SEISMIC RESISTANCE OF LOW RISE BUILDING

Transcription

1 SEISMIC RESISTANCE OF LOW RISE BUILDING Abdul Haris 1 and Amrinsyah Nasution 2 1 Doctor Candidate at Civil Engineering Study Program ITB 2 Professor in Structural and Construction Engineering, Faculty of Civil Engineering and Environmental ITB ancedin@bdg.centrin.net.id ABSTRACT: There are hundred-thousands low rise building which are built every years through out Indonesia. It might be designed by unfamiliar-with-seismic-design engineers, even by non engineers. Therefore, simplified seismic resistant design manual is absolutely a need. In this paper, requirements for seismic resistance in Indonesian Concrete Code (SNI) was extracted for more simple guidance manual. Study of seismic resistance design is carried out by taking the advantages of limited choices of typically low rise building parameters in Indonesia such as maximum column-to-column span, build ability of structural members dimension, reinforcement bar diameter, economical constraint. More effective way to understand how one can design a seismic resistance low rise building is by simple approach through illustration drawing which is completed by charts, tables and simple explanation. 1. INTRODUCTION Earthquake is nature phenomena that is part of the nature itself. Delicate earthquake in general is not sensed by human although its frequencies reach thousand times in a year around the world. Moderate earthquakes that occur hundreds times in a year is less felt by human. Strong earthquake in part of populated area that instigates severe damages, happens only in numbers a year. Do we need to design buildings that can resist a 200 or 500 year strong earthquake, while services of the building are only about 30 to 50 years? Seismic resistance building design that will not be damaged even for rarely strong earth-quake produces a very large structure elements. It is not viable as well as uneconomical. Term seismic resistance refers to design criteria of building that can be damaged during earthquake, but it will not be total collapse as for safety of life. Seismic resistance is translated as tahan gempa in Indonesian language which has a certain connotation. Therefore, public and even engineers often misunderstand that seismic resistant building is about building that never damage, even under strong earthquake. Intensive socialization of what is seismic resistance meant is required by explaining proper information which is more easily understood and adopted. One should realize that in developing country such as Indonesia, there are so many low rise buildings built in earthquake prone areas without any structural seismic resistance in order to stand strong earthquake. A number of strong quakes for the last ten years has triggered people to start to realize importance of preparing their properties for seismic resistance design. It is difficult to expect that engineers fully understand about seismic resistant design concept especially in conjunction with ductility concept. It is more realistic to give simply information which is more easily understood and adopted. This paper is intended to explore a number of geometric alternatives that becomes simple practice guidance using charts and tables. Scope of work is limited for low rise building supported by open frame structure. There are hundred-thousands this type of building scattered throughout Indonesia which mostly are not designed by seismic resistant building approach. Back to Table of Contents 606

