In te current study, wind-induced torsional loads on low and medium eigt buildings were examined in te boundary layer wind tunnel. uilding model (scal

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1 Te Sevent International Colloquium on luff ody Aerodynamics and Applications (AA7) Sangai, Cina; September 2-6, 2012 Wind-induced torsional aerodynamic loads on low and medium eigt buildings M. Elsarawy, T. Statopoulos and K. Galal Faculty of Engineering and Computer Science, Concordia University, Montréal, QC, Canada ASTRACT: Since tere is limited information about wind-induced torsional loads on buildings, wind tunnel tests were carried out on a series of buildings wit low and medium eigts. Four buildings (scale 1:400), aving te same orizontal dimensions but different eigts (6, 12, 25 and 50 m), were tested in a simulated open terrain exposure for different wind directions (i.e. from 0 o to 180 o every 15 o ). Te syncronized wind pressure measurements on te rigid building model allowed estimating te instantaneous sear forces and torsional moments. All results were normalized and presented in terms of mean and peak values of sear and torsional coefficients representing two load cases (torsion and sear load). Furtermore, te experimental results were compared wit te existing torsion- and searrelated provisions in te National uilding Code of Canada (NCC 2010), te American Society of Civil Engineers Standard (ASCE/SEI 7-10) and te European Code (EN ). Te results demonstrated significant discrepancies among te provisions of tese wind standards from one side and te wind tunnel results from anoter in evaluating torsional wind loads on low and medium eigt buildings. Te findings of tis study could assist to improve te analytical metods to evaluate wind-induced torsional loads on low and medium eigt buildings. KEYWORDS: Torsion, wind loads, codes, low buildings, medium eigt buildings, structural design 1 INTRODUCTION Te common caracteristics of wind-induced loads on building envelopes continuously vary in temporal and spatial dimensions. Te variation of local wind pressures on building claddings and te total effective wind forces (base sear/overturning moment) on te main structural building systems of low and medium rise buildings ave been investigated extensively in te past few decades (Krisna, 1995, Statopoulos and Dumitrescu, 1989, and Sanni et al, 1992). However, studies on wind-induced torsional loads on low and medium eigt buildings are very limited. Discrepancies ave been found wen wind-induced torsional load provisions for low and medium eigt buildings in tree well known building codes and standards (American, Canadian, and Eurocode) were compared by Elsarawy et al Wind-induced torsion as been measured in te wind tunnel for tree low-rise buildings wit different aspect ratios (lengt/widt = 1, 2, and 3) in open terrain exposure by Isyumov and Case, Te study suggested tat applying partial wind loads, similar to tose implemented for te design of tall buildings, would improve te design of low buildings until more pertinent data become available. Recently, tree low rise buildings aving te same orizontal dimensions but different roof angles (0 o, 18.4 o, and 45 o ) located in open terrain exposure were tested by Elsarawy et al Te results were also compared to te current wind load provisions and it was found tat te American standard introduces torsional moment in line wit te experimental data, wile te Canadian and Eurocode provisions underestimate te torsional moment on low buildings

2 In te current study, wind-induced torsional loads on low and medium eigt buildings were examined in te boundary layer wind tunnel. uilding model (scaled at 1:400) as been used to represent four actual buildings aving te same orizontal dimensions but wit different eigts. All buildings were tested in open exposure for different wind directions. Te syncronized wind tunnel measurements were presented in terms of sear, and torsional coefficients. Furtermore, te experimental results were compared wit wind provisions in te NCC 2010, ASCE/SEI 7-10 and EN WIND TUNNE STUDIES All experiments were carried out in a boundary layer wind tunnel wit a working section approximately 12.2 x 1.80 m and an a adjustable roof eigt ranging between 1.4 and 1.8 m. A turntable of 1.2 m diameter is located on te test section of te tunnel and allows testing of models for any wind direction. A new automated Traversing Gear system as been installed to give te capability of measuring wind caracteristics at any spatial location around a building model inside te test section. A geometric scale of 1:400 as been recommended for te simulation of te most important variables of te atmosperic boundary layer under strong wind conditions. 2.1 uilding models Te basic building model used for te experiments was fabricated from plexiglass and scaled at 1:400. Figure 1 sows te model and te location of 146 pressure taps on its side walls. Te roof does not ave any pressure taps, since te uplift force does not contribute to torsion or orizontal sear forces. Te model was tested at different building eigts representing four actual buildings wit eigts (6, 12, 25 and 50 m). Model dimensions and te tested building eigts are given in Table 1. Test 1 (H = 6 m) Test 2 (H = 12 m) mm Test 3 (H = 25 m) mm 37.5 Test 4 (H = 50 m) ,5 30,5 30,5 152,5 mm 30,5 16, ,5 32, mm 16,25 Figure 1. uilding model and 146 pressure taps location. Table 1. Model dimensions and uilding eigts tested uilding Dimensions Scaled (1:400, mm) Actual (m) Widt () engt () Tested eigts () 15, 30, 62.5, 125 6, 12, 25,

