story mid-rise building on top of a hill.
Slide 22
Mixed Occupancy Combinations
Mid-rise buildings will permit occupancy combinations including assembly, mercantile, low- and medium-hazard industrial major occupancies and storage garage combinations.
The diagram on the left applies to a building with a Group C major occupancy (residential) and the diagram on the right applies to a building containing a Group D major occupancy (business and personal services).
The main difference between the two is that Group F industrial occupancies are not permitted in Group C residential buildings with the exception of storage garages for the parking of motor vehicles. In Group D major occupancies (business and personal services) some industrial occupancies are permitted.
Both building types permit a storage garage to be located on the first three storeys.
Slide 23
Sprinkler Protection
Historically in combustible construction, fire spreads through concealed spaces and balconies. Sprinkler protection requirements are increased for mid-rise combustible construction buildings.
Most concealed spaces within floor and roof assemblies will need to be sprinklered.
Additionally, exterior balconies will need to be sprinklered to address fire spread on the surface of the exterior wall assembly from one storey to the other.
What if a designer chooses to restrain himself to the more stringent requirements for mid-rise combustible buildings and apply them to a 4-storey combustible building because of the larger permitted building area? Then the permission to use NFPA 13R would still apply within the scope of the NFPA standard for residential occupancies, effectively having a 4-storey building with increased building area (up to 2250 m² vs. the current 1800 m²) with an NFPA 13R sprinkler system.
Using sprinklers to suppress the spread of fire beyond the point of origin has proven to be a very effective means of limiting the overall risk in buildings
Slide 24
Building Envelope
There are no changes to the building envelope requirements of the NBC (Part 5) applicable to mid-rise buildings of combustible construction, since this Part contains performance-based criteria rather than prescriptive requirements.
However, what is good for a 4-storey building may not be not good for a 6-storey building. Several explanatory notes were amended to draw attention on specific issues for designers and engineers.
In short, higher building means higher exposure to elements and higher loads.
Slide 25
Building Envelope
Wood shrinkage, which is related to moisture content of the wood, is a well known mechanism that is predictable.
With respect to the building envelope, there are no new requirements but there is a reference to CSA O86 in an explanatory Note to address shrinkage.
CSA O86 specifies a method for calculating building movement due to changes in moisture content.
Slide 26
Wood-frame buildings are known to perform well in earthquakes because wood buildings:
- are lighter,
- can flex (i.e., nailed wood connections allow the building to flex thereby absorbing energy), and
- have redundancies (i.e., numerous small connections rather than few large ones and attachment of sheathing to numerous wood joists and studs).
However, certain elements can create problems especially when looking at 5- and 6-storey combustible buildings. Therefore, type 4 and 5 irregularities and soft storeys were identified as important issues to be considered.
Slide 27
Two types of irregularities are not permitted when seismic force levels are higher than a threshold value prescribed in the Code. These irregularities are related to how to drive force loads through the structure in a seismic event.
Type 4 irregularities are related to in-plane discontinuity in vertical lateral force resisting elements. The diagram you see on the screen is a building section where the shear walls, shown in blue, are in the plane of the screen. In the left image, horizontal forces resulting from an earthquake would find a direct path into the ground. In the image on the right, the shear wall panel is discontinued at the lower storey, which may cause building collapse during a seismic event.
For mid-rise buildings greater than 4 storeys, type 4 irregularities are not permitted when seismic force levels are higher than a trigger value.
Slide 28
Type 5 irregularities are related to out of plane offsets.
An example of a type 5 irregularity is when wood panel shear walls are shifted (offset) to a different plane at a storey as seen by the red shear wall panels.
This is not allowed in mid-rise combustible construction over 4 storeys when seismic force levels are higher than a threshold value.
The prohibition of these irregularities is intended to ensure that the expected structural response is maintained at a reasonable level by well-defined lateral load systems.
Slide 29
What is a soft storey? Envision a building where there are retail occupancies on the first storey with large windows. This storey is potentially a soft storey.
Some of the most susceptible structures to earthquake damage are soft-storey buildings.
A soft storey refers to the existence of a building floor that presents a significantly lower stiffness than the others. Large openings in the walls such as windows, doors and garage doors can leave too little wall area to provide adequate resistance to the earthquake forces. In an earthquake, ground shaking causes such structures to sway and sometimes collapse. Building Codes have recognized that buildings with soft storeys have to be carefully designed and detailed to resist earthquakes.
Traditionally wood buildings have the potential to exhibit soft storey behaviour under earthquake. The effects are not so pronounced for regular buildings which are 4 storeys or less in height. But that is not the case for 5- and 6-storey combustible buildings.
Slide 30
In building A, you see the earthquake response in a normal building and in building B, the response of a building with a soft storey, which could lead to collapse.
Storey strength is less than that in the storey above leading to formation of a sway mechanism in the weak storey.
It was decided that the simplest method to reduce the potential of catastrophic collapse of 5- and 6-storey mid-rise combustible buildings would be to specify an increase in the minimum force level.
Depending on which method you use, different safety factors are introduced.
Slide 31
The other slides have addressed almost all of the firefighter's concerns. However, we have to deal with the building when it is under construction. These types of building are in perfect configuration for fire, kind of like a wood crib where oxygen is readily available to the fire.
Measures are introduced in the NFC to address the three most common causes of fires on construction sites:
- arson (which is the leading cause of fire on construction and demolition sites),
- hot works, and
- smoking
We need to consider how to begin suppressing a fire as fast as possible in order to limit the probability that a fire will spread to an adjacent property. To address these objectives the approach uses the basic principle of risk management. Provisions are either added to reduce the probability of something bad occurring or to reduce the consequence after something bad occurs.
Slide 32
Arson is the leading cause of fire on construction sites.
One of the most obvious measures that could easily be put in place is fencing to limit access to the site.
Fencing, boarding, or barricades was already required by Part 8 of the NBC.
Slide 33
Smoking is the second and third leading causes of fires on construction sites.
It is now required to have identified smoking designated areas for mid-rise combustible buildings under construction.
The previous edition of the NFC addressed smoking but with a different perspective: it prohibited areas where smoking was permitted, while the new requirements identify and restrict smoking to designated areas.
This better reflects the current practice and was found to be a better proactive approach to control smoking on construction and demolition sites.
Changes were also introduced to address hot works, however because these changes were made to address hot works in all buildings not just mid-rise buildings you will have to either watch the presentation on the changes to the NFC or refer to your handbook for more information.
Slide 34
Special attention was given to disposal chutes and disposal bins where combustible materials could easily be ignited from arson and through the disposal chutes, spread the fire into the building, potentially resulting in a major conflagration. The chute could act similar to a chimney.
Two options in the NFC 2015:
- 2 m clearance to a disposal bin, or
- a noncombustible chute.
Slide 35
Most 911 calls come from workers on their personal cell phones. The problem is that when the call comes in to the fire services the information about the jobsite is unreliable. In a new building construction the street address may not even be known by the worker and the building would be referred to by the name. Project names have little meaning to the fire services and may delay the response.
To help construction personnel better identify which stair or section of the building they are located in, temporary signage is now required at every floor in a stairway commonly used to access each floor.
Also, signage is required outside of the building.
The information required on the signs should help firefighters more efficiently gain access to the site, to the building and to the floor where the fire was identified.
Slide 36
As soon as construction materials arrive on site, a water supply must be provided.
Water supply is not defined, but it must be "adequate for firefighting."
It may be municipal water supply, a water tank or even a water truck. Whatever solution suits the needs of the local fire department.
Slide 37
A stairway needs to extend upward as each floor is installed in the new construction. The same step configuration, handrail and guard height design for permanent stairs are applicable to the construction stairways. This doesn't mean that such stairs will meet all the applicable requirements for stairs in terms of evacuation but it provides a reasonable level of safety for firefighters gaining access to the floor area for firefighting purposes.
The stairway does not need to be temporary scaffolding as shown in the image. It could be the finished stair in the building as long as the step configuration, handrail, and guards meet the minimum requirements.
The construction stairways are so that firefighters can readily have access to the fire floor.
Slide 38
We have covered several topics but the biggest three takeaways from this presentation are as follows:
There are new design options in the Code that permit the construction of mid-rise buildings of combustible construction.
Once finished, mid-rise combustible buildings have the same hazards as other buildings using conventional construction techniques.
There is a specific recipe for these buildings that needs to be followed.
Thank you. This concludes the presentation on mid-rise combustible construction.
Slide 39
I have covered a lot of information in today's presentation. The handbook is a useful resource if you want to review the topics from this presentation in more detail.
It covers the majority of technical changes that were implemented in the 2015 National Building Code, National Fire Code, National Plumbing Code and National Energy Code for Buildings.
The handbook can be purchased on NRC's virtual store as a downloadable PDF or as a hard copy.
Slide 40
Contact Information
| Alternative title | Mid-rise Combustible Construction (National Building Code of Canada: 2015 and National Fire Code of Canada: 2015) |
|---|---|
| Download |
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| DOI | Resolve DOI: https://doi.org/10.4224/40002092 |
| Author | Search for: 1; Search for: 1; Search for: 1 |
| Speaker | Search for: Hyde-Clarke, Sefton1 |
| Affiliation |
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| Format | Video, Learning Object |
| Subject | Codes & guides; construction; building; fire; NRCCode |
| Publication date | 2015 |
| Publisher | National Research Council of Canada |
| Related publication | View items (3) |
| Language | English |
| Export citation | Export as RIS |
| Report a correction | Report a correction (opens in a new tab) |
| Record identifier | 43fbde03-d784-4eab-a020-fe572d4248b5 |
| Record created | 2021-05-04 |
| Record modified | 2022-06-21 |
- Date modified: