Seismic Design (Part 4 of the NBC)
Seismic Design (Part 4 of the NBC) - Transcription
Slide 1
Hello, my name is Jitender Singh and I am the Technical Advisor for the standing committee on earthquake design.
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
Let's talk about earthquakes. Welcome to Subsection 4.1.8. of the 2015 NBC dealing with earthquake provisions.
Here is the first fact:
Earthquakes do not kill people, buildings do.
Second fact:
The two most important variables affecting earthquake damage are:
- the intensity of ground shaking, and
- the quality of engineering of structures in the region.
The good news is that the NBC deals with both and much more.
The outline shows the changes in NBC 2015 that we are going to cover in this presentation:
- Seismicity - earthquake force on a structure
- Low hazard zones
- Inclined structure
- Flexible diaphragm
- Pallet racks, elevators and glazing
- Seismic isolation
- Supplemental damping
Handbook: pages 29-34
This is not the complete list.
This is the outline.
I will introduce the changes.
The details are in the handbook.
Slide 6
What you see here is a number of different buildings and the equation that governs their behaviour during an earthquake.
The two main concepts here are time period and site coefficient.
Time period describes the time in seconds (or fractions of a second) that is needed to complete one cycle of a building to wave back and forth when put in motion by an earthquake. Natural time periods vary with height and weight and seismic restraint. But in general they are:
- about 0.1 seconds for a one-storey building,
- 0.5 seconds for a four-storey building, and
- taller buildings of between 10 and 20 storeys will swing at periods of about 1 to 2 seconds.
The site coefficients capture the effect of soil underneath a building. When earthquake waves pass through soft soil, for example, they can get amplified by a factor of 1.5 to 6 as compared to the case when the building is on rock.
The equation:
S(Ta) MvIEW
(RdRo)
is a staple for seismic design engineers. It provides a simple relationship to find earthquake forces on a building. Part of the force depends on the location and the other part depends on the properties of the building.
Slide 7
Let's take a look at the changes to location first.
The 2015 NBC provides spectral hazard values for 679 locations in Canada in Table C-3. Many of them have changed.
The reason that these values change from one code to another is that over time, researchers are getting better at estimating the real hazard as they get more data from earthquakes around the world.
But to be clear, the real hazard at a location from an earthquake is constant, it does not change, and we just do not know the real value.
What we show in the NBC is the best estimate of the real hazard based on our current data and knowledge. For example, the 2010 NBC was based on an earthquake catalogue up to 1990s. The catalogue used for the 2015 NBC includes earthquakes up to 2010.
This along with all the data and knowledge acquired from earthquakes around the world led to substantial changes to the spectral hazard values in the 2015 NBC.
This pie chart shows the changes in spectral hazard values for all 681 (679) locations in the 2015 NBC at time period of 0.2 seconds.
This time period is typical of 2 to 3 storeys buildings.
The new values are lower in 546 locations and higher in 133 locations.
Out of these 133 locations, 79 locations are in low hazard zones and therefore the impact is not significant.
However, 54 locations are in moderate to high hazard zones, most of them in British Columbia.
Slide 8
This slide shows the geographical distribution of the changes on the spectral hazard at 0.2 second time period.
The shades of gray are representative of the level of hazard. A darker gray means a higher hazard and a lighter gray a lower hazard.
The triangles represent a change in hazard value:
- a light blue triangle shows a decrease and a bigger blue triangle means a bigger decrease, and
- red shows increase and the size is also related to the magnitude of increase.
So if we look at Vancouver Island or the Saguenay, in Quebec, we find big red triangles in dark gray locations. This means a significant increase in hazard in an already high hazard area.
So, combining this with the numbers we learned in the last slide, it means that there are 133 red triangles and 54 of them are in darker shades of gray.
And that is a problem because it means a significant increase in an area with moderate or high hazards.
Let's see what caused these changes.
Slide 9
What you see here is a cross-section of the earth's crust around Vancouver and Seattle. This is called the Cascadia fault. And because the Juan de Fuca plate (on the left) slides under the North American plate, it is called a subduction zone.
Most recent data shows that a Cascadia subduction event could produce earthquakes of a magnitude 9. That is almost 20 times bigger than previously thought.
