The main sources of lateral loads on buildings are either strong winds or earthquakes. These lateral forces are resisted by the buildings’ Lateral Load Resisting Systems (LLRSs). Adequate design of these systems is of paramount importance for the structural behaviour in general. Basic procedures for design of buildings subjected to lateral loads are provided in national and international model building codes. Additional lateral load design provisions can be found in national and international material design standards. The seismic and wind design provisions for engineered wood structures in Canada need to be enhanced to be compatible with those available for other materials such as steel and concrete. Such design provisions are of vital importance for ensuring a competitive position of timber structures relative to reinforced concrete and steel structures.
In this project a new design Section on Lateral Load Resisting Systems was drafted and prepared for future implementation in CSA O86, the Canadian Standard for Engineering Design in Wood. The new Section was prepared based on gathering existing research information on the behaviour of various structural systems used in engineered wood construction around the world as well as developing in-house research information by conducting experimental tests and analytical studies on structural systems subjected to lateral loads. This section for the first time tried to link the system behaviour to that of the connections in the system. Although the developed Section could not have been implemented in CSA O86 in its entirety during the latest code cycle that ended in 2008, the information it contains will form the foundation for future development of technical polls for implementation in the upcoming editions of CSA O86.
Some parts of the developed Section were implemented in the 2009 edition of CSA O86 as five separate technical polls. The most important technical poll was the one on Special Seismic Design Considerations for Shearwalls and Diaphragms. This technical poll for the first time in North America includes partial capacity design procedures for wood buildings, and represents a significant step forward towards implementing full capacity-based seismic design procedures for wood structures. Implementation of these design procedures also eliminated most of the confusion and hurdles related to the design of wood-based diaphragms according to 2005 National Building Code of Canada. In other polls, the limit for use of unblocked shearwalls in CSA O86 was raised to 4.8 m, and based on the test results conducted during the project, the NLGA SPS3 fingerjoined studs were allowed to be used as substitutes for regular dimension lumber studs in shearwall applications in engineered buildings in Canada.
With the US being the largest export market for the Canadian forest products industry, participation at code development committees in the field of structural and wood engineering in the US is of paramount importance. As a result of extensive activities during this project, for the first time one of the AF&PA Special Design Provisions for Wind and Seismic includes design values for unblocked shearwalls that were implemented based on FPInnovations’ research results. In addition, the project leader was involved in various aspects related to the NEESWood project in the US, in part of which a full scale six-storey wood-frame building will be tested at the E-Defense shake table in Miki, Japan in July 2009. Apart from being built from lumber and glued-laminated timber provided from Canada, the building will also feature the innovative Midply wood wall system that was also invented in Canada. The tests are expected to provide further technical evidence for increasing the height limits for platform frame construction in North America.
Building construction - Design
Earthquakes, Effect on building construction
Glued joints - Finger
Grading - Lumber
spIn this report, the seismic performance of 6-storey wood frame residential buildings is studied. Two building configurations, a typical wood-frame residential building and a building to be tested under the NEESWood project, were studied. For each building configuration, a four-storey building and a six-storey building were designed to the current (pre-April 6, 2009) 2006 BC Building Code (BCBC) and to the anticipated new requirements in the 2010 National Building Code of Canada (NBCC), resulting in four buildings with different designs. The four-storey building designed to the current 2006 BC Building Code served as the benchmark building representing the performance of current permissible structures with common architectural layouts.
In the design of both four-storey and six-storey buildings, it was assumed that the buildings are located in Vancouver on a site with soil class C. Instead of using the code formula, the fundamental natural period of the buildings was determined based on the actual mass and stiffness of wood-based shearwalls. The base shear and inter-storey drift are determined in accordance with Clauses 18.104.22.168.(3)(d)(iii) and 22.214.171.124.(3)(d)(iv) of BCBC, respectively.
Computer programs DRAIN 3-D and SAPWood were used to evaluate the seismic performance of the buildings. A series of 20 different earthquake records, 14 of the crustal type and 6 of the subcrustal type, were provided by the Earthquake Engineering Research Facility of the University of British Columbia and used in the evaluation. The records were chosen to fit the 2005 NBCC mean PSA and PSV spectra for the city of Vancouver.
For representative buildings designed in accordance with 2006 BCBC, seismic performance with and without gypsum wall board (GWB) is studied. For representative buildings designed in accordance with the 2010 NBCC, the seismic performance with GWB is studied. For the NEESWood building redesigned in accordance with 2010 NBCC, seismic performance without GWB is studied. Ignoring the contribution of GWB would result in a conservative estimate of the seismic performance of the building.
