The ability of these systems to take up shrinkage is the reason we can
hold down these mid-rise structures effectively.
When you're designing a mid-rise wood frame building with five or six stories, as opposed
to a three or four storey wood frame building, your hold-down loads will double so you can go
from 25 PSA for a four storey wood frame structure to as much as fifty or sixty kips, for a six storey
structure, so there will probably need to be more of a through rod, continuous rod system.
The purpose of a continuous tie rod system, is to help a mid-rise structure from overturning,
during a high wind or seismic event. How a tie rod system works in structure,
it basically is a continuous rod all the way up both ends of a typical shear wall,
before we go over how the system works, I'd like to go over the components.
So here we have all threaded goes up all the way through the structure, standard rod couplings,
bearing plates, shrinkage cap, seen devices and then the system repeats
itself as we all go all the way up the structure. Our system terminates in structure we frame in
a mid wall header here, which keeps our device away from potential interferences of rip joists,
above the double top plate. What we have here is installation shop drawing sample of a six storey
structure, showing the continuous thread tie rod system, originally the concrete going all
the way up the structure to the top storey here. So what this drawing also shows is every component
call out and also all the compression wood requirements of the tie rod system.
The tension loads are cumulative as we work our way down the structure,
the bottom stories have much heavier thread rods and they will get lighter as we work our way up,
the compression members required to counteract the overturning moment
decrease as we work our way up the structure. What we have here is a tie rod system that
does not have a shrinkage compensating device and on this side one that does.
The purpose of a continuous tie rod system is to keep the building from swaying during
a seismic or high wind event. In order to do this effectively we must have tight connections
but what happens when shrinkage occurs, there is a gap between the nut and the plate.
This small gap will translate to several inches of horizontal drift, in the mid-rise structure,
conversely on the system that does have a take -up device, what happens during shrinkage is
that the part will compensate for whatever shrinkage occurs in the mid rise structure.
What we have demonstrated is the effects of shrinkage on both systems. If a seismic
event were to occur, this side will rack in order to close this gap before it's engaged.
The slide hammer effect could cause premature damage to the bottom plates of each shear wall.
On the other side the take-up device remains tight during the
seismic event which maintains the integrity of the system.
We're here today with Thomas Leung, the principal of Thomas Leung Structural Engineers. Thomas,
you've designed a few mid-rise building so far?
Yes in the past two years we've done a few, and recently we're doing about three or four,
currently I would like to show you one of them. We're here with a proposed building,
currently still under design. It is the architect is Ramsay Worden Architects
and the developer is Mosaic. It is probably one of the first five
storey wood frame building in Vancouver. So after we received the plans from the architect,
we review the plans, we allocate shear walls, and if it doesn't fit our requirements we ask the
architect to juggle the spaces somewhat and then in conjunction with the other consultants,
such as the mechanical engineer, we decide where the locations of these shear walls
are gonna be using tributary areas. We distribute the loads and allocate,
the tributary forces, assuming the floor diaphragms and the roof diaphragms are flexible.
So now I have X direction and I have Y direction properties for the shear walls,
then I put them together, I can generate a three-dimensional model, and here it
is folks and that's my representative of the building, under the equivalent
static load method, to excite the building. We go to apply the loads from the building code,
to find, say, find its natural frequencies. The picture you're seeing is largely exaggerated
to give the understanding of the vibration. After we understand the building we also have
to look at the actual deflection of this building. I have to compare the results from the 3D model,
and compare that to my original hand calculation and reiterate that process.
Elevator shaft, elevator core being a rigid element, is not compatible in a
lateral seismic behavior with wood. We have to come up with a device,
it permits the wood structure to slide horizontally, both horizontally and vertically.
We have proposed in this project, a hybrid masonry wall on either side of the elevator
shaft and the back wall is being constructed with solid wood, and the gap is exaggerated
to show the movement, but this back wall that has the wood component, would permit
the movement in the up and down direction to be the same as the rest of the building.
Mid rise construction is very attractive to the pre-manufactured component industry,
where mass repetition is key. Manufacturing in a controlled environment,
sheltered from our Canadian winter is great for maintaining quality control.
Prefabrication and mid-rise construction go together hand in hand, in that they
require a higher level of coordination, attention to detail, and quality control.
In the prefabrication industry we divide our projects into three stages:
design, production, and erection. During the design stage, our team works
with the architect, the engineer of record and the owner, to resolve issues, connection details,
seismic restraint issues, prior to us putting the project into production.
During the production process information is sent directly from our layout program
to our automated production equipment, for assembly, packaging and delivery.
Materials that would typically end up in a waste bin on a standard job site,
are recycled back into wall panels as blocking and other components.
Pre-manufactured, ready to install components also cut down on on-site construction waste and
dramatically speed up on-site construction times. Many factors, controllable and not, affect
the construction schedule, but when you have limited on-site storage space, prefab
components with just-in-time delivery are just another reason to consider prefab walls.
Installation of prefabricated panels is faster,
more efficient and higher quality than standard framing practice.
When the building code limits the use of structural or architectural wood in certain applications,
code consultants can often find sustainable alternative solutions,
which can be used to permit wood use, while fulfilling the intent of the code.
My name is Andrew Harmsworth, our company GHL Consultants Limited is a building
code consultant and fire engineering firm. Fire engineers and building code consultants have
backgrounds of the building code and specialist education in fire science and fire safety.
Our role is to understand not only the specific building code requirements,
but also to understand how fires start, how fires affect building and how people respond
to fire. That is how the occupants respond to fire alarms, how they exit the building, and
how to assist firefighters responding to a fire. Code consultants use this knowledge of how fires
burn along with computer based fire modeling and actual fire tests, to provide a wide variety of
code compliant solutions for their clients. One beautiful example of this is the
Earth Sciences Building at the University of British Columbia.