Structural Design (Part 4 of the NBC)
Structural Design (Part 4 of the NBC) - Transcript
Slide 1
In this presentation, we will introduce the most significant changes to Part 4 of the National Building Code (NBC).
Part 4 deals with structural design and earthquake design of buildings.
This presentation deals only with structural design, such as design for wind and snow, but not with the earthquake loads.
Slide 2
This presentation is part of a series of 13 presentations on the 2015 editions of Codes Canada.
Before I begin with the technical content of this presentation, I will 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/territorial authorities to become law.
This may mean 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 is 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 is shown on the slide:
- It typically starts with someone requesting a Code change.
- It continues with technical committees developing proposed changes.
- It involves a public review and the final 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:
The presentations cover only the changes from 2010/2011 to 2015 Codes and not how to use or interpret the Codes in general.
The presentations contain only the significant changes - the details are in the handbook. Each presentation contains a reference to the relevant pages in the handbook.
The presentations stay strictly within the scope of the National Codes and do not cover provincial or territorial variations.
Slide 5
In total, 38 changes have been introduced in Part 4 of the 2015 NBC in the area of structural design.
This presentation focuses on the most significant changes in the following areas:
- New load combination factors for situations where snow and live loads may occur at the same time,
- Changes to the design and testing of guards,
- Changes to the design procedures that deal with snow loads and wind loads,
- New standard referenced for the structural design of glass in buildings, and
- New standard for the design of repair garages.
Handbook pages 23-29
Slide 6
Before we go into the details of each of the Code changes, I would like to point out one large, overarching change.
When structural design engineers apply the requirements of Part 4 of the NBC, they often also refer to the Structural Commentaries that are published shortly afterwards.
The commentaries have graphics, formulas and tables to support the designers when determining the right load distributions for specific cases.
Over the years, some of the graphics in the Structural Commentaries have become part of the necessary design procedure.
The Structural Commentaries may however not be legally binding in all jurisdictions.
The design procedures on snow and wind loads in the Structural Commentaries are therefore now provided directly in the main body of the Code - including the drawings - which is a first in the development of the National Building Code.
Slide 7
What you see here is an example of a load combination gone bad.
This is a parking deck in New Jersey that collapsed while a snow plow truck was removing snow from the top floor of the parking garage.
This collapse could be blamed on the simultaneous occurrence of a “heavy weight of snow” and live loads due to parked cars. This combinations effect may not have been accounted for in the design of the slab.
Here in Canada, the heavy amount of snow that occurred in 2008 in eastern Quebec and in southern Ontario caused some concern with the load combinations in the 2010 NBC.
The concern is with situations where the ratio of live load to snow load is close to 1, and in areas with high snow loads.
The companion load factor for the live load and the snow load has therefore been increased:
- from 0.5 to 1 when both loads have to be considered together, and
- from 1.0 to 1.5 in situations where the live load represents a storage area or equipment area.
Slide 8
We will now review changes to the design and testing of guards.
Slide 9
This is an image of a guard being tested for its lateral strength.
Guards need to be designed to address both strength and serviceability.
Of course - testing is one way to demonstrate Code compliance.
Previously the inward pull load on a guard was required to be equal to the outward push load, which is not very logical for most applications.
Now, the required lateral inward guard load is reduced to half the outward load.
In addition, a new provision now requires a maximum deflection for pickets in a guard to ensure safety in use.
Slide 10
This is the “Glacier Skywalk” in the Canadian Rockies guaranteeing a breathtaking view.
As you can imagine when you look at the glass guard up there, the design of exterior guards can be quite challenging because the wind pressures that the guard has to withstand can be quite high.
So, it is very important to address the wind loads properly in the design of exterior guards, such as guards on balconies.
Of course, it is also important to consider wind loads in combination with the specified live loads for strength and serviceability.
This is why, specific design wind loads for balcony guards are now provided in the NBC.
Slide 11
Let's now review changes to the design procedures that deal with snow loads.
Slide 12
The third set of changes relates to snow load data.
Snow loads on roofs are based on ground snow load values.
The ground snow values in Table C-2 were updated by Environment Canada based on measurements up to 2012.
As a result, ground snow load values:
- remained unchanged for about 85% of the locations,
- have increased for 11% of the locations, and
- have decreased for 4% of the locations.