2 2. OVERVIEW a. Damage Structure b. Total damage structure Figure 1 Damages of low rise building (source: Sigit, Wayan and Gunawan). An overview of seismic resistant building design should be started from design philosophy. It involves uncertainty oft earthquake risk, economical constraint, architectural consideration and the most important is human life. So far, modern technology has not been exactly determined when and where earthquakes will occurs. What technology can do is to predict the risk level for some areas based on the earthquakes history record. Based on this record, codes issue seismic risk zone. Nevertheless, from seismic resistant design perceptive, it is more useful to have seismic risk zone than to know the D-day. 2.1 Design Philosophy Design philosophy for seismic resistant building is developed from earthquake risk during its service life. At a delicate/weak earthquake, all structure s elements in building will not be damaged. At a moderate earthquake, some structure s elements in building may have minor damaged that need only renovation or retrofitting. At strong earthquake, structure of building will not be totally collapse, although the elements are damaged. This condition is possible if the collapse is designed for ductile collapses mechanism, of which hinges joints at element s joints are created at a certain hierarchy. 2.2 Frame Structure In general, low rise building supported by open frame structure. We rarely find it is equipped by shear wall, especially in Indonesia. It is hiperstatic structure which has redundant force in its statically equilibrium equation. The advantage of hiperstatic structure is having many failure mechanism alternatives, so one can choose one alternative which will give maximum energy absorption. Frames with high redundancy can be modelled to develop plastic hinges near joints in a certain hierarchy to be a ductile structure. Exception for single element structure such as pole, chimney etc., the ductility concept is not prevail. In this type of structure, plastic hinges precisely are not allowed to be developed. 2.3 Ductility Earthquake induce energy into flexible structure. Generally, induced energy of small earthquake is absorbed by developing elastic stress in structure material. In the other hand, moderate-strong earthquake induce significant energy into the structure. We have known that mostly, structure is designed remain elastic under gravity load. As a consequence, its dimension is very large under strong-earthquake loads to assure its material remain elastic. However, it is unrealistic to have elastic structure in all possibilities in conjunction with earthquake risk during its service life. Engineers must make a decision what they do in design in order to prepare to those possibilities. Since human life is the main concern, structural ductility become a main issue in seismic resistant building design. It means that building will damage when strong quake occurs but remain in tack/stand. Therefore, building ductility represent the capacity of the building structure to absorbs Back to Table of Contents 607

3 induced energy by developing inelastic deformation. Many definitions refer to ductility term such as material ductility, rotational ductility, structural ductility, etc. Material ductility should be understood before one can fully understand term of structural ductility. Without ductile material we never able to have ductile structure. Carbon steel is the most ductile of steel materials, even though it is less-strength compared with other kind of steel. Higher strength steel such as high-strength low-alloy, heat treated carbon steel and heat treated alloys steel, have lower ductility than carbon steel respectively. Ductility of homogen material is measurement by single action test. For instance, carbon steel ductility is determinated by comparing collapse and yield deformation in a tension test. In resisting moment, longitudinal reinforcement bar is dominant in tension or compression. In resisting moment, overreinforced concrete member is brittle material because its failure is initially by concrete failure. In other hand, underreinforced concrete is more ductile because its failure initially by yielding of reinforcement bar. Ductility F max F max Δ Δ Energy dissipation of elastic action Energy dissipation of idealized elastic-plastic action Figure 2 Typical energy dissipation in structures. Structural Ductility is not always automatically achieved by setting all member using ductile material. Structural ductile also depends on failure mechanism. No need to equipped all section member to become ductile. The most critical stresses of beams under earthquake loading will generally be at and near intersection with supporting column. This region required special attention to maintenance ductile against inelastic deformation. It is termed as plastic hinges. Large portion of induced energy is absorbed by plastic hinges and other small portion by elastic deformation. Therefore, in open frame structure critical region is modelled as plastic hinges. Special effort is intended to maintenance its strength or stiffness and ductility during earthquake. Plastic hinges will be developed during earthquake, one after onether depending on configuration of section capacity of structural elements beam and column. Ductility measurement: There are many kind of methods to measure material, element or structure ductility. Steel ductility is measured by tension test, comparing longitudinal deformation at collapse and yielding. Ductility of structural member such as beam is measured based on its moment vs curvature curve. Structure ductility of low rise building is measured by comparing roof lateral deformation when it collapses and its first hinges developed. Back to Table of Contents 608

