Capítulo 4. Análisis de resultados
4.1. Presentación de resultados
4.1.5. Resultados de las notas de campo
A comparison of test results to the ASCE7-05 (2006), was carried out to determine the design wind speeds based on the strength of the toe-nail connections. Due to the significant variability in toe-nail capacity, a factor of safety (FOS) must be selected in
order to make a useful comparison to a design standard. Studies by Reed et al. (1997) used FOS of 2 to 3 along with the 5 and 10 percentile values from the failure capacity distributions, while Cheng (2004) used a factor of safety of 2.5.
The full scale test house described in Chapter 2.1.1 was used to obtain the design wind forces from ASCE 7-05 (2006). It is unclear whether toe-nail connections should be treated as Main Wind Force Resisting Systems (MWFRS) or as a Components and Cladding (C&C) element in the ASCE 7-05 (2006). Consequently, the design forces at RTWC ―S3‖ were obtained by treating toe-nail connections as both a Main Wind Force Resisting System (MWFRS) and a Components and Cladding (C&C) element. The design terrain was assumed to be category C, with wind directionality factor (Kd),
topographic factor (Kzt), and importance factor (I) all equal to 1. The ASCE7-05 (2006)
wind speeds are then calculated and presented in Table 5.2, using the toe-nail capacity for each assumed factor of safety and a dead load due to the weight of the roof of 250 N, for both MWFRS and C&C loads using the FOS of Reed et. al. (1997). It is noted that the 5th and 10th percentile peaks correspond well with the failure capacities found for toe-nail connections with missing nails reported in Table 4.2. In addition, since the wind tunnel data for the building under consideration is available, GCpeq values are calculated from
this data, using the procedure outlined by St. Pierre et al. (2005) and presented in Eq. (3.1). For the current test house the value ‗a‘ in the ASCE 7-05 that defines the different
regions of the roof is 1.0m and is defined by 10% of the least horizontal dimension. This means that using a tributary area approach to calculate the loads on connection ―S3‖ using MWFRS the tributary area (3.2 m2) is within Zone 3E, with a GCp value of -0.69.
However, when using the C&C portion of the code, the tributary area for connection ―S3‖, is spread over 3 zones: Zone 1 44%, Zone 2 45% and Zone 3 11%. This results in an area average GCp of -1.47. The wind speeds shown in Table 5.2 show that the
MWFRS loads calculated for this particular connection are unconservative compared to that obtained from the wind tunnel data. In contrast the C&C loads are shown to be conservative compared to the wind tunnel data. Considering the wind speeds calculated using the C&C loads and the wind tunnel pressure data, the toe-nail connections tested do not have sufficient hold down capacity for the highest loaded region of the test house, in any wind region given by the ASCE7-05 (2006). It is noted that the wind tunnel wind speeds meet the 90 mph wind speed region, provided no factor of safety is assumed. Wind speeds calculated using the MWFRS coefficients from the ASCE7-05 (2006) show that toe-nail connections can be used in several wind regions, when using the 5th and 10th percentile toe-nail strength. Caution should be taken however, since these results are significantly unconservative as compared to the wind tunnel data.
Table 5.2 ASCE 7-05 Design Wind speed calculated using both the MWFRS and C&C coefficients from the code as well as from wind tunnel pressure data
FOS Toe-nail Design
Capacity (kN)
ASCE7-05 Design Wind Speed (mph) MWFRS C&C Wind Tunnel
0 2.8 108 64 92
2 1.4 80 47 65
3 0.93 67 40 65
5th Percentile 1.9 91 53 76
10th Percentile 2.2 97 57 82
Other factors, such as internal pressure and load sharing were not considered in the present analysis and both can have a significant effect on the calculated failure wind speed. In the case of internal pressures, dominant openings in the windward wall can
significantly lower the failure wind speed (Kopp et al. 2008). At the present time the amount of load sharing that occurs for wood frame structures is unknown, although this is examined to some extent in chapter 7.0. The results presented in Table 5.2 assume that there is no load sharing between connections, which is a conservative assumption. The wind speeds presented in Table 5.3, have been calculated assuming perfect load sharing between all connections on the roof. Or, in other words, that the load at every connection is the same. As discussed in Chapter 1.1, this assumption is likely unconservative for the worst loaded connections. In contrast, to the no load sharing case, by assuming perfect load sharing both the MWFRS and C&C loads from the ASCE7-05 (2006) are conservative compared with the wind tunnel data for RTWC ―S3‖. This is likely the result of the ASCE7-05 (2006) over-predicting the pressure coefficients in the field of the roof for this particular structure. The wind speeds for the MWFRS and C&C loads have increased in comparison to the worst loaded connection case shown in Table 5.2, however the changes are small. In the case of the wind tunnel data, the wind speeds are substantially higher. This result is not surprising since the low spatial correlation of the wind loads on the roof are well known and have been shown earlier in Inset B of Figure 1.5. However, the added benefit of significant load sharing between connections is that the FOS can be reduced from that used when considering a single connection.
Table 5.3 ASCE 7-05 Design Wind speed calculated using both the MWFRS and C&C coefficients from the code as well as from wind tunnel pressure data, assuming perfect
load sharing between toe-nail connections on the roof
FOS Toe-nail Design
Capacity (kN)
ASCE7-05 Design Wind Speed (mph) MWFRS C&C Wind Tunnel
0 2.8 112 84 124
2 1.4 82 61 91
3 0.93 70 52 77
5th Percentile 1.9 94 70 104