Wind Load Provisions of the Revised Building Code in Japan

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1 Wind Load Provisions o the Revised uilding Code in Japan Hisashi Okada 1), Yasuo Okuda 2) and Hitomitsu Kikitsu 3) Astract This paper descries the new wind load provisions in the revised Japanese uilding code. The provisions were estalished in June, 2000 and enorced immediately. In the reorm, design wind loads were deined on the results o extreme statistical analysis o wind velocities. Various new technical concepts such as terrain categories were introduced in the reorm. In the revision works, simpliication o the expressions was carried out as well as removal o unclearness on the criteria. In this paper major revision works are presented in this paper. Key words: uilding code, Design wind speed, Terrain category Wind pressure coeicient 1. INTRODUCTION The Japanese uilding Code is composed o the uilding Standard Law, the uilding Standard Law Enorcement Order, etc. The uilding Standard Law, in which ojectives o the code, deinitions o terms, undamental concepts and so on were descried, was revised in The revision was the one toward into the perormance-ased regulation. Following the revision, technical regulations such as load provisions, structural calculation provisions, etc. were reormed in They are in the uilding Standard Law Enorcement Orders (cainet orders), the uilding Standard Law Enorcement Regulations (Ministry o Land, Inrastructure and Transportation [= The ormer is Ministry o Construction] orders) and Notiications o Ministry o Construction. The revision on wind load provisions was very drastic, ut it was ased on AIJ recommendation 1). In this paper, major technical works that were carried out or the revision are presented. 2. NEW WIND LOAD PROVISION 30m/s 32m/s 34m/s 36m/s 38m/s 40m/s 42m/s 44m/s 46m/s Figure 1 Map o datum wind speed The radical revision o the wind load provisions was carried out or the new uilding code. The revision was ased on AIJ recommendation. The main revisions are those on levels o load, clear separation o loads into or structural rames and or external uilding components, introduction o exposure actor and introduction o gust eect actor. (1) Level o wind load In the old provisions, wind load level was ased on a record at Muroto promontory in a historical huge typhoon, Muroto in We have a lot o data at present, which can give us reliale estimation o expected value o wind speed. So 1)Director, Structural Engineering Department, uilding Research Institute, Tsukua, Iaraki , Japan 2) Senior Researcher, Ditto 3)Chie Oicial, uilding Guidance Division, Housing ureau, Ministry o Land, Inrastructure and Transportation, Kasumigaseki, Tokyo, , Japan

2 the statistical analysis was carried out. Utilizing the result, Datum wind speed was prescried on 50 years return period values in each place in Japan in the new provisions. Datum wind speed was deined as 10 minute average wind speed at 10m height aove the ground over the lat open terrain. Taking uncertainty o typhoons into consideration, the minimum wind speed or datum wind speed was set. As the result, it is ranged rom 30 to 46m/s. A map or datum wind speed is shown in igure 1. Design wind load or no damage was prescried on datum wind speed. Design wind load or no collapse design was prescried as 1.6 times o the no damage design wind load. They are corresponding to 50 years and 500 years return periods values respectively. (2) Load estimation ormula Wind loads or no damage design are given y the ollowing ormulas. The ormula or structural rames is W 2 r 2 o = 0.6E G V C (1) where, W = wind load (N/m 2 ), E r =vertical distriution coeicient or mean wind speed, G =gust eect actor, V 0 = datum wind speed (m/s) and C =wind orce coeicient. While the ormula or external uilding components is W = q ˆ (2) c C where, W c = wind load (N/m 2 ), q =mean velocity 2 2 pressure 0.6 r o and Ĉ =peak wind orce coeicient. (3) Category or exposure actor Terrain categories are deined with descriptive expressions and/or photographs showing the typical examples in AIJ recommendation as well as the other Standards 2),3). The deinition was thought to have a ear to produce misjudgment. Hence, the clear deinition rom the viewpoint o administrative uilding control was required and developed as ollows. The terrain categories are regulated with whether the site is inside o the city planning area or not, uilding height and the distance rom sea ront or lakeront. Figure 2 shows diagrammatically the categories. The area o Category I is designated in the area o Category II and the area o Category IV is designated in III y regional governments. In AIJ recommendation, Category V where tall uildings heavily concentrate is categorized. ut it was not categorized in the provisions ecause it was decided not to e necessary rom the viewpoint o uilding control. Height o uilding 13m Figure 2 Categories o exposure actor (4) Vertical distriution co. or mean wind speed E r or vertical distriution coeicient or mean wind speed is given with the ollowing equation. Z Er = 1.7 Z G α α H Z (3) H = 1.7 H > Z Z G where, H = mean height o roo, and the other parameters are shown in tale 1. Figure 3 shows the igures o E r. Terrain category Height o uilding (a) Outside o city planning area 31m 13m 0m 200m 500m Distance rom shoreline or lakeront () Inside o city planning area Tale 1 Parameters o E r Z (m) Z G (m) α I II III IV

