Resultados de la tercera fase: los focus group

A typical 2-story wood frame, brick veneer house shown in Figure 2.1, was built at the Insurance Research Lab for Better Homes following the Ontario building code, by Building Science Students from Fanshawe College in London Ontario. The house plan dimensions are 9.0 m by 8.9m with an eaves height of 8.0 m and a gable ended roof with a roof slope of 4:12 (φ= 18°). The roof overhangs the external wall of the house on all sides by approximately 0.61m. The prefabricated roof trusses are spaced 0.61m on center and connected to the wall top plate using standard toe-nail connections. The roof is sheathed with 9mm plywood, and asphalt shingles. The walls were built with standard 2 x 4‘s at 0.41 m centers and the exterior walls are covered with 25mm foam insulation which was then covered by a brick veneer. Figure 2.3 shows a picture of the house under construction prior to the installation of the roof sheathing and brick veneer. The construction of the house was intended to approximate typical construction in the region.

As such, no attempt was made to enforce any particular quality control measures that would not be implemented in standard construction, although construction errors and structural details were documented during and after construction.

Figure 2.1 Photograph of the test house surrounded by the steel reaction frame used to mount the air bags and PLAs.

Through this documentation of the structure it was observed that 2 different types of twisted shank nails had been used for the toe-nail RTWC, either 12d (length: 82.6mm, shank diameter: 2.87 mm) or 16d (length: 88.9 mm, diameter: 3.33 mm) nails. Overall, the use of each nail appeared to be equal. The use of 2 nails likely occurred because different students having different sized nails available. While the number of nails per connection varied from connection to connection, the average number of nails per

connection is three, however, it should be pointed out that this is a simple count of the number of nails and not a reflection of the quality of the nailed connection. Chapter 9 of the National Building Code of Canada NBCC (2005), prescribes that the roof trusses should be held down with three 82mm nails. So, the current test appears to meet the prescriptive requirement for RTWC of the NBCC (2005). Figure 2.2 provides a side by side photograph of each of these nails. The documentation of the RTWC also included photographs of each connection from both sides so that errors in construction or construction defects are noted and to provide a basis of comparison once testing had commenced and damage occurred to the house.

Figure 2.2 Photograph of the two types of nails used for the toe-nail RTWC on the test House. The grid shown is 6mm by 6mm, above is the 12d nail, below is the 16d nail.

Figure 2.4 shows a photograph of one toe-nail connection prior to testing. From the photograph it is observed that one nail has been nailed at too shallow an angle, likely reducing the hold down capacity of the connection compared with a ―perfect‖ connection. APPENDIX B provides further photographs for the other RTWC prior to testing. The house was inspected by 30 building inspectors from across southern Ontario who, on average, found it to be typical of construction within the region. The documentation of the structural details of the house also revealed that there were additional RTWC between the roof trusses and the internal walls. While these additional connections may be present in typical construction, it is not guaranteed. Moreover, the number and location of these connections are dependent on the internal floor plan of the house. Since the connections may not exist in typical construction, or may even be removed during renovations it was decided to remove these internal connections after construction had been completed. While an attempt was made remove all of these connections, it was discovered upon removal of the roof that some connections had been missed, although an exact count and location of the extra connections to the internal walls is not available.

Prior to testing of the roof the shingles were removed and the roof sheathing was screwed to the roof trusses to prevent failures of these elements and to allow the attachment of the air-bags to the roof (the air-bags will be discussed in Chapter 2.1.2). This reduces the dead load of the roof, but does not significantly alter the overall structural system. Moreover, since the dead load is evenly distributed over the entire roof, the effect on each individual connection is the same. As a result the results can be re-interpreted in post processing to account for the missing weight.

Figure 2.3 Photograph of the test house under construction. Photograph shows the house prior to the installation of the brick veneer and roof trusses.

The test house has a total of 16 roof trusses, for a total of 32 RTWC. Figure 2.5 provides a schematic drawing of the truss layout, the naming convention, along with cardinal directions, which will be used through the remainder of the thesis.

Figure 2.5Truss layout and naming conventions, with laboratory cardinal directions

The goal of the current experiments is to monitor the response of the ‗as built‘ house under realistic wind loads. As such, an effort was made to make as few modifications to the house as possible. In order to monitor the movement of the house during the experiment, displacement transducers were mounted on the brick veneer shown in Figure 2.6, to measure the displacement of every roof to wall connection, with the exception of the north and south connections on truss 17. Truss 17 was the gable end wall on the Western side of the house and during construction the truss was cut so that the truss did not overhang the North or South walls; as a result, it was not possible to measure the displacement of this truss. Independent measurements from the ground ensured that the brick veneer remained stationary throughout all of the tests. In addition, 16 video

cameras as shown in Figure 2.6, were placed at various positions around the house and were also used to monitor the response.

Figure 2.6 Photograph of the displacement transducers mounted on the brick veneer to measure the displacement of the roof to wall connections