3 Te Sevent International Colloquium on luff ody Aerodynamics and Applications (AA7) Sangai, Cina; September 2-6, Terrain simulations An open-country exposure was simulated in te wind tunnel. Figure 2 sows te flow approac profiles of mean wind velocity and turbulence intensity measured using a 4-ole Cobra probe (TFI) for te simulated terrain exposure. Te gradient wind velocity is 13.6 m/s at a eigt of z g =70 cm. Te power law exponent for te wind velocity profiles simulated in tese tests was Altoug, it is not common for medium eigt buildings to be situated in open terrain, tis exposure was used as a kind of conservatism since iger loads are expected to act on buildings. Te pressure measurements on te models were conducted using a system of miniature pressure scanners from Scanivalve (ZOC33/64Px) and te digital service module DSM All measurements were syncronized wit a sampling rate of 300Hz on eac cannel for a period of 27 sec (i.e. about 1 our in full scale). 2.3 Analytical approac Figure 3 sows a scematic representation of external pressure distributions on building envelope at a certain instant, te exerted sear forces (F X, F Y ) and torsional moment (M T ). Pressure measurements are scanned simultaneously. Te instantaneous wind force at eac pressure tap is calculated according to f i,t = (p i,t A effective ) f j,t = (p j,t A effective ) (1) were P i,t, and P j,t are instantaneous pressures measured at eac pressure tap. Te wind forces exerted at pressure tap locations in X- and Y-directions are noted by f i,t and f j,t, respectively. For eac wind direction, te orizontal force components in X- and Y-directions, and te total base sear, are evaluated according to N X = M F f i,t F Y = f j,t i=1 j= V F X FY (2) were N and M are te numbers of te pressure taps on te longitudinal and transverse directions, respectively. All tese forces are normalized wit respect to te dynamic wind pressure at te mean roof eigt as follows: C vx FX q FY C vy q V C V (3) Were q = dynamic wind pressure at mean roof eigt (kn/m 2 ), = minimum orizontal building dimension (m), and = mean roof building eigt (m). Te torsional coefficients (C T ) and equivalent eccentricity (e) are evaluated based on C MT M e (%) T x T q *V 100 (4) were = building lengt NCC 2010 specifies wind loads on low buildings (mean roof eigt, < 10 m, or < widt, and < 20 m) and medium-eigt rigid buildings ( < 60 m, / < 4, lowest natural frequency, f n > 1 Hz). On te oter and, ASCE/SEI 7-10 identifies low buildings as ( < 18 m and < ) and medium-eigt rigid buildings as aving f n > 1 Hz. In EN , low buildings were defined as tose wit < 15 m wile buildings wit frames, structural walls wit less tan 100 m are introduced structurally as rigid buildings. In tis study all tested buildings were assumed to be structurally rigid and follow te limitations stated in te tree wind load standards. q 1211