A major reason for the spectral hazard changes along the west coast is because of a future Cascadia subduction event.
I would like to give you more numbers to illustrate the significance of this:
- some experts estimate that there is a 30 per cent chance that British Columbia could be hit with a significant earthquake in the next 50 years, and
- a magnitude 9 earthquake off the coast of Vancouver could lead to $75 billion dollar in losses. However only about $20 billion of that is currently insured.
Slide 10
Now that we have looked at location specific changes let us review the changes related to site coefficients which capture the effect of soil in an earthquake.
Slide 11
The 2010 NBC had two site coefficients.
One acceleration based coefficient for short periods - called Fa and one velocity based coefficient for long periods - called Fv.
This was a simplification because in reality the seismic response of a building is different for different time periods which depends on height of the building.
The 2015 NBC now provides site coefficients for a range of periods, which takes our approach closer to the real value of hazard.
It provides more accurate site factors for our main equation.
Slide 12
We looked at how factors that are location dependent changed in the 2015 NBC.
We will now talk about the properties of the building itself.
The only change to the building portion of the equation were higher mode factors. The mode factors are represented by the Mv in the equation.
Slide 13
The black shapes show different ways in which a building can oscillate in an earthquake over a green shape of the normal building.
The first picture is called the first mode or the fundamental mode.
The analysis is simple for the first mode but gets complicated as we move to the second or higher modes.
If a designer wanted to find the exact force on a building due to earthquake, he would have to analyse a building for each mode and then sum up the resulting force.
But this would be a lot of work especially for smaller or moderately high buildings from 10 to 15 storeys.
Therefore, to keep it simple, the NBC provides the equation based on the first mode and then requires that the resulting force is multiplied by a factor to account for all the other modes. This factor is called the higher mode factor.
The values of Mv in NBC 2015 have changed, because the value of Mv depends on hazard values or more specifically the shape of the hazard curve.
This factor has changed because the shape of the hazard curve was modified and the effect of the modes depend on the hazard. And updated values for Mv for the new hazard have been provided.
Slide 14
Next on the list is the change related to the low hazard zones. There is no term like low hazard zones in NBC 2015 - what is given are new triggers with cutoff values. Low hazard zone refers to all locations where the value of the trigger is below the cutoff value shown in NBC 2015.
The 2010 NBC did not require seismic design for buildings in some locations, for example in Manitoba, large parts of Alberta and in Saskatchewan.
This relaxation has been withdrawn and all locations in Canada now require seismic design.
However, recognizing the low magnitude of hazard at these locations, the NBC now has a simplified standalone procedure.
Slide 15
Here is a map similar to the one we saw before.
This map shows low hazard areas in Canada in light blue.
Slide 16
What you see on-screen is a map of the Prairies and the Rockies showing past earthquakes as red dots. We see a lot of dots in the Rockies to the left, which is the seismically active zone in Canada, but if you look between the blue lines in the Prairies region you will notice that there have been earthquake events in such low hazard zones.
One of the red dots in the blue circle shows that an earthquake with a magnitude 5.5 was recorded in 1909 south of Regina.
Slide 17
This was of course news headline on May 16, 1909 and there were accounts of frightened residents throughout Winnipeg.
The isoseismal lines on the picture show how the tremors were felt across the Prairie region.
It was the first time a quake had been known to strike the area.
It was estimated at 5.5 on the Richter scale and originated from the 300-kilometre Hinsdale fault line in Montana.
It remains the largest historical earthquake known in North America's Great Plains.
Even looking at large earthquakes in stable regions around the globe, research shows that a magnitude 6.5 earthquake happens once in 10 years in one such stable locations around the world.
This highlights the reasons why the NBC eliminated the exemption from seismic design.
Slide 18
There is another important reason for withdrawing the 2010 NBC exemption.
If you remember the previous slide, the hazard has decreased for 546 locations.
The decrease in hazard at some of these locations would have exempted such locations from seismic design if the exemption provided in 2010 NBC was continued in 2015 NBC. The map on your screen shows the location that would have gone from full-blown seismic design in NBC 2010 to no seismic design because of drop in hazard.
Many of these locations are urban areas with significant populations such as London and Kitchener.
Considering the risk, the Committee felt it prudent not to exempt such populated areas from seismic design in a reasonably active seismic region.