4 Structural Ductility 2 2 Ductile structure Strong column-weak Beam 1 1 Ductile Failure Mechanism Brittle Structure (Soft Storey) 3 4 Soft storey Mechanisme (Brittle) Capacity Design Figure 3 Failure mechanism in structures. Capacity design is intended to have ductile structure by setting section capacity of structural members in order to absorb induced energy as much as possible. Plastic hinge is initially expected at one of critical regions at end beams. We avoid formation of plastic hinges at columns end before as many as possible hinges plastic at end beam have been developed. Therefore, there are overstrength factor for flexural strength relative to that of beams meeting join. Overstrength factor is taken from 1.2 until 1.5 depend on which codes one adopts. In capacity design, one does not only detail member section proportional to load level but also setting overstrength factor which is called as strong column-weak beam concept. This concept is absolutely effective, especially for low rise building with relative longer span such as school, auditorium, show room and multi purpose building. The next step is to equipped critically regions or plastic hinges by special detailing. 2.5 Beam Earthquake design requires a specific function of beams, especially at beams end which are as well as plastic hinge regions. It is required more stringent lateral reinforcement than those for member designed for gravity loads or the less critically stresses part of members in earthquake-resistant structures. A closely spaced transversal reinforcement also support longitudinal compressive reinforcement against inelastic buckling and confine concrete core. A number of test has shown that confinement will increases concrete strength and ductility. Back to Table of Contents 609

5 Capacity Design Elastic or Proportional Design Capacity Design Structural members is designed proportionally under grafity and wind loads except moderate-strong earthquake load. Strong earthquake is absorbed only through limited plastic hinges at column ends as column im-mediately become unstable before next plastic hinges developed. Structural members is designed proportionally under gravity and wind loads, including moderate-strong e- arthquake load Strong earthquake is absorbed by developing very large elastic deformation of material. There are no plastic hinges deformation. Structural members is designed proportionally under grafity and wind loads, including reduced moderate -strong earthquake load. Columns has overstrength factor to beams, in order to assure hierarchy of plastic hinges deformation yields ductile failure mechanism Strong earthquake is absorbed by developing plastic hinges as many as possible at beam ends. The next plastic hinges is designed at column ends near fixed support. Figure 4 Elastic and capacity design of structures. However, performance plastic hinge during earthquake loading is not only influenced by variables mentioned earlier but also level of nominal shear stress. Bertero indicates that when the nominal stress exceeds about 3 fc ' some reduction in ductility is initiated as well as stiffness when subjected to loading associated with strong earthquake response. As consequence of applying capacity design concept, shear force is calculated based on the maximum probable flexural strength the beams ends. 2.6 Column Capacity of column represented by P-M diagram interaction. For resisting moment frame, the flexure capacity of column is reduced by the present of axial load. Critically stressed regions caused by lateral load from earthquake loading is assumed will be developed at column ends near beam-column joints. It is not totally true by considering inelastic action as well as dynamic response where higher modes present, especially for tall frame building. As mentioned earlier, column overstrength is required in order to avoid brittle failure mechanism such as soft storey mechanism. It is worth noting that in three dimensional we need to consider biaxial moment capacity. In monolithic reinforced concrete, flexural capacity of beam increases caused by participation of slab reinforcement over influenced width. Therefore, one has to calculate overstrength factor based on the flexural strength capacity of T or L beam. Back to Table of Contents 610

6 Column, Beams and Joints + Figure 5 Pattern of internal forces elements. Special transverse reinforcement is also required at critical stress parts as a consequence strong column-weak beam approach. As a result, confinement effect from transverse reinforcement increases concrete core. After as many as plastic hinges at beam ends are developed, one can expect the next plastic hinges at column will be developed near fixed support in order to continue ductile inelastic deformation. 2.7 Beam-Column Joint Joint is important member in conjunction with ductile behaviour. Its failure mechanism is dominated by shear. Under earthquake loading, joints disruption must be avoided by special detailing in order to maintenance join integrity and its stiffness caused by join crack and losing of bonding between concrete and longitudinal anchorage. In the last decades, shear action is explained by struts and tie approach which is developed from what we call truss approach. Concrete cracks under stress tension form struts following such direction perpendicular to principal direction. Adequate reinforcement act as ties in struts and ties mechanism. Since column and beam longitudinal reinforcement bars are placed through the joints, detailing is carried out by inserting reinforcement bars which will completely forming struts and tie mechanism as well as to confine joint core concrete. A number of test and experiments show that additional horizontal bar at mid joint increases joint shear capacity by developing truss mechanism. Slip between bar and concrete at joint can be reduced by using bar with small diameter. Stub is often used when anchorage regions is not wide enough. 3. SIMPLIFIED SEISMIC REQUIREMENT Seismic resistant building technology has not completely socialized, especially in developing countries such Indonesia. Few engineers are familiar with earthquake resistant design. Concept of seismic resistant building design is totally different from ordinary low rise building design which dominated by gravity load. It has not easy to fully understood by practitioners. Therefore, one needs to simplify the requirement for low rise building by considering: Back to Table of Contents 611