4 Pressure tap F II F I M T Measured pressures r j F X M T F Y r i f i, t f j, t Wind direction Figure 2. Wind velocity and turbulence intensity profiles for open terrain exposure. Figure 3. Instantaneous wind pressure distributions, generated wind forces (F X, F Y ) and torsional moment (M T ). 3 COMPARISON WITH PREVIOUS STUDIES A comparison wit a previous study by Amin and Auja, 2012 for building ( = 37.5, = 15, = 90 m, scale 1:300, 140 pressure taps) was made using te wind tunnel measurements for building model ( = 61 m, = 39 m, = 50 m, scale 1:400, 146 pressure taps). Figure 3 sows te evaluated mean torsion for bot buildings for different wind directions. Te mean torsion coefficient was calculated based on te following formula i n C Mean ase Torsion / (q ) ( C x A x r )/ (5) T Mean i 1 P Mean effective Were n=number of pressure taps, C P Mean = mean pressure coefficient, A effective = presented area by te pressure tap, r i = te distance between pressure tap and te building center. i Figure 3. Mean torsional coefficient measured by Amin and Auja, 2012 and te current study Data sow relatively good agreement for te variation of te mean torsional coefficient measured in te two studies. Te differences seen between te two studies for some wind directions may be attributed to te difference in building dimensions, te scale used, te number of pressure taps. 4 EXPERIMENTA RESUTS 4.1. Variation of torsion and sear coefficients:

5 Te Sevent International Colloquium on luff ody Aerodynamics and Applications (AA7) Sangai, Cina; September 2-6, 2012 Figure 4 sows te variation of mean and peak sear coefficients (C vx,c vy ) wit wind direction for te two buildings (6 and 50 m) tested in simulated open-country exposure. Te maximum sear forces in x-direction occur for wind directions from 0 o to 45 o ; wile in y- direction wen wind is almost perpendicular to building face, i.e. 90 o. For te tested buildings, te peak sear coefficients ave increased by about 50% wit increasing building eigts form 6 to 50 m. 0 o Y X 90 o Figure 4. Variation of sear coefficients (C VX, C VY ) wit wind direction for building models corresponding to 6 and 50 m eigts. Altoug, te determination of te sear coefficient is important to propose equivalent wind loading, identification of orizontal distribution of tese wind loads on building structural system still requires information about te torsional moment. Te variation of mean and peak torsional coefficients wit wind direction is presented in Figure 5 for buildings (6, 12, 25, and 50 m) in open-country terrain exposure. As a result of te building models aving symmetric sapes, mean torsions are zero for wind directions perpendicular to building face, i.e. 0 o and 90 o. However, tere are significant maximum and minimum torsional coefficients for tese wind directions due to te lack of wind pressure correlation over te building envelope in te orizontal direction. Te maximum torsional moment occurs for wind directions from 15 o to 45 o for te first tree buildings (6, 12, 25 m) wile for te 50 m building, two peaks appear at wind directions 30 o and 75 o. Tis may be attributed to different caracteristics of wind flow interactions wit buildings wit eigt lower tan 25 m compared to buildings wit eigts greater tan 25 m. For better understanding in tis regard, a flow visualization study is sceduled as a part of future tasks in te current researc project Most critical torsion and sear coefficients As it is very well known, te distribution and te magnitude of wind forces on building envelope are linked to te magnitude of torsional moment. Terefore and based on te wind tunnel measurements, two load cases are presented. Case A sows maximum torsion (C T Max. ) and corresponding sear (C V Corr. ) wile Case sows maximum sear (C V Max. ) and corresponding torsion (C T Corr. ). For simplicity, torsional loads can be treated analytically by introducing wind forces (V) wit equivalent eccentricity (e) as sown in Figure 6. Tables

