11th fNTERNAT10NAL BRICKlBLOCK MASONRY CONFERENCE

TONGJ1 UNIVERSITY, SHANGHAI, CHINA, 14 - 16 OCTOBER 1997

A NEW Ali (ARCHITECTURAL INSTlTUTE OF JAPAN) STANDARD FOR STRUCTURAL DESIGN OF

MEDIUM-RISE GROUTED MASONRY BUILDING STRUCTURES

Koji YOSHIMURAI, Kenji KIKUCHI2,

Jyoji BASAKI3, Takeshi OKAMOT04 and Shiro KAI5

1. ABSTRACT

Based on the provisions of a new AIJ (Architectural Institute of Japan) standard which will be published in 1997, a structural design methodJor medium-rise reinforced concrete (R/C) grouted masonry builçlings is introduced. Special emphasis is placed on the seismic design method adopted in the proposed new design standard.

2. INTRODUCTION

In accordance with the current AIJ Standards for Structural Design of Masonry Structures, the maxirnum stories or height of the masonry buildings which can be designed and constructed are limited up to three-stories or eleven meters in height, respectively. Based on the recent extensive development in research and construction technique in the tield of the RlC grouted masonry structures (Figure 1), establishment of a new design standard for structural design of medium-rise RlC grouted masonry buildings with less than tive stories or sixteen meters in height is now being discussed in the AIJ Masonry Committee, and a new Structural Design Guideline will be published by the end of 1997. Herein, a structural system adopted in the ordinary RlC grouted masonry buildings in Japan is introduced flfSt, and then the structural design methods including seismic design, which are provided in the new AIJ Standards, are brief1y presented. Since there were almost no severe structural damage to this kind of RlC grouted masonry buildings even during the 1995 Hyogoken-Nanbu earthquake [1], ·the masonry buildings with this type of boxed-wall structural system are expected to beco me more popular in Japan.

Keywords: Grouted masonry building structure; Seismic design; New AIJ Standard. I. Professor of Structura1 Engineering, Oita University, Oita, JAPAN, Chairman of AIJ Masonry Committee. 2. Assodate Professor of StructuraJ Engineering, Oita University, Oita, JAP AN, Secretary Df AIJ Masonry Committee. 3. Structural Engineer, lida Design and P1anning Office, Oita, JAPAN. 4. Structura1 Engineer, Sato·gumi Co., Oita, JAPAN. 5. Graduate Student Df Oita University, Oita, JAP AN.

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Figure 1. Grouted masonry building under construction in Oita City, Japan

3. GROUTED MASONRY BUILDING SYSTEM IN JAPAN

One ofthe typical R/C grouted masonry str\1cturaI systems adopted in the ordinary residentiaI buildings in Japan is schematically illustrated in Figure 2, where grouted masonry bearing walls, R/C wall giróers and cast-in-place R/C 1100r slabs are shown respectively [2]. As shown in the figure, each of the masonry bearing walls is composed pf concrete hollow masonry unit with or without mortar joints, reinforcing bars (Re-bars) and grout materiaIs, and all of the celÍs of the hollow masonry units are completely and continuously tilled with grouts such as concrete or a mÍXture of cementítious materiaIs and aggregate, and so on.

RC wàll girder

Grout concrete is fully cast into void area

Cast-in-place concrete ------------­Vertical reinforcement

--_____ RC floor-slab

Masonry wall

Horizontal (bed) joint

Vertical (head) joint

~al reIDfonoeffi'nt oi wall ,dg'