In the 2006 BCBC and 2010 NBCC, the inter-storey drift limit is set at 2.5 % of the storey height for the very rare earthquake event (1 in 2475 year return period). Limiting inter-storey drift is a key parameter for meeting the objective of life safety under a seismic event.
For 4-storey and 6-storey representative wood-frame buildings where only wood-based shearwalls are considered, results from both DRAIN-3D and SAPWood show that none of the maximum inter-storey drifts at any storey under any individual earthquake exceed the 2.5% inter-storey drift limit given in the building code. With DRAIN-3D, the average maximum inter-storey drifts are approximately 1.2% and 1.5% for 4-storey and 6-storey buildings designed with 2006 BCBC, respectively.
For the NEESWood wood-frame building, none of the maximum inter-storey drifts at any storey under any individual earthquake exceed the 2.5% inter-storey drift limit for 4-storey building obtained from SAPWood and 6-storey building obtained from DRAIN-3D and SAPWood. For any 4-storey building analysed with DRAIN-3D, approximately half of the earthquakes resulted in the maximum inter-storey drifts greater than 2.5% inter-storey limit. This is partly due to the assumptions used in Drain-3D model in which the lumped mass at each storey is equally distributed to all the nodes of the floor. As a result, the total weight to counteract the uplift force at the ends of a wall would be much smaller than that anticipated in the design, thus causing hold-downs to yield and large uplift deformations to occur.
Based on the analyses of a representative building and a redesigned NEESWood building situated in the city of Vancouver that subjected the structures to 20 earthquake records, 6-storey wood-frame building is expected to show similar or smaller inter-storey drift than a 4-storey wood-frame building, which is currently deemed acceptable under the current building code.
Building construction - Design
Building construction - Specfications
Earthquakes, Effect on building construction
This report summarizes the existing knowledge on building movement related to wood-frame construction. This knowledge includes fundamental causes and characteristics of wood shrinkage, instantaneous and time-dependent deformations under load, major wood-based materials used for construction and their shrinkage characteristics, movement amounts in publications based on limited field measurement, and movement estimations by construction practitioners based on their experience with wood-frame construction. Movement analysis and calculations were also demonstrated by focusing on wood shrinkage based on common engineering design assumptions, using six-storey platform buildings as examples. The report then provides engineering solutions for key building locations where differential movement could occur, based on the literature review as well as a small-scale survey of the construction industry.
The report emphasizes the importance of comprehensive analysis during design and construction to accommodate differential movement. Most building materials move when subjected to loading or when environmental conditions change. It is always good practice to detail buildings so that they can accommodate a certain range of movement, whether due to structural loading, moisture or temperature changes. For wood-frame buildings, movement can be reduced by specifying materials with lower shrinkage rates, such as engineered wood products and drier lumber. However, this may add considerable costs to building projects, especially when specifications have to be met through customized orders. Producing lumber with a lower moisture content adds significant costs, given the additional energy consumption, lumber degrade and sorting requirements during kiln drying. Specifying materials with lower moisture content at time of delivery to job site does not guarantee that wood will not get wet during construction, and excessive shrinkage could still be caused by excessively long time of exposure to rain during construction. On the other hand, effective drying can occur during the period between lumber delivery and lumber closed into building assemblies. Appropriate measures should be taken to ensure lumber protection against wetting, protected panel fabrication on site, good construction sequence to facilitate air drying, and supplementary heating before closing in to improve wood drying.
This report also provides recommendations for future work, including field measurement of movement and construction sequencing optimization, in order to provide better information for the design and construction of wood buildings, five- and six-storey platform frame buildings in particular.
Preliminary simulation was carried out using hygIRC and WUFI, both 1-D hygrothermal models, to analyze moisture performance of rainscreened wood-frame walls and cross-laminated timber (CLT) walls for the climates in Vancouver and Calgary. The major results are as follows.
In order to provide baseline knowledge, preliminary comparisons between hygIRC and WUFI were conducted to investigate the effects of climate data, wall orientations and rain intrusion on the performance of the rainscreened wood-frame walls based on Vancouver’s climate. hygIRC tended to produce almost constant moisture content (MC) of the plywood sheathing throughout a year but WUFI showed greater variations, particularly when the ventilation of the rainscreen cavity was neglected. Rainscreen cavity ventilation provided dramatic drying potentials for wall assemblies based on the WUFI simulation. hygIRC indicated that east-facing walls had the highest moisture load, but the differences between orientations seemed negligible in WUFI when the rainscreen cavity ventilation was taken into account. When 1% of wind-driven rain was simulated as an additional moisture load, hygIRC suggested that the rainscreen walls could not dry out in Vancouver, WUFI, however, indicated that they could dry to a safe MC level in the summer.