6 and 3, for Case A and Case, sow te evaluated coefficients for te critical wind directions for wic te maximum torsion and sear were measured. For te 50 m building eigt, te most critical torsional moments ave been measured for wind azimuts 30 o and 75 o. Tus, it quite significant for te wind provisions to cover tese critical torsions for acieving te anticipated proper design for low and medium eigt buildings. H 0 o Y X 90 o Figure 5. Variation of mean and peak torsion coefficients wit wind direction for four different building eigts (6, 12, 25,and 50m) 90 o F X F Y M T e Wind 0 o V Figure 6. Horizontal wind force and torsional moment and its equivalent eccentric force. Te maximum equivalent eccentricity as been reported of about 16% of building largest orizontal dimension for building wit eigt 50 m. Te maximum ratio between te corresponding sear (associated to maximum torsion, Case A) to te maximum base sear (full load - Case ) was 74% for te 25 m ig building. As indicated in many past wind tunnel studies, wind-induced torsion always exists even for wind direction for wic te maximum full sear force occurs. Te current study demonstrates tat maximum sear is mostly associated wit equivalent eccentricity about 5%. Tis is in line wit te following statement given in ASCE/SEI 7-10, (Commentary part, C27.4.6), wind tunnel studies often sow an eccentricity of 5% or more under full (not reduced) base sear. Te designer may wis to apply tis level of eccentricity at full wind loading for certain more critical buildings even toug it is not required by te standard. Te present more moderate torsional load requirements can in part be

7 Te Sevent International Colloquium on luff ody Aerodynamics and Applications (AA7) Sangai, Cina; September 2-6, 2012 justified by te fact tat te design wind forces tend to be upper-bound for most commo Table 2. Case A: Maximum torsion (C T Max. ) and corresponding Sear (C V corr. ) uilding Wind tunnel measurements eigt (m) Wind azimut C T Max. C V Corr. e (%) 6 30 o o o o o o o o o o Table 3. Case : Maximum sear (C V Max. ) and corresponding torsion (C T corr. ) uilding Wind tunnel measurements eigt (m) Wind azimut C V Max. C T Corr. e (%) 6 0 o o o o o o o o INSTANTANEOUS WIND FORCES ON UIDING SURFACES Te wind flow caracteristics (i.e. attaced flow, separation, and reattacment) around buildings are critical for te determination of torsional moment (M T ). Te non-uniform distribution of te generated wind loads in te orizontal directions is te main reason for generating torsional moment. Figure 7 sows measured integrated wind forces on building wit eigt 25 m in X and Y axes for te critical wind directions, i.e. 0 o, 90 o, and 30 o as te peak torsional moment occurs. F4 F3 F1 Y 0 o X F2 30 o 90 o uilding (=25m) Figure 7. Horizontal forces in X and Y directions for azimuts 0 o, 30 o, and 90 o for wic maximum torsions were measured for building wit eigt = 25 m. Since all buildings ave symmetric sapes and structural systems, te center of rigidity is located at te middle of building plan. Identification of te wind forces (F1 to F4) around te vertical building axis may allow better understanding for te relation between te generated torsion and wind direction. In case of perpendicular wind (0 o and 90 o ), te forces (F1 and F2) vary wit te same magnitude ranges and te trend line for tese values as almost slope 1:1, and te correlation coefficient is very small (0.003). Similarly, te F3 and F4 values vary in