Slide 19
The green dots represent locations where the buildings can be designed using the simplified procedure. The new NBC values and requirements ensure that buildings in all such regions have the capacity to withstand some shaking.
Slide 20
This is a snapshot of the new simplified approach for these locations provision. I know - it doesn't look simple. The details of where the simplifications are in this equation are in the handbook.
The entire procedure takes up to 2 pages in the 2015 NBC while the full earthquake design requirements take up 30 pages.
The method also has some limitations.
For example, buildings of unreinforced masonry over 30 m cannot be designed with this method for high importance or post disaster buildings.
And buildings with a cold-formed steel structure must be less than 15 m in height in order for the simplified method to apply.
It is conservative but the designer always has the option of using the full set of earthquake design requirements.
This method should result in a structure that will behave almost elastically and suffer little damage in an earthquake.
Slide 21
Buildings with inclined columns or large projecting balconies are becoming commonplace in our cities.
These buildings pose a different kind of challenge for seismic engineers.
This section will explain how the 2015 NBC now addresses them with an entirely new set of requirements.
Slide 22
Here are a few examples of buildings with gravity induced lateral demand or GILD.
In an earthquake situation, buildings like these behave similar to a ratchet.
They drift in one direction but do not return to their neutral position and like a ratchet, this shift from a neutral position becomes more and more extreme.
The large residual displacements can lead to instability and eventual collapse.
NBC 2015 defines the tendency to ratchet in one direction in terms of a parameter called alpha and provides different requirements depending on the value of alpha
Slide 23
Another type of special but common buildings are buildings with flexible diaphragms.
A diaphragm in a building can be the roof or a floor such as the steel deck of a warehouse or concrete floor of a building.
These new provisions apply to single storey buildings with large steel or wood roofs where the roof is much larger than the height of the building
Slide 24
On this picture of a typical industrial area, you see a few examples.
Large warehouses which are long but not very high are fairly common in our landscape.
Typically, the time period of a building depends on its height.
However, for such long but low structures with steel or wood roofs, the size of the roof also influences the time period.
If you think about the ground underneath these buildings shaking, you can imagine that these buildings react with more flexibility.
This flexibility is now recognized in the NBC and allows for more cost-effective designs.
Slide 25
In the 2010 NBC, the time period did not change with the size of a diaphragm. The blue line shows a relationship as per the NBC 2015. In this case the time period depends on the length of the diaphragm - the longer the diaphragm the higher the time period, which means a reduction in the design seismic force.
A comparison of the 2010 and 2015 methods in a building in Richmond, BC resulted in a 15% reduction of seismic force with the new method.
This translates directly into cost savings for the design and construction of these buildings. Even more so for retrofits and seismic upgrades of existing buildings.
Slide 26
We will now look at seismic provisions for pallet racks, elevators and glazing.
These belong to what are called elements of structures, non-structural components and equipment in NBC. According to a study such elements account for almost 75% of the loss in an earthquake.
The 2015 NBC has expanded the list of such elements to cover steel pallet storage racks, elevators, escalators, and glazing systems.
Slide 27
Steel pallet storage racks are a $175 million dollar industry in Canada.
Steel pallet racking systems are currently being designed using U.S. standards or outdated Canadian standards. None of these standards are compatible with the 2015 NBC.
Slide 28
This is a picture of damage to pallet racks during the 1994 Northridge earthquake.
This could potentially be significant risk to life safety.
As a result, new provisions have been added to the 2015 NBC to satisfy requests from regulators to provide guidance for such structures in codes and standards.
The new NBC provisions, along with new information in CSA's structural design standard, fill the gap.
Slide 29
This illustration is a simple representation of how a building deforms laterally during an earthquake.
If this happens, any window attached to the structure must also deform by the same amount.
Brittle material like glass in a window cannot tolerate significant deformations and cracks when the gap between the frame and the glass closes due to the deformation.
As a result, glass in windows can break, fall down and hurt people on sidewalks below.
Glazing failures were relatively common in high-rise buildings in Mexico City in the 1985 Earthquake.
The 2015 NBC now mandates that glazing systems be designed to accommodate these effects in moderate to high hazard areas.
Slide 30
On the right side of this slide, you see a drawing of a typical elevator with all its elements.