7 - Dimension of column or beam section is relative small and there are very limited bar configuration alternative which we can choose as well as no many choices in selecting bar size. - As long as there is no extraordinary architecture expression, there are limited structure layout alternatives. - Code of Indonesian Concrete 2002 likely is intended as a standard for high-rise building purposes. Therefore, there are too many check-points must be carried out even for simple low rise building design. - In ATC 40 reports, in general, building with fundamental period less than 1.0 second such as lower rise building has its fundamental modal mass participation factor is dominant. As a result, we can apply simple lateral distribution load in static equivalent approach. - In general, risk increasing as a consequence of unsymmetrical plan is more easily under-stood The intensity of strong earthquake in Indonesia increases for last ten years. Low rise building collapses are reported from areas throughout Indonesia. School buildings, show room and other low rise buildings with relatively long span will have soft storey mechanism if it is designed according to elastic concept and assumption about building ductility factor is never reached. 4. EXAMPLE OF SIMPLE CHARTS, TABLES In capacity design approach, beam s longitudinal reinforcement and dimension are selected based on the moment output from structure analysis using trial beam section as proportional design does, except that it is included earthquake forces and assume ductility level. Ideally, earthquake forces is calculated based on time history record. Nevertheless, the computation is required special effort and consuming computational time. Instead, static equivalent and response spectrum methods are more often used. Transverse reinforcement is detailed referred to selected beam section. By using the table, one just selects appropriate section from the table. Additional reinforcement at the joint follows hoops patern of the column ends. Crossties is avoided in order not having congested reinforcing bar placement. Tables and charts example are carried out by numerical methods using MATLAB version 7.1. Example of tables are shown bellow. Requirements check points are simplified as shown in Table 1. No Requirement Table 1 Simplified requirement. Simplified Requirement Note I. General I.1 Strength Reduction Factor Table Included in charts & tables I.2 Dynamic Analysis No need Replace by static equivalent 1.3 Push over analysis No need approach 1.4 Overstrength factor 1.5 Minimum compressive strength Table and Chart 1.6 Minimum yield strength Table and Chart It is limited by tables and charts, explicitely II. Beam I.1 Transverse reinforcement Table Included in charts & tables I.2 Longitudinal reinforcement Table Included in charts & tables 1.3 Axial load Level No need Included in charts & tables I.4. Minimum or maximum bar No need diameter Included in charts & tables 1.5 Axial Load Level No need III Column Back to Table of Contents 612

8 III.1 Transverse reinforcement Table and Chart Included in charts & tables III.2 Longitudinal reinforcement Table and Chart Included in charts & tables III.3 Axial load Level No need Included in charts & tables III.4. Minimum or maximum bar No need diameter Included in charts & tables III.5 Minimum column size No need Included in charts & tables IV. Beam-Column Joint IV.1 Additional reinforcement Table and Chart Included in charts & tables IV.2 Axial load Level No need Included in charts & tables IV.3 Stress Level No need Included in charts & tables IV.4. Minimum or maximum bar No need diameter Included in charts & tables Table 2 Bare beam sections. Rectangular Bare Beam Sections, fy= 40 MPa and fc = 20 MPa DETAIL b (mm) h (mm) Top rfc 3D13 5D13 6D13 8D13 5D16 Bottom rfc 2D13 3D13 4D13 4D13 3D16 Trvs rfc φ8-100 φ8-100 φ8-100 φ8-100 φ8-100 d (mm) ρ (%) M (kgf-m) DETAIL b (mm) h (mm) Top rfc 6D16 4D16 5D16 8D16 Bottom rfc 3D16 2D16 3D16 4D16 Trvs rfc φ8-100 φ8-100 φ8-100 φ8-100 d (mm) ρ (%) M (kgf-m) There are only bare beam sections in this paper. In the complete manual, there are more sections, including T and L sections. The use of T and L section are intended to accommodate monolith structure where well anchorage slab reinforcement will increases flexural capacity. Back to Table of Contents 613

9 The next step in common design procedure is selecting column section using combination of axial force output and maximum flexural capacity of beam with overstrength factor and then checks and details beam-column joint. By using chart one just selects the appropriate column section. Example of charts is shown bellow. Table 3 25x25 cm 2 column sections. f y = 40 MPa and f c = 20 MPa DETAIL b (mm) h (mm) Top rfc 3D13 2D16+4D13 6D16 Bottom rfc 3D13 2D16+4D13 6D13 Trvs rfc φ8-100 φ8-100 φ8-100 d (mm) ρ (%) (Pu.) / 0.65.A.'0,85.fc (Pu.e/h) / 0.8.A.0,85.fc' f y = 40 MPa and f c = 20 MPa Table 4 30x30 cm 2 column sections DETAIL b (mm) h (mm) Top rfc 2D19+4D16 6D19 2D22+4D19 Bottom rfc 2D19+4D16 6D19 2D22+4D19 Trvs rfc φ8-100 φ8-100 φ8-100 d (mm) ρ (%) (Pu.e/h) / 0.8.A.0,85.fc' 5. FUTURE WORK (P u.) / A.0,8 5.fc' Development an integrated seismic resistant design manual contains simple requirement, tables and charts especially for low rise building including steel and timber buildings. Ideally, it is supported by statistic and probabilistic analysis in conjunction with low rise building parameters trend in Indonesia. Pushover analysis is also required in order to have structural ductility of some simple typically low rise buildings Back to Table of Contents 614

10 International International Conference Conference Earthquake on Earthquake Engineering Engineering and Disaster and Mitigation, Disaster Mitigation Jakarta, April , CONCLUSIONS 1. There are hundred-thousands low rise building which are built every years through out Indonesia. It might be designed by unfamiliar-with-seismic-design engineers, even by non engineers. Therefore, simplified seismic resistant design manual is absolutely a need. In this paper, requirements for seismic resistance in Indonesian Concrete Code (SNI) was extracted for more simple guidance manual. 2. Study of seismic resistance design is carried out by taking the advantages of limited choices of typically low rise building parameters in Indonesia such as maximum column-to-column span, build ability of structural members dimension, reinforcement bar diameter, economical constraint. 3. More effective way to understand how one can design a seismic resistance low rise building is by simple approach through illustration drawing which is completed by charts, tables and simple explanation. 7. LITERATURE ACI Commite 318 (2003). Building Code Requirements for Structural Concrete (318M-99) and Commentary (318RM-99), American Concrete Institute, Farmington Hills Michigan. Applied Technology Council (1996). ATC-40 seismic evaluation and retrofit of concrete buildings, Volume 1, California Seismic Safety Commission, Report No. SSc 9601, November Budiono, B. (1999). Lecture Note of Earthquake Resistant Design, Institut Teknologi Bandung. Derecho, A.T. Seismic design of reinforced concret structures, Chapter 9, The Seismic Design Hadbook, 2 nd Edition, Farzad Naeim (Editor). Imran, I. (1999). Lecture Note of Advanced Reinforced Concrete Structure, Institut Teknologi Bandung. Nasution, A. (2000). Lecture Note of Reinforced Concrete Structure, Institut Teknologi Bandung. Nasution, A. and Hasballah (2000). Numeric Methods, Penerbit ITB, Bandung. SK SNI (2002). Tata Cara Perencanaan Ketahan Gempa untuk Bangunan Gedung, Badan Standardisasi Nasional. Back to Table of Contents 615