? Cast-in-place concrete

RC footing beam

Regular unit Concrete-form masonry unit

Figure 2. Atypical grouted masonry

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The first stI\lctural design standard for this type of masopry building !,>ystem was established and published in 1952 as one of the AIJ Standards for Structural Design of Masõnry Structures [2], where maximum stories or height of the masonry buildings are limited up to three-stories or elev.en meters in height, respective1y. Such a maximum story- and height-limitatjons of the masonry buildings inc1uding R/C grouted masonry buildings are still in use and are specified in the current AIJ Structural Standards for Masonry Building Structures [2] . In order to develop more seismic R/C grouted masonry building structures, however, a large number of theoretical and experimental studies were conducted during the period fram 1984 through 1988 as a part of the US-Japan Coordinated Earthquake Research Program on Reinforced Masonry (RM) Buildings [3]. During this five-years' project , a five-siory full-scale grouted masonry or reinforce~ masonry (RM) building was tested on a laboratory test fIoor of the BRI (Building Research lilstitute) of the Ministry of Construction of Japanese 'Governrnent, and since then the 'seismic safety of the medium-rise grouted masonry buildings has been verified [4]. Based OI;l the results of these research projects, guide1ines and their commentaries for structural design and construction of the medium­rise grouted reinforced masonry (RM) buildings were published in 1991 [5 and 6]. Based on the fac{ that there were almost no severe structural damage to this type of R/C grouted masonry búildings even during the 1995 Hyogoken-Nanbu earthquake (see Fígure 3), and also based on the recent extensive development in research and construction technlques in the field of the grouted masonry building structures, establishment of a new design standard for structural design of medium-rise R/C grouted masonry buildings with less than five stories or less than sixteen meters in height is now being discussed in the AIJ Masonry Committee (the Chairman ofthis Committee is the first author ofthis paper), and a new guideline for structural design will be published by the end of 1997. Herein, the structural design-methods inc1uding seismic design method ofthemedium-rise R/C grouted masonry buildings, which are provided in the draft of a new AIJ Structural Design Guideline, are briefly introduced, and a design example of a five-story grouted masonry residential building is also presented.

Figure 3. Surv.ived grouted maSL 'lfy buildings in Kobe City

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4. STRUCI'URAL DESIGN FOR MEDIUM-RISE GROUTED MASONRY BUILDINGS

Followings are the brief description of the structural design methods provided in the new AIJ Guideline for Structural Design of Medium-rise Grouted Masonry Buildings, which will be published in 1997. AlI of the design provisions including minimum requirements for low-rise masonry buildings which are specificd in the current AIJ Standards for Structural Design of Masonry Buildings [2] are not described in the present papeL

4.1 Scope

Objective of this proposed Guideline is to present the design standards for structural design of the medium-rise RlC grouted masonry building structures as well as the seismic design for both medium and strong motion earthquakes specified in the present Building Standard Law (BSL) of Japan [7]. Herein, the definition ofthe medium-rise buildings is the four- and five-story masonry buildings whose height is less than sixteen meteIS in height. The maximum story height in each story is also limited up to 3.0 meters, except for less than 3.5 meters in the first story. It is noted that the dead plus live loads for deterrnining the design earthquake forces are assumed to be 0.013 MPa (1,300 kg!m2 ) in this Guideline. Structural design methods for one-, two- and 'three-story masonry buildings lower than eleven meters in height are unchanged, and are given in the current AIJ Design Standards [2].

4.2 Fundamental Seismic Design Concept

For ali of the four- and five-story reinforced grouted masortry buildings which are designed in áccordance with the design provisions of this Guídeline, seismic safcty of the dcsigned building must be investigated for both medium and severe earthquakes specified in the current BSL [7]; that is: For medium design earthquake forces with standard base shear coefficient, Co=0.2 or more, allowable stress design based on the elastic structural analysis shall be performed for determining the design forces and required reinforcement for ali the structural members. In addition to such an ordinary elastic design for ali the structural members, story-drifts less than 1/2,000 rad. And eccentricity caused by the same medium design earthquake forces must be investigated, as well as the lateral stiffness distribution along the height of the designed building. . In addition to the first-stage of structural design based on the elastic design technique as shown above, seismic safety of the designed building must be investigated against severe earthquake forces (Qud) having a standard base shear coefficient of Co=1.0 or more, which are necessary for determining the required ultimate lateral strength (Qun) in each story of the designed building. In accordance with the design provisions of the current BSL, the required ultimate lateral strength (Qun) can be ca1culated by the following equation;

Qun = Ds Fes Qud (1)

where (Ds) is a required strength reduction factor depending on the deformability or deformation capacity, and is given by the function of ductility factor ( f1 ) of each story. In equation form;

Ds = 1/(2 f1 -1)1/2 (2)

In case of the ordinary RlC building structures, the values of (Ds) are specified between 0.30 to 0.55 according to the requirements of the current BSL. In the second-stage of the

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structural design of medium-rise R/C grouted masonry buildings, however, it is recommended in the new AIJ Guideline that the value of (Ds) shali be more than 0.55; in other words, the ultimate lateral strengths of ali the stories (Qu) shali be designed so as to be larger than the required strengths (Qun) calculated by using the values of Ds=0.55 or more. (Fes) in the above equation is an increasing facto r for required ultimate lateral strengths which depend on the eccentricity and irregularity in lateral stiffness along the height of the designed building (7]. In addition, it is recommendcd in the new AIJ Design Guideline that the R/C grouted masonry building shali be designed to fail in an acceptable failure mode such as the beam­mechanism in its ultimate state when a very hig earthquake attacks the designed building. One of the recommended failure modes is shown in the design example (Figure 8) of the present paper.