During an earthquake, elevators and their elements are sensitive to storey drift and acceleration.
In the 1994 Northridge Earthquake, close to 900 hydraulic elevators suffered damage.
In several cases, doors jammed, cars came out of rails, guide rails were bent and tie downs on several oil tanks failed.
This is why elevators and escalators have been added to the 2015 NBC.
Slide 31
Two new Articles have been added to the 2015 NBC that address seismic isolation.
Slide 32
This photo shows the Tomb of Pasargadae in Persia, which is modern day Iran.
The tomb was built around the 6th century BC.
It is the oldest base-isolated structure in the world.
It has a wide stone and mortar foundation, which has been smoothed at the top.
On top of the lower smooth foundation is a second foundation - built of wide, smoothed stones linked together - forming a plate that slides back and forth over the lower foundation.
This tomb has survived a Richter scale 7 earthquake without any damage.
Slide 33
While the Persians used stone and mortar and a lot of labour in smoothing the foundation, we now use “isolation devices.”
Shown here on the right is the typical construction of a lead rubber bearing, a type of isolation device made of layers of rubber laminated with steel and having a core of lead.
The rubber is the spring component with the lead providing the damping.
On the left, you see the installation of such a bearing under a column of a building.
Slide 34
Here are two graphs that show the effect of base isolation.
The graph on the left shows how seismic isolation results in a significant decrease in base shear due to increase in time period of the building.
The graph on the right shows that deformations increase substantially for the isolated building.
This seems counterproductive as higher drift means that there is a likelihood of greater damage.
Slide 35
The seismically isolated building on the right of your screen has more drift, but the drift is concentrated at the isolation interface.
The building above the isolation interface is not affected, and as a result there would be a lot less damage in a seismically isolated building.
Seismic isolation is a great option particularly if you are constructing post-disaster buildings in moderate or high hazard areas.
For example, all hospitals in Chile are required to be seismically isolated.
Seismic isolation is now addressed in two new articles in the 2015 NBC and can be used for seismic design.
Slide 36
Another new Article in the 2015 NBC addresses supplemental damping.
Slide 37
Damping is a principle that is around us every day. For example, car and truck suspensions, and shock absorbers.
If they were not installed, our cars and trucks would swing up and down after each pothole.
We can use this principle for designing buildings to withstand seismic loads.
The picture on your screen shows a damping device used as a diagonal brace. Buildings inherently have damping of 1 to 5% and in comparison cars tend to have 20 to 25% of critical damping and truck suspensions between 30 to 40%.
However with supplemental damping, this can be increased up to 25 to 40%.
The advantages of supplemental damping are:
- decreased base shear and inter-storey shear of up to 40%, and
- reduced dynamic displacement of over 50% reduction in many cases.Two new articles have been added in NBC 2015 with provisions for supplemental damping.
Slide 38
Here is a snapshot of what we talked about:
- Calculations for earthquake forces on buildings have changed.
- All buildings in Canada now have to be designed to resist earthquake forces.
- Buildings with inclined columns and large cantilevers require special attention.
- Long, low-rise buildings can be designed for lower loads.
- Elevators, glazing and pallet racks must conform to the NBC seismic requirements.
- Seismic isolation and supplemental damping can be used as an acceptable solution.
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
| Autre titre | Seismic Design (Part 4 of the National Building Code of Canada: 2015) |
|---|---|
| Téléchargement |
|
| DOI | Trouver le DOI : https://doi.org/10.4224/40002099 |
| Auteur | Rechercher : 1 |
| Orateur | Rechercher : Singh, Jitender1 |
| Affiliation |
|
| Format | Vidéo, Object d'apprentissage |
| Sujet | Codes et guides; construction; bâtiment; NRCCode |
| Date de publication | 2015 |
| Maison d’édition | National Research Council of Canada |
| Publication connexe | |
| Langue | anglais |
| Exporter la notice | Exporter en format RIS |
| Signaler une correction | Signaler une correction (s'ouvre dans un nouvel onglet) |
| Identificateur de l’enregistrement | 68eb101d-92f3-4fa8-8e54-41521e77c277 |
| Enregistrement créé | 2021-05-04 |
| Enregistrement modifié | 2022-06-21 |
- Date de modification :