8 te same manner for te same wind directions. Tis justifies te fact of te zero mean-torsion measured wile aving relatively maximum and minimum torsions for tese wind directions. For wind direction 30 o, te magnitude range of (F 2 ) generated on te side close to wind is about double te force range of (F 1 ) generated on te far side. Te trend line for te F1 and F2 values as slope equal to 2:3 wile te correlation coefficient is Accordingly, tis could explain wy te maximum torsion occurs for wind direction 30 o. 6 CODE PROVISION COMPARISONS WITH WIND TUNNE RESUTS Te results of te wind tunnel tests (Case A and Case ) for te tested building eigts were compared to te values for base sear force and torsional moment evaluated by NCC 2010, ASCE/SEI 7-10 and EN NCC 2010 In NCC 2010, te static metod (called erein NCC 2010-S1) is introduced for low buildings wile te simplified metod (called erein NCC 2010-S2) is proposed for rigid buildings wit intermediate eigt. Te static metod calculations for te torsional and sear coefficients were derived based on figure I-7 of NCC 2010, Commentary I, were te external peak (gust) pressure coefficients (C p C g ) are provided for low buildings. ikewise, for te simplified metod, te external pressure is taken from Figure I-15, Commentary I. Partial and full load cases were considered to estimate maximum torsion and corresponding sear, as well as maximum sear and corresponding torsion. Calculations were carried out considering te open terrain exposure. Static metod values were increased by 25% to eliminate te implicit reduction (0.8) due to directionality issue ASCE/SEI 7-10 Te tree analytical procedures stated in ASCE/SEI 7-10 to evaluate wind loads were applied for tis comparison. Te envelope metod (ASCE 7-10-E) appropriate for low buildings ( < 18 m and < ) were and are te mean roof eigt and te least orizontal dimension respectively, figure is used to get te external pressure coefficients (GC pf ). Te basic (transverse) and torsional load cases presented in ASCE 7-10, figure are used to estimate te maximum torsional moment and te maximum base sear. Directional metods- Part 1 and Part 2 (called in tis paper ASCE 7-10-D1 and ASCE 7-10-D2), proposed in ASCE/SEI 7-10 to be used for all building eigts, are also considered in tis comparison. External pressure coefficients were collected from figure Pressure coefficients are provided in table for buildings wit eigt up to 48.8 m. ASCE 7-10 calculations were carried out considering te open terrain exposure C. Also, te directional factor was taken as EN Te Eurocode defines one unified analytical metod tat can be used for predicting te wind forces on all building types regardless of eigt. Torsional effects are taken into account by applying non-uniform pressures and forces, as sown in EN , Figure 7.1. A triangular wind load is applied on te windward surface wit a rectangular load on te leeward face of te building. External pressure coefficients for vertical walls of rectangular plan buildings are calculated using Figure 7.5 and Table 7.1 available in section 7, wile for te external pressure of duo-pitc roofs, values are provided in te same section (Figure 7.8 and Table 7.4a). All ASCE/SEI 7-10 values were multiplied by and EN values by in order to consider te effect of te 3-sec and te 10-min wind speed respectively in comparison to te mean-ourly wind speed in NCC

9 Te Sevent International Colloquium on luff ody Aerodynamics and Applications (AA7) Sangai, Cina; September 2-6, 2012 Figure 8 summarizes te results for Case A (see Table 2). Peak torsional coefficients, corresponding sear, and equivalent eccentricity are evaluated eiter by te wind tunnel study or by te provisions of standards considered. Case (see Table 3) comparison results, maximum sear, corresponding torsion and equivalent eccentricity are presented in Figure 9. In te first instance, European code sows very good agreement wit torsions measured in te wind tunnel for low and intermediate eigt buildings, see Figure 8. However, te applied wind load (i.e. corresponding sear) and te corresponding equivalent eccentricity are different from te measured values, given tat torsional coefficients are always products of force coefficients times eccentricities. For low buildings, te envelope metod in ASCE/SEI 7-10 sows relatively good agreement wit te measured torsion, altoug decreasing te eccentricity from about 18% to 15% will improve its performance. Also, te static metod in NCC 2010 underestimates torsion on low buildings significantly. On te oter and for medium eigt buildings, te wind load procedures in te NCC 2010 and te ASCE/SEI 7-10 overestimate torsion in some case significantly. Regarding Case, Figure 9 indicates clearly tat te static and envelope metods for low buildings in NCC 2010 and ASCE/SEI 7-10 respectively succeed to predict maximum sear forces. All oter metods in te tree provisions overestimate sear forces on low and medium eigt buildings, as sown in Figure 9. ased on te results presented in Figures 8 and 9, it could be recommended tat applying 75% of te full wind loads (i.e. maximum sear measured in Case ) wit equivalent eccentricity 15% will improve torsion evaluation for low and medium eigt buildings. Equivalent eccentricity (e (%)) Torsional coefficient (C T Max. ) Case A ASCE D2 NCC S2 ASCE D1 EN uilding eigt (m) Case A NCC S2 ASCE D1 and D2 EN Corresponding sear coefficient (C VCorr. ) Case A EN ASCE D2 NCC S2 ASCE D1 uilding eigt (m) NCC S2 ASCE D1 ASCE D2 EN Peak ase Torsion CT Max. q Corresponding asesear CV Corr. q e (%) Peak ase Torsion *Corresponding asesear C TMax. 0 uilding eigt (m) e C VCorr. Figure 8. Comparison of torsional load case evaluated by NCC 2010, ASCE/SEI 7-10, and wind tunnel tests (Case A: maximum torsion and corresponding sear)

