Total uplift force coefficient comparisons

In this last part of the uplift force verification process, the wind tunnel simulation is incorporated to the finite element analysis in order to estimate the total uplift force. As previously discussed, full-scale data were acquired using both pressure and force sensors, whereas wind tunnel tests produced only envelope pressures. In addition to the verification of the wind tunnel simulation in the form of pressure coefficient

1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00

1.E-03 1.E-02 1.E-01

SF /σ 2 n /V Full Scale Estimated (FEA) 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00

1.E-03 1.E-02 1.E-01

SF /σ 2 n /V Full Scale Estimated (FEA)

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comparisons, the total instantaneous uplift force on the building was compared to the available full-scale load data by integrating the measured envelope pressures obtained in the wind tunnel over the roof surface. Consequently, the total uplift force coefficient was calculated, and compared to that calculated directly by the load cell data, for each upstream terrain case using the following equation:

p,i eff ,i f ,z

c A

c

A





where cp,i: instantaneous pressure coefficient, Aeff,i: effective roof pressure tap area (m2) and A: horizontal projected area of the roof (m2). It should be noted that dynamic and attenuation effects occurring as the wind load is transferred from the various structural and non-structural components to the foundation walls are not considered in the wind tunnel approach (instantaneous static analysis). Similarly to the previous comparisons, for the full-scale calculations the mean force values were based on a 10-minute average and the instantaneous peak force values on a 3-second gust, both obtained from the available stationary full-scale records. The field data were filtered to retain only those for wind speeds over 4 m/sec (at 10 meters height). The dynamic pressure was always averaged on a 10-minute basis and was referenced to the roof height. Moreover, field data were integrated over a wind angle of attack of 10-degree range to account for the higher standard deviation values of the wind direction and to be directly compared to wind tunnel tests carried out using intervals of 10 degrees. To account for the varying characteristics of the full-scale results, the range (maximum and minimum values) of the integrated values of each set of data was considered in addition to the mean values.

The comparison of the mean total uplift force coefficients presented in Figure 6.27, shows that all three wind tunnel upstream exposure configurations are close to the range

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of the field values. The discrepancies are somewhat higher for the South-West to North- West region (230 to 270 degrees) where the mean full-scale values are up to 35% and 40% higher compared to the light and heavy suburban terrain wind tunnel tests respectively. The peak uplift force coefficient comparison is presented in Figure 6.28 and shows better agreement compared to the mean values. Both positive and negative peak wind tunnel results are in most cases within the range of the field data. The agreement for both positive and negative peak uplift force coefficients is particularly improved for the light suburban terrain simulation. It should be noted that the full-scale values are always lower compared to the heavy suburban terrain wind tunnel tests. This finding is quite interesting, considering that the full-scale mean and peak pressure coefficients were in better agreement with the heavy suburban wind tunnel case. As indicated above the use of wind tunnel pressure coefficient in the finite element analysis does not incorporate any attenuation effect similar to those described in the previous sections. Therefore, by using the pressures obtained in the wind tunnel the total uplift force may be overestimated by at least 20% (see section 6.5.2) and this could justify the discrepancies between the heavy suburban wind tunnel force values and those obtained by the foundation load cells.

In addition to the experimental findings, the estimated total uplift force coefficients derived from the NBCC 2005 building code and ASCE 7-10 standard were also plotted in Figure 6.28. For the NBCC 2005 calculations, the external peak composite pressure-gust coefficients (CpCg) from Figure I-7 were used to calculate the total uplift force coefficient. The averaging period for the reference wind pressure was adjusted from hourly to 10-minute mean (Figure C26.5-1, ASCE 7-10). In a similar manner, the external pressure coefficients (GCpf) from Figure 28.4-1 (ASCE/SEI 7-10) were

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considered and the total uplift force coefficient was computed. For this comparison the velocity pressure was adjusted to account for the averaging period of 10 minutes instead of the 3-sec gust considered in the ASCE standard. It should be noted that on the estimation of the total uplift force using NBCC and ASCE 7-10 two cases were considered, those with and without the contribution of the internal pressures. For the case of full-scale force measurements, the load cells capture the total effect including both external and internal pressures applied on the wall and roof surfaces whereas in the wind tunnel experiments only external pressures are considered.

As Figure 6.28 indicates (note that NBCC* and ASCE* refer to the cases where internal pressures were considered), the estimated NBCC 2005 and ASCE 7-10 values appear to be conservative in relation to the field measurements. However, wind tunnel values, particularly for the case of the heavy suburban terrain, exceed the recommended code provisions. The ASCE 7-10* uplift force coefficient value of -2.03 is higher (absolute value) than the open and light suburban terrain wind tunnel curves and for most of the directions is also higher than the heavy suburban terrain wind tunnel curves. On the other hand, the NBCC 2005* value of -1.60 compares slightly worse than ASCE with the heavy suburban terrain values, whereas the light suburban terrain wind tunnel force coefficients is again below the estimated NBCC 2005 values. This underestimation will become even more critical if the NBCC 2005 values are adjusted for the exposure using the factors provided in Sentence (5) of section 4.1.7.1 (NBCC 2005). For the particular building the exposure factor, which is equal to 0.90 for open terrain and 0.70 for rough terrain, would further reduce the estimated force coefficient by an additional 30% resulting in values that are lower than the experimental findings. It should be noted that

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similar observations (non-conservative code recommendations – particularly for the ASCE 7 standard) have been reported by other studies in the past (e.g. Liu et al 2009, Caracoglia and Jones 2009, Mensah et al 2010).

As previously addressed by Zisis and Stathopoulos (2009) the discrepancies between the experimental procedures can be partially attributed to the relatively complex surrounding region. The analysis of wind velocity and direction data from the weather tower indicated a non-uniform variation of the basic exposure parameters (power law exponent, turbulence intensity and roughness length) with respect to the wind direction. Significant roughness amplification was denoted for the wind direction range from 240 to 300 degrees which coincides with the region where the discrepancies between the full- scale and wind tunnel values are of higher order. Finally, higher fluctuations of the wind direction in the field data should also be addressed and considered accountable for discrepancies between the wind tunnel and field values.

Figure 6.27 Mean total uplift force coefficient comparison.

-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0 30 60 90 120 150 180 210 240 270 300 330 360 C f,m ea n

Wind Direction (deg.)

Wind Tunnel (α=0.16) Wind Tunnel (α=0.22) Wind Tunnel (α=0.28)

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Figure 6.28 Minimum and maximum peak total uplift force coefficient comparison.

-3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 0 30 60 90 120 150 180 210 240 270 300 330 360 C f, p eak

Wind Direction (deg.)

Wind Tunnel - Cf,min (α=0.16) Wind Tunnel - Cf,max (α=0.16)

Wind Tunnel - Cf,min (α=0.22) Wind Tunnel - Cf,max (α=0.22)

Wind Tunnel - Cf,min (α=0.28) Wind Tunnel - Cf,max (α=0.28)

Full-Scale (minimum) Full-Scale (maximum)

Full-Scale (range)

ASCE 7-10

NBCC 2005 NBCC 2005* ASCE 7-10*

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