4.3 Quality of Masonry Units, Re-bars, Mortar and Concrete

Minimum requirements for quality of concrete masonry units and Re-bars are specified in the Japanese Industrial Standard, lIS A5406, lIS G3112 and JIS G3117, respectively. Among them, the minimum compressive strength of the masonry units shall be more than 40 MPa (408 kg/cm2). The compressive strengths of mortar and grouts including cast-in­place concrete shall be more than 34 MPa (350 kg/cm2) and 24 MPa (240 kg/cm2),

rcspectively.

4.4 Aliowable Stresses and Material Strengths of Masonry

The compressive strength of masonry (Fm) shall be determined bya set of three masonry prisms based on tests at 28 days, and shall be more than 24 MPa (240 kg/cm2

). If the compressive strengths of the masonry determined by the prism tests are larger than 26 MPa (270 kg/cm2

), the compressive strength of 26 MPa (270 kg/cm2) shali be adopted.

Aliowable stresses of the masonry used for the frrst-stage of the structural design of medium­rise R/C grouted masonry buildings are given by the function of the compressive or material strength of the masonry (Fm). In accordance with the provisions of the new AIJ Design Guideline, the aliowable compressive strengths of the masonry in long- and short-term loading are Fm/6 and Fm/3, respectively, which are determined basf;d on the analysis results obtained from a large number of calculations conducted against the typical medium-rise RlC grouted masonry buildings having various size and shape of the bearing wali arrangement. The aliowable shear stresses of the masonry are 4(Fm/180)1f2/ 1.5 and 4(Fm/180)1f2 for long­and short-term loading, respectively, which are from the conservative values specified in the Guide\ine for Structural Design of Medium-rise Reinforced Masonry (RM) Buildings [5]. Aliowable stresses and material strengths of ali other materiais used for the structural design of medium-rise R/C grouted masonry buildings are the same with the corresponding design standards.

4.5 Arrangement of Bearing Walls

Ali the bearing walls shali be arranged in good balance on the horizontal fioor diaphragms within the eccentricity limitations of (Re) is less than 0.15 and each of the floor area surrounded by the bearing walls shali be less than 45 m2

, where "bearing walis" are detined as the walls designed to resist in-plane lateral shear forces and moments as well as axial forces including vertical gravity loads. In addition, ali the upper-story bearing walls are recommended to be placed just above the adjacent lower-story bearing walls so as that the

985

irregularity in lateral stiffness along the height of the building is within the limitation of lateral stiffness ratio (Rs ~ 0.6). In both of the longitudinal and transverse directions of each story of the building, the values of"WalI Rate" shalI be more than the values given in Table 1, where wall rate is defined as the total horizontallength (cm) of alI the bearing walls located in one direction is divided by the flQor area (m2

).

Table 1. Minirnum values of walI rate (cm/m2)

Location Four-story building Five-story building

5 th story 15

4 th story 15 18

3 th story 18 20

2 th story 20 20

1 st story 20 20

4.6 Structural Details of Bearing WalIs

The minirnum requirements given in all the bearing walIs are in the folIowings: (1) . Horizontallength shalI be more than 100 cm. (2) . Total thickness of the bearing wall shall be more than 20 cm. By considering the

minirnum requirements for this wall thickness and horizontal walI-length per unit floor area given in Table 1, the maxirnum value of the average (or mean) shear stresses induced in the bearing walls can be easily determined. The result is r w =0.2 x 12700 x 5 /(20 x 20)= 0.3 MPa (3.25 kg!cm2) in casewhen a five-story grouted masonry building without any eccentricity in plan is subjected to the design medium earthquake forces with the standard base shear coefficient of Co = 0.2. It is worthy of note that the maxirnum value of the average shear stresses induced in the grout sections within th~ masonry bearing walI sections becomes as Te = 0.2 X 12700 X 5/(20 X 14)= OA MPa (4.64 kg!cm2). It can be understood that these values of ( r w) and ( Te) are very conservative. by taking into account of the allowable shear stresses specified herein and corresponding other AlI Design Standards.