10 Sear coefficient (C V Max. ) Equivalent eccentricity (e (%)) Case EN ASCE D2 NCC S2 ASCE D1 uilding eigt (m) Case NCC S2 & ASCE D1 and -D2 & EN Corresponding torsion coefficient (C TCorr. ) Case NCC S2 & ASCE D1 and -D2 & EN uilding eigt (m) NCC S2 ASCE D1 ASCE D2 EN Peak asesear CV Max. q Corresponding ase Torsion CT Corr. q Corresponding ase Torsion e (%) *Peak asesear C TCorr. 0 uilding eigt (m) e C VMax. Figure 9. Comparison of sear load case evaluated by NCC 2010, ASCE/SEI 7-10 and wind tunnel tests (Case : maximum sear and corresponding torsion). 7 CONCUSIONS Wind-induced torsion was measured in te wind tunnel for four buildings aving te same orizontal dimensions wit different eigts ranged from 6 m to 50 m. In addition, te experimental results were compared wit wind provisions in NCC 2010, ASCE/SEI 7-10 and EN Te comparison results demonstrate te following: a) For low buildings: Te static metod in NCC 2010 underestimates torsion significantly. Te envelope metod in ASCE/SEI 7-10 sows relatively good agreement wit te measured torsion. EN sows good agreement wit te wind tunnel results. b) For intermediate eigt buildings: Wind load procedures in NCC 2010 and ASCE/SEI 7-10, overestimate torsion wile EN sows good agreement wit te wind tunnel results. Until more experimental data become available, it could be recommended tat applying 75% of te full wind loads wit equivalent eccentricity 15% will improve torsion evaluation for low and medium eigt buildings. 8 ACKNOWEDGMENT Te autors are grateful for te financial support received for tis study from te Natural Sciences and Engineering Researc Council of Canada (NSERC)

11 Te Sevent International Colloquium on luff ody Aerodynamics and Applications (AA7) Sangai, Cina; September 2-6, REFERENCES 1 NC 2010, Structural Commentaries (part 4), Issued by te Canadian Commission on uildings and Fire Codes, National Researc Council of Canada, ASCE/SEI 7-10, Minimum design loads for buildings and oter structures. Publised by te Structural Engineering Institute of ASCE, Reston, VA, EN , Eurocode 1, 2005: Actions on Structures General actions Part 1-4: Wind actions, European Standard 4 P. Krisna, Wind loads on low-rise buildings a review. Journal of Wind Engineering and Industrial Aerodynamics, 1995, 54-55, T. Statopoulos and M. Dumitrescu-rulotte, Design recommendations for wind loading on buildings of intermediate eigt, Canadian Journal of Civil Engineering, 1989, 16, R. A. Sanni, D. Surry, and A. G. Davenport, Wind loading on intermediate eigt buildings, Canadian journal of civil engineering, 1992, 19, M. Elsarawy, T. Statopoulos, and K. Galal, Evaluation of wind-induced torsional loads on buildings by nort American and European codes and standards, Proceedings of te 2011 Structures Congress, Sponsored by ASCE/SEI, as Vegas, Nevada, USA, 2011, April N. Isyumov and P. C. Case, Wind-Induced torsional loads and responses of buildings, in: Proceedings of te 2000 Structures Congress, Sponsored by ASCE/SEI, Piladelpia, Pennsylvania, USA, 2000, May M. Elsarawy, T. Statopoulos, and K. Galal, Wind-Induced torsional loads on low buildings, Journal of Wind Engineering and Industrial Aerodynamics, 2012, ttp://dx.doi.org/ /j.jweia J. A. Amin and A. Auja, Wind-induced mean interference effects between two closed spaced buildings, KSCE Journal of Civil Engineering, 2012, 16 (1),