The discrepancies in material property data between the two models and between different databases in WUFI (even for the same wood species) were found to be very large. In terms of wood sorption data, large differences existed at near-saturated RH levels. This is a result of using pressure-plate/membrane methods for measuring material equilibrium moisture content (EMC) under high RH conditions. The EMC of wood at near-100% RH conditions measured with these methods can be higher than 200%, suggesting wood in construction would decay without liquid water intrusion or severe vapour condensation. The pressure-plate/membrane methods also appeared to be highly species-dependent, and have higher EMC at a certain RH level for less permeable species, from which it is relatively difficult to remove water during the measurement. The hygrothermal simulation in this work suggested that such a species bias caused by testing methods could put impermeable species (most Canadian species) at a disadvantage to permeable species like southern pine during related durability design of building assemblies.
In terms of using CLT for construction in Vancouver and Calgary, the WUFI simulations suggested that the use of less permeable materials such as EPS (expanded polystyrene insulation), XPS (extruded polystyrene insulation), self-adhered bituminous membrane and polyethylene in wall assemblies reduced the ability of the walls to dry. On the other hand, permeable assemblies such as those using relatively permeable insulation like semi-rigid mineral wool (rock wool) as exterior insulation, instead of less permeable exterior insulation materials, would help walls dry. The simulation also suggested that using CLT products with initially low MC would significantly reduce moisture-related risks, which indicated the importance of protecting CLT and avoiding wetting during transportation and construction.
In addition, the simulation found that indoor relative humidity (RH) conditions generated by the indoor RH prediction models included in hygIRC and WUFI varied greatly under the same basic climate and building conditions. The intermediate method specified in ASHRAE Standard 160 P resulted in long periods of saturated RH conditions throughout a year for the Vancouver climate, which may not be representative of ordinary residential buildings in Vancouver.
The simulation in this study is preliminary and exploratory. It would be arbitrary to recommend one model over the other based on this report or use the simulation results directly for CLT wall assembly design without consultation with building science specialists. However, this work revealed more opportunities for close collaborations between the wood science and the building science communities. More work should be carried out to develop appropriate testing methods and assemble material property data for hygrothermal simulation of wood-based building assemblies. Model improvement and field verification are also strongly recommended, particularly for new building systems such as CLT constructions.
In this study market opportunities for treated glue-laminated (glulam) products were investigated in the industrial wood sector. The main benefits of treated glulam are through-product treatment and the ability to manufacture treated products in shapes and sizes that do not fit into common treating chambers. These attributes provide for very durable and large glulam structures that are appropriate for outdoor use. For these reasons bridges, power poles, and sound abatement barriers were investigated. These are markets where wood has lost market share to or is being challenged by concrete and steel substitutes.
The vehicular bridge market was once heavy to the use of wood. Today wood accounts for only 7% of the number bridges in the US and less than 0.9% of the actual surface area of bridges in place. In interviewing municipalities in Canada it is clear that wood is not the preferred material with many wood bridges being replaced by concrete. Further, none of the municipalities contacted were planning wood bridges. However, wood bridges are still being installed. In the US 0.9% of the bridges installed by area in 2007 were wood. This is good news as wood is holding its market share. Steering clear of high volume or large bridges, local bridges are well suited for wood as they are plentiful, small in scale, and many are in disrepair. If 20% of local bridges were built with wood in Canada this would have equalled approximately $51 million in wood bridge construction in 2007.
Municipalities are much more open to the use of wood for pedestrian bridges and overpasses. Their quick construction and aesthetics are positive attributes in this application. One municipality contacted is planning multiple wood pedestrian bridges in the next five years. However, for the purpose of this market review there is little published information on pedestrian bridges.
Noise abatement barriers are a good high-volume technical fit for treated glulam. Increases in traffic and current road infrastructure improvements will lead to more demand for sound abatement in the future. This market is dominated by concrete, but at a very high price. If treated glulam can give adequate durability and sound performance properties it would be approximately 20% cheaper than concrete. The market for sound barriers in Canada could utilize up to 10 mmbf of wood per year to construct 80 km of barrier. This product can also be marketed as a high-performance acoustic fence for residential markets.
Treated glulam was also considered for utility poles. It is transmission grade poles where glulam would best fit the market as the demand is for longer poles which are more difficult to get in solid wood. This type of pole is where wood is currently being displaced by tubular steel. If glulam poles were used in 25% of the replacement transmission poles per year this could equal 8 mmbf. Light poles or standards are another market to consider. While this is a relatively low volume market glulam light standards are a premium product in European markets.