(3) . Thickness of each shelI of the concrete masonry unit shalI be less than 3 cm. (4) . By considering the above Requirements 2 and 3, thickness ofthe grout concrete inside

the masonry bearing walls shalI be more than 14 cm. (5) . Shear reinforcement ratios (psh and psv) provided in the bearing walIs in lower two (or

three) stories ofthe four-story (or five-story) buildings shall be more than 0.20 %, and shall be more than 0.15 % in the upper two stories, where definition' of the shear feinforcement ratios (psh and psv) are the total cross-sectional areas of the shear Re­bars provided in horizontal or vertical directions which are divided by the vertical Of horizontal cross-sectional areas of the bearing walI section, respectively. The values of (psh and psv) in the third-story bearing walls shall be more ' than 0.20 % in five-story btlilding and 0.15 % in four-story building, respectively. The minirnum requirements for these shear reinforcement were determined based on the designed shear forces carried by the unreinforced masonry bearing walls àgainst the medium design earthquake forces with the standard base shear coefficient of Co = 0.2.

(6) . The minirnum requirements for flexural reinforcement provided along the walI edges

986

are also given in the new AIJ Guideline, which were determined based on the member forces against the medium design earthguake forces with the standard base shear coefficient of Co = 0.2.

Validity of the determined wall reinforcement required in ali the bearing walis for both lateral shear and flexural design forces shall be investigated in the second-stage of the seismic structural design against severe earthquakes, where most of the structural members shall be designed so as to avoid the brittle shear failure mode before developing their flexural moment capacity .

4.7 Structural Oesign of Wall Girders

Structural design of the wali girders (or coliar beams) which are reguired to be provided along the top of ali the bearing walis are very important, because excessive bending and/ar shear forces are expected to be induced especialiy in the ultimate state of the building. Foliowings are the minirnum requirements specified in the new AIJ Guideline for structural design of medium-rise RlC grouted masomy buildings. (1) . Width (b) and depth (D) of the wali girders shali be more than the thickness of the

adjacent bearing walis (or 20 cm) and 60 cm, respectively. (2) . Clear span to depth ratio (lo/D) shali be more than 1.5. (3) . Flexural (or longitudinal) reinforcement shali be provided in top and bottom of ali the

beam sections. The minirnum flexural reinforcement ratio (pt) shali be 0.2 %, or more, and at least two deformed flexural Re-bars with bar size of 013 (or #4 bars) or more, but less than 019 (#6), shali be provided. Oefinition of the flexural reinforcement ratio is pt = ai / bd, where (at) is the cross-sectional area of the flexural Re-bars provided in top ar bottom ofthe beam section and (d) is the distance between extreme compression fiber and centroid of the tensile flexural Re-bars.

(4) . Required shear reinforcement ratio (psv) shali be more than 0.30, and ali the Re-bars of 010 (#3) or 013 shali be used as the shear reinforcement in the wall girders. In case of the second and third-floor wali girders whose clear span to depth ratio (lo/D) is less than 2.0, shear Re-bars of 013 shali be placed at least 10 cm spacing.or less along the beams.

5. Oesign Example

Figures 4 and 5 are the typical floor plan and one of the elevations of a five-story RlC grouted masomy residential building which is selected as an example for a structural and seisrnic design presented in the new AIJ Guideline for Structural Design of Medium-rise Gro~ted Masomy Building Structures. In thls design example, RlC grouted masomy bearing walls with 20 cm thickness are provided along through the height of the building. Amount of wali rate in longitudinal and transverse directions are 22.1 and 25.3 (cm/mZ

), respectivel y, which are more than the reguired minirnum wall rates shown in Table 1 of this paper. In accordance with the provisions and recommendations provided in the new AIJ Guideline and its Commentary, the structural design including seisrnic design of this selected model-building was conducted, and ali of the adopted structural design technigues, design calculation processes and obtained results are presented in detail in the latter part of the new AIJ Guideline. In the first -stage of the structural design ofthis building, four different structural analyses based on the conventional simplified method and direct stiffness method using electronic computers are performed against medium design earthquake forces with Co = 0.2, and analysis results are compared each other. In these structural analyses, mechanical modelirig for ali the bearing walls and wall girders are conducted by adopting the colurnn and beam

987

line models with rigid zones around their connections, respectively. Based on the precise analysis results by a computer, maximum values of the calculated story-drift (R) and eccen tric ity ratu (R e) are 0.11 ( X 10 - 3 rad) and 0.01, respectively, and minirnum lateral stiffness ratio (Rs) obtained is 0.79, ali of which are within the lirnitation of the specified requirements.