3 Height (m) Er Terrain Category Figure 3 E r (=Vertical distriution coeicient or mean wind speed) (5) Gust eect actor Gust eect actor is used to convert 10-minute average velocity pressure to instantaneous wind orce. Dynamic eect such as resonance etween dynamic ehavior o structural rame and wind load luctuation is taken into consideration with the gust actor. Tale 2 shows gust eect actors prescried in the provisions. The gust eect actor was derived with the detailed procedure II o AIJ recommendation, assuming the ollowing values; Datum wind speed ; 35m/s uilding; Aspect ratio =1, 2 and 4 Natural requency; 40/H T (Hz), where, H T = uilding height(m) Damping coeicient =0.02 The provisions prescrie wind tunnel tests or actual measurements are availale to determine gust eect actor. Tale 2 Gust eect actor Terrain Mean height o roo category H 10m 10<H<40m 40m H I II 2.2 due to linear 2.0 III 2.5 interpolation 2.1 IV (6)Wind Force coeicient Wind orce coeicient or structural rame is deined as ollows. C = C pe C pi (4) where, C pe = external wind pressure coeicient, C pi = internal wind pressure coeicient. External wind pressure coeicients or the ollowing shape uildings and structures are shown in the provisions. They are closed uildings having pitched roo, lat roo, mono-slope roo, arched roo, multi span pitched roo and multi span saw-tooth roo, unenclosed uildings having pitched roo, ree roos o pitched roo and wing type roo, lattice structure, netlike structure and cylindrical structure. Figure 4 shows the external wind pressure coeicients o closed uilding having lat roo and pitched roo as the examples. 0.8kz a (a) lat roo rectangular uilding 0.8kz see tale a H () pitched roo rectangular uilding Figure 4 External wind pressure coeicient or closed type uilding In Figure 4, H, and a in Figure 4 are deined as ollows; H = mean height o roo

4 = readth o uilding a = min(, 2H) And k z is deined as ollows; in case o H Z k z = 1. 0 in case o H > Z, Z Z, k z Z = H 2α 2α Z Z > Z, k z = H Tale 4 shows internal pressure coeicients prescried in the provisions. Wind tunnel tests are availale to get external and internal pressure coeicients. Tale 3 External pressure coeicient or roo o pitched roo roo pitch plus coeicient minus coeicient less than Liner interpolation may e used or the roo pitch etween the roo pitches shown the aove Tale 4 Internal wind pressure coeicient Closed Unenclosed uilding uilding Open in Open in leeward 0 and (7)Peak wind orce coeicient Peak wind orce coeicients are given y the ollowing equation. Cˆ = Cˆ pe Cˆ pi (5) where, Ĉ pe =peak external pressure coeicient, Ĉ pi = peak internal pressure coeicient. Here, an attention should e paid to the peak internal pressure coeicient. It is not the one to deine the real peak internal pressure ut the contriution o internal pressure to the maximum instantaneous wind orces. Wind tunnel tests are availale to determine the peak wind orce coeicients, too. External peak pressures coeicients are given y the products o C pe and G pe in case they are plus. ut in case that they are minus, they are given y external peak pressure coeicients themselves. Tale 5 and 6 show the C pe and G pe or pitched roos, mono-slope roos and multi span saw tooth roos. Tale 7 shows the minus peak external pressure coeicients or pitched roos. Tale 8 shows peak internal pressure coeicients. Tale 5 Plus C pe or pitched roo, mono-slope roo and multi span saw tooth roo roo pitch C pe Liner interpolation may e used or the roo pitch etween the roo pitches shown the aove. For roo pitch less than 10 degree, plus C pe should not necessary to e considered. Tale 6 G pe or pitched roo, mono-slope roo and multi span saw tooth roo in case o plus C pe. Terrain Mean height o roo category H 5m 5<H<40m 40m H I 2.2 due to linear 1.9 II 2.6 interpolation 2.1 III and IV Tale 7 Minus peak external pressure coeicient or pitched roo, mono-slope roo and multi span saw tooth roo Part roo pitch A C D Part A,, C and D are indicated in igure 5. Liner interpolation may e used or roo pitch other than shown. a in igure 5 is smaller one etween smaller dimension o uilding plan and 2 times o uilding height.

5 D 0.1a C 3) ASCE Standard, ANSI/ASCE 7-95, 1996 A 0.1a H Figure 5 Classiication or minus peak external pressure coeicient o pitched roo Tale 8 Peak internal pressure coeicient Closed uilding peak external pressure 0 peak external pressure 0 <0 Unclose open in 1.5 d uilding open in leeward CONCLUSION Wind load provisions in the new Japanese uilding code were descried. In the wind load provisions, terrain categories were deined with clear deinitions rom the viewpoint o uilding control. The design wind speeds were ased on 50 years return period or no damage design and 500 years return period or no collapse. They are 10-minute average wind speed at 10m height aove the ground over lat open terrain. To develop the new provisions, some simpliications were carried out. As one o the results, gust eect actors or structural rame were deined with terrain category and uilding height. Wind load estimation ormulas were prescried or structural rame and or external uilding components separately. Some wind pressure coeicients shown in the provisions were presented here. Reerence 1) AIJ Recommendations or Loads on uildings Architectural Institute o Japan, ) Australian Standard, AS , SAA Loading Code Part 2, 1989