This report, written by FPInnovations professionals, presents the results of a technological and economic analysis focusing on manufacturing cross-laminated timber (CLT). The purpose of the report is to provide specialized information on existing technology and manufacturing processes for the benefit of potential prometers who might be interested in implementing industrial projects.
A study was conducted with the primary objective of examining the efficacy of a standard block shear test method to assess the bond quality of cross-laminated timber (CLT) products. The secondary objective was to examine the effect of pressure and adhesive type on the block shear properties of CLT panels. The wood material used for the CLT samples was Select grade nominal 25 x 152-mm (1 x 6-inch) Hem-Fir. Three adhesive types were evaluated under two test conditions: dry and vacuum-pressure-dry (VPD), the latter as described in CSA standard O112.10. Shear strength and wood failure were evaluated for each test condition.
Among the four properties evaluated (dry and VPD shear strength, and dry and VPD wood failure), only the VPD wood failure showed consistency in assessing the bond quality of the CLT panels in terms of the factors (pressure and adhesive type) evaluated. Adhesive type had a strong effect on VPD wood failure. The different performance levels of the three adhesives were useful in providing insights into how the VPD block shear wood failure test responds to significant changes in CLT manufacturing parameters. The pressure used in fabricating the CLT panels showed a strong effect on VPD wood failure as demonstrated for one of the adhesives. VPD wood failure decreased with decreasing pressure. Although dry shear wood failure was able to detect the effect of pressure, it failed to detect the effect of adhesive type on the bond quality of the CLT panels.
These results provide support as to the effectiveness of the VPD block shear wood failure test in assessing the bond quality of CLT panels. The VPD conditioning treatment was able to identify poor bondline manufacturing conditions by observed changes in the mode of failure, which is also considered an indication of wood-adhesive bond durability. These results corroborate those obtained from the delamination test conducted in a previous study (Casilla et al. 2011).
Along with the delamination test proposed in an earlier report, the VPD block shear wood failure can be used to assess the CLT bond quality. Although promising, more testing is needed to assess whether the VPD block shear wood failure can be used in lieu of the delamination test. The other properties studied (shear strength and dry wood failure), however, were not found to be useful in consistently assessing bond line manufacturing quality.
A study was conducted with the primary objective of gathering information for the development of a protocol for evaluating the surface quality of cross-laminated timber (CLT) products. The secondary objectives were to examine the effect of moisture content (MC) reduction on the development of surface checks and gaps, and find ways of minimizing the checking problems in CLT panels. The wood materials used for the CLT samples were rough-sawn Select grade Hem-Fir boards 25 x 152 mm (1 x 6 inches). Polyurethane was the adhesive used. The development of checks and gaps were evaluated after drying at two temperature levels at ambient relative humidity (RH).
The checks and gaps, as a result of drying to 6% to 10% MC from an initial MC of 13%, occurred randomly depending upon the characteristics of the wood and the manner in which the outer laminas were laid up in the panel. Suggestions are made for minimizing checking and gap problems in CLT panels. The checks and gaps close when the panels are exposed to higher humidity.
Guidelines were proposed for the development of a protocol for classifying CLT panels into appearance grades in terms of the severity of checks and gaps. The grades can be based on the estimated dimensions of the checks and gaps, their frequency, and the number of laminas in which they appear.
A study was conducted with the primary objective of examining the efficacy of delamination test using cylindrical core specimens to assess the bond quality of cross laminated timber (CLT) products. A prototype coring drill bit was fabricated to prepare a cylindrical-shaped specimen, the height of which corresponds to the full thickness of the CLT panel. A secondary objective was to examine the effect of pressure, adhesive type, number of plies, and specimen shape on the delamination resistance of CLT panels. The wood material used for the CLT samples was Select grade nominal 1 x 6-inch Hem-Fir boards. Examples of three adhesive types were evaluated, which were designated as A, B, and C. The delamination tests used were as described in CAN / CSA O122-06 and EN 302-2.
Cylindrical specimen extracted as core was found satisfactory as a test specimen type for use in delamination testing of CLT product. Its efficacy was comparable to that of a square cross-section specimen. The former is recommended as it can be extracted from thicker panels and from any location in the panel. It would also be more convenient to plug the round hole.
Adhesive type had a strong effect on delamination resistance based on the two delamination tests used. Adhesive A exhibited the greatest delamination resistance, followed in decreasing order, by adhesives C and B. It should be noted that no effort was made to find the optimum CLT manufacturing parameters for each type of adhesive. Therefore the relative rankings of the adhesives tested may not be representative. However, for the purposes of this study, the different performance levels from the three adhesives are useful in providing insight into how the proposed delamination test responds to significant changes in CLT manufacturing parameters.