Figure 4. Typical fioor plan of a five-story RlC grouted masomy residential building

Figure 5. Elevation of a five-story R/C grouted masonry residential building for de.sign example

After finishing the first-stage structural design for medium earthquake loading which is based on the allowable stress design method, the second-stage structural design based on the ultirnate strength design method was performed for investigating the seismic safety of the designed building against severe earthquakes. Figure 6 shows a failure mode observed in one of the RlC grouted masomy frames, in which reinforcement details of ali the structural member sections were determined based on the precise computer analysis during the frrst­stage structural designo In this figure, there are three bearing walls shown by the symbol of ( X ) which are expected to fail in brittle shear failure mode befare developing their ultirnate fiexural moment capacity. The location of the fiexural ar pl~tic hinges are shown in Figure 6 by using the symbols ( • ). Other structural members including ali the wall girders in this

988

frame do not fail in such a shear failure mode but are expected to fail in ductile manner. Since the similar failure mechanisms were obtained in other frames of this model building, additional amount of shear reinforcement were provided in ali the members which are expected to fail in brittle shear failure modes. By providing such an additional shear reinforcement , all the brittle structural members have completely disappeared as being observed in Figure 7.

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(uni! in kN · m and kN ) Fraxne (Yl)

Figure 7. Failure mechanism after providing additional shear reinforcement in brittle members

Up to this stage of the present structural calculations, ali the bottom of the first-story bearing walls (or columns ) have been assumed to be fIxed supports. After this stage, however, various types of final failure modes can be selected depending on the reinforcement details adopted in the R/C foundation beams. Figure 8 shows one of the solutions of the expected failure mechanism in Frame (Yl) of this model building. The ultimate lateral strengths (Qu) determined in each story of this building are approximately 1.22 to 1.98

989

larger than the required lateral strengths (Qun), which are specified in this new AIJ Structural Design Guideline.

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6. CONCLUDING REMARKS

By emphasizing the seisrnic design method especially, the structural design procedures adopted in the new AIJ Guideline for Structural Design of Medium-rise Grouted Masomy Building Structures are introduced, as well as a structural design example of a five­story R/C grouted masomy building.

7. REFERENCES

[1]. Bruneau, M. and Yoshimura, K., "Damage to Masomy Buildings Caused by the 1995 Hyogo-ken Nanbu (Kobe, Japan) Earthquake," Canadian Joumal of Civil Engineering, VoI. 23, No. 3,1996, pp.797-807.

[2]. AIJ (Architectural Institute of Japan) Masomy Committee, "AIJ Standards for Structural Design of Masomy Structures (1989 Edition)," 1994, in English. (Spanish version is also available in CENAPRED (National Center for Disaster Prevention) in Mexico D.F., MEXI CO).

[3]. Okamoto, S., Okada, T., Yamazaki, Y. and Baba, A, "Development of Medium-Rise Reinforced Masomy Systems," Proc. of 9th World Conference on Earthquake Engineering, VoI.8, pp.1165-1170, Tokyo-Kyoto, Japan, 1988.

[4]. Yamazaki, Y., Kaminosono, T., Teshigawara, T. and Seible F., "The Japanese 5-Story Full Scale Reinforced Concrete Masomy Test - Pseudo Dynamic and UltÍrnate Load Test Results -," The Masomy Society Joumal, July-December, 1988, Vol.7, NO.2.

[5]. Society for the Promotion of RM Buildings, "A Guideline and Commentary for Structural Design of Medium-rise Reinforced Masomy (RM) Buildings," 1991, in Japanese.

[6]. Society for the Promotion of RM Buildings, "A Guideline and Commentary for Structural Construction of Medium-rise Reinforced Masomy (RM) Buildings," 1991, in Japanese.

[7]. The Building Center ofJapan, ''The Building Standard Law of JAPAN, " edited by the Housing Bureau of the Ministry of Construction of Japanese Governrnent, 1990, in English.

990