The greatest proportion of increases was for locations in the Canada's North.
In addition, the Code now properly reflects that wind is not very effective in removing snow from large roofs.
The Code still permits a reduction based on the wind exposure factor, but limits this reduction to rural areas.
Slide 13
What you see highlighted on this picture is a snow drift from a higher portion of the roof to the lower portion.
The drift has substantially increased the snow load on the lower roof portion.
Snow drift is an important parameter to address when designing snow load on roofs.
Not addressing it properly may cause the roof to collapse especially when heavy snow is involved.
The snow density is commonly used in drift calculations.
The NBC 2015 now uses the formula in the American Society of Civil Engineers, ASC 7 for determination of snow density as this formula has been found to be consistent with observations in Canada.
Slide 14
Although it is fairly common practice in Canada to remove snow from roofs after heavy snowfalls, snow removal cannot be relied upon when designing roofs for snow loads.
This is why any reduction of the design snow load on the basis of snow removal by any means is no longer permitted.
Slide 15
Here you see the effect of snow sliding on a slippery roof.
As a matter of fact, it is probably a mixed effect of drifting and sliding.
Both effects are accounted for in the design of lower roofs.
Now the requirements for the design of sliding snow on slippery upper roofs apply to all slippery roof slopes of the upper roofs, such as sheet steel roofing.
Slide 16
More on drifting snow. In this case we are talking about a specific provision to account for snow accumulation on roofs.
The calculation method for the accumulation factor, previously called “shape factor”, is now provided directly in the NBC.
Imagine a building with a multilevel roof as you see in the icon on the right top.
The large diagram to the left of it illustrates the calculation of the accumulation factor.
The calculation has been improved to better account for three main cases.
The arrows indicate the wind direction.
The blue areas highlight the source of the blowing snow.
The red squares indicate where the drift creates an issue:
- The first case on the left side shows drifting snow from an upper level roof into a roof step.
- The second case in the middle shows drifting snow up against a roof projection.
- The last case on the right side shows snow drifting over a lower level roof into a roof step.
The Code now also addresses snow drifts around inside and outside roof corners.
Slide 17
This is a transmission tower built from steel lattice that was destroyed during the ice storm in Quebec in 1998.
Since then Environment Canada has developed a new model to compute ice loading on vertical and horizontal surfaces and around cables.
This model addresses this kind of failure by determining the ice loading on lattice structures and other exposed building components.
The information for this is not directly in the 2015 NBC but in a referenced standard.
This concludes the changes that relate to snow loads.
Slide 18
I will now talk about changes to the design procedures that deal with wind loads.
Slide 19
As I explained earlier for snow, a lot of accepted practice from the Structural Commentaries on Wind Load Design has also been moved directly into the Code and has been used to clarify requirements.
The first set of changes deals with structural design methods.
There are three acceptable design methods: static, dynamic and wind tunnel procedure.
These three design methods for determining wind loads are now more clearly defined.
The Code now clearly states for which buildings each method is required.
It also sets out limits for each method.
For example, the wind tunnel method is now required for buildings where the building is 6 times as high as its effective width.
Note that Computational Fluid Dynamics or CFD could also be used in wind engineering.
However, it still has some limitations.
In fact, the ability of CFD to achieve the appropriate level of accuracy and reliability in the context of wind engineering in buildings has not been verified
There is no standard method that defines appropriate CFD procedures.
Slide 20
I would like to provide a bit more details on changes to clarify the use and application of the dynamic procedure.
The 2015 NBC requires the use of the dynamic procedure for:
- buildings whose height is greater than 4 times their minimum effective width,
- buildings higher than 60 m, or
- other buildings whose lowest natural frequency is between 1 and 0.25 Hz.
The Code now specifies different exposure and external gust effect factors for use with the dynamic procedure, while the pressure coefficient is the same as in the static procedure.
Slide 21
Here is a look at how the wind tunnel method is applied.
On the left side of this slide, you see the wind tunnel mockup at the design stage for the Petronas Twin Towers in Kuala Lumpur.
On the right side, you see a picture of the finished buildings.
You can see the strength of the wind tunnel procedure in addressing not only the wind load design of the building itself, but its interaction with the environment it is built in.