Pressure used in fabricating the CLT panel showed a strong effect on delamination resistance as demonstrated for one of the adhesives. Delamination resistance decreased with decreasing pressure. The effect of the number of plies in the CLT panel was dependent upon the type of adhesive, and this was probably related to the adhesive’s assembly time characteristic. These results provide support as to the effectiveness of delamination test in assessing the moisture durability of CLT panels. It was able to differentiate the performance in delamination resistance among different types of adhesives, and able to detect the effect of manufacturing parameters such as pressure and increased number of plies in CLT construction.
The test procedure described in CAN / CSA O122-06 appears to be reasonable in the delamination resistance assessment of CLT panels for qualification and quality control testing. Based on the results of the study along with some background information and guidelines, delamination requirements for CLT panels are proposed. The permitted delamination values are greater than those currently specified for laminated and fingerjoined lumber products. This is in recognition of the higher bond line stresses when bonded perpendicular laminations (i.e. CLT) are exposed to the delamination wetting and drying cycles, as opposed to parallel laminations (i.e. glulam or fingerjoints).
The number of occupant complaints received about annoying low-frequency footstep impact sound transmission through wood floor-ceiling assemblies has been increasing in proportion with the increase in the number of multi-family wood buildings built. Little work has been conducted to develop solutions to control the low-frequency footstep impact sound transmission. There are no code provisions or sound solutions in the codes. Current construction practices are based on a trial and error approach. This two-years project was conducted to remove this barrier and to successfully expand the use of wood in the multi-family and mid- to high-rise building markets. The key objective was to build a framework for the development of thorough solutions to control low-frequency footstep sound transmission through wood floor-ceiling assemblies.
Field acoustic tests and case studies were conducted in collaboration with acoustics researchers, builders, developers, architects, design engineers and producers of wood building components.
The field study found that:
1. With proper design of the base wood-joisted floors and sound details of the ceiling:
With no topping on the floor, the floor-ceiling assembly did not provide sufficient impact sound insulation for low- to high-frequency sound components ;
Use of a 13-mm thick wood composite topping along with the ceiling did not ensure satisfactory impact sound insulation;
Even if there was the ceiling, use of a 38-mm thick concrete topping without a proper insulation layer to float the topping did not ensure satisfactory impact sound insulation ;
A topping system having a mass over 20 kg/m2 and composed of composite panels and an insulation layer with proper thickness achieved satisfactory impact sound insulation.
2. The proper design of the base wood-joisted floors was achieved by the correct combination of floor mass and stiffness. The heaviest wood-joisted floors did not necessarily ensure satisfactory impact insulation.
3. Proper sound ceiling details were found to be achieved through:
Use of two layers of gypsum board;
Use of sound absorption materials filling at least 50% of the cavity ;
Installation of resilient channels to the bottom of the joists through anchoring acoustic system resulted in improved impact sound insulation than directly attaching the resilient channels to the bottom of the joists.
A four-task research plan was developed to thoroughly address the issue of poor low-frequency footstep impact insulation of current lightweight wood floor-ceiling assemblies and to correct prejudice against wood. The tasks include: 1) fundamental work to develop code provisions; 2) expansion of FPInnovations’ material testing laboratory to include tests to characterize the acoustic properties of materials; 3) development of control strategies; and 4) implementation.
The laboratory acoustic research facility built includes a mock-up field floor-ceiling assembly with adjustable span and room height, a testing system and a building acoustic simulation software.
The preliminary study on the effects of flooring, topping and underlayment on FIIC of the mock-up of the filed floor-ceiling assembly in FPInnovations’ acoustic chamber confirmed some findings from the field study. The laboratory study found that:
A topping was necessary to ensure the satisfactory impact sound insulation;
The topping should be floated on proper underlayment;
Topping mass affects impact sound insulation of wood framed floors;
A floating flooring enhanced the impact sound insulation of wood framed floors along with the floating topping.
It is concluded that:
1. even if the studies only touched the tip of the iceberg of the footstep impact sound insulation of lightweight wood-joisted floor systems, the proposed solutions are promising but still need verification ;
2. with proper design of the base wood floor structure, the proper combination of flooring, and sound ceiling details along with proper installation, the lightweight wood floor-ceiling assembly can achieve satisfactory impact sound insulation ;
3. this study establishes a framework for thoroughly solving low-frequency footstep impact sound insulation problem in lightweight wood-joisted floor systems.
Solutions will be developed in the next phase of this study as planned and the study will be conducted under NRCan Transformative Technology program with a project dedicated to “Serviceability of next generation wood building systems”.