Phenomena such as wind channeling and buffeting require the use of the wind tunnel method in order to properly address these effects.
The Code also requires that the wind tunnel procedure be used in accordance with a standard specifically written for this purpose.
Slide 22
Here we see an array of antennas and satellite receivers that are anchored to the building.
A new procedure for such exterior non-building components has been introduced to:
- include the effects of icing, and
- account for the group effect in the design where there are a number of similar components and loads that could be transferred from these elements to the building structure.
Slide 23
Let's take a trip to Quebec City.
You see some buildings located at the bottom of the hill, and you see the Chateau Frontenac at the top of the hill.
Naturally, the buildings in these different locations will not be exposed to the same wind loads.
Previously, the topography was accounted for in the exposure factor through a specific formula in the Structural Commentaries.
A separate topographic factor is now included in the basic formula for wind pressure in the Code, which accounts for wind speed increases on hills or escarpments and therefore compensates for this effect.
Slide 24
An important element in determining wind loads are the pressure coefficients.
The pressure coefficients are constant values selected based on building geometry and wind direction.
The Code now defines the external pressure coefficients separately for the overall building and for the cladding.
And the determination of wind loads on cladding systems is now addressed regardless of the wind direction.
This image shows a model of a building.
You see the main structure exposed at the top and you see the cladding being installed in the lower storeys.
Both the main structure and the cladding system have to be designed for wind loads.
Slide 25
This is the Museum of History in Gatineau right across from Parliament Hill.
If you have been there, you may remember that the space between the two buildings is quite open and leads right down to the river, which means space for lots of wind.
If the door in this building is opened when a gust is going by, it may produce significant internal wind pressure.
Internal wind pressure has always been part of the design procedure, but the table of internal pressure coefficient values that was previously in the Structural Commentaries is now in the main Code.
Slide 26
You'll find hangars in every airport in the world. This hangar is in Berlin Tempelhof.
They are described as large structures enclosing a single unpartitioned volume.
For this type of structure, the internal pressure takes significant time to respond to changes in external pressure, which reduces the internal gust factor.
The Code now contains the formula for an internal gust factor for these structures, which was previously in the Structural Commentaries.
Slide 27
The next change introduces a new standard referenced for the structural design of glass in buildings.
Slide 28
What you see here is a structure entirely made of glass.
The Canadian standard to design glass has not been updated for many years.
To provide more flexibility for structural designers and to harmonize with U.S. Codes, an up-to-date American standard has been added for the design of structural glass.
Load adjustment factors for wind were added to make sure that both Canadian and US standards result in consistent and acceptable designs.
Slide 29
Finally, we will talk about a new standard for the design of repair garages.
Slide 30
The floor of a repair garage is exposed to the harmful effect of salt, which could significantly affect the durability of the concrete floor.
As a result, NBC 2015 requires that repair garages have to offer the same corrosion protection as parking structures.
Slide 31
Here are the highlights for structural design changes on Part 4 of the NBC:
- many of the Structural Commentaries' material for snow and wind loads - including graphics - was moved into the NBC;
- the 2015 NBC has more stringent load combination factors where snow and live loads may occur together;
- ice loading and wind loads on exterior elements are now addressed by referencing a CSA Standard;
- design wind loads are now provided for balcony guards;
- wind load now includes a topographic factor;
- application for the three permissible wind design procedures is clearly described now;
- a current US standard can now be used for the structural design of glass; and
- repair garages also need to be designed to account for corrosion protection.
Slide 32
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 33
Contact Information
| Alternative title | Structural Design (Part 4 of the National Building Code of Canada: 2015) |
|---|---|
| Download |
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| DOI | Resolve DOI: https://doi.org/10.4224/40002098 |
| Author | Search for: 1 |
| Speaker | Search for: Singh, Jitender1 |
| Affiliation |
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| Format | Video, Learning Object |
| Subject | Codes & guides; construction; building; NRCCode |
| Publication date | 2015 |
| Publisher | National Research Council of Canada |
| Related publication | |
| Language | English |
| Export citation | Export as RIS |
| Report a correction | Report a correction (opens in a new tab) |
| Record identifier | 637196e5-abba-45ea-810a-1665a391039d |
| Record created | 2021-05-04 |
| Record modified | 2022-06-21 |
- Date modified: