SPACE FRAME AND SPACE TRUSS IDEALIZATIONS FOR BAMBOO BUILDING’S STRUCTURAL ANALYSIS
Suantoro Wicaksono
Post Graduate Student of Gadjah Mada University
Morisco
Civil Engineering Lecture of Gadjah Mada University
Ashar Saputra
Civil Engineering Lecture of Gadjah Mada University
ABSTRAK:
Compared to other construction material, bamboo has excess that is can be created become a beautiful building in esthetical but strong in structural. In line with technological development at bamboo especially at pickling technology and tacking on bamboo, now we can build a bamboo building more than three floors. In doing a building structural analysis, we usually used Space Frame idealisation or Space Truss idealisation. No one compared Space Frame idealisation and Space Truss idealisation, especially to bamboo building. At this journal compared between analysis by using Space Frame idealisation and Space Truss idealisation to bamboo building Heart of School and Suspension Bridge. Heart of School is three storey building that function for school. And Suspension Bridge is a bridge for pedestrian. From analysis result known that both structures analysed, displacement and member forces of Space Truss idealisation are smaller than Space Frame idealisation.
Key Words: bamboo building, Space Frame, Space Truss
1 INTRODUCTION
Compared to other construction material, bamboo has excess that is can be created become a beautiful building in esthetical but strong in structural. In past years using of bamboo as component of construction is limitedly because of social factor, where public opinion that is often express that building made from bamboo is building for poor people. In line with technological development at bamboo especially at pickling technology and tacking on bamboo, and awareness of public to be more using friendly material, now we can build a bamboo building more than three floors.
As does at wood, the biggest weakness of bamboo as component of constructions are at joint. To improve effort of strength of bamboo joints has been done by Morisco and Mardjono (1995, 1996) by adding cement mortar and wood as filler at bamboo cavity at joints. Continued equipment applied is gusset plate and bolt from steel.
In doing a building structural analysis, we usually used Space Frame idealisation or Space Truss idealisation. No one compared Space Frame idealisation and Space Truss idealisation, especially to bamboo building. At this journal compared between analysis by using Space Frame idealisation and Space Truss idealisation to bamboo building Heart of School and Suspension Bridge. From comparison of the idealisations can choose which analysed that fit for bamboo building.
2 BAMBOO AS CONSTRUCTION MATERIALS
2.1 Mechanical properties of bamboo
To be able applied as construction material, we must know the bamboo mechanical properties. Mechanical properties of bamboo that need to be known are tensile strength, compressive strength, shear strength, flexural strength, and relation between tension and strain. Theoretically bamboo mechanics characters depends on ( Frick, 2004):
· Bamboo type related to flora
· Bamboo age when hewing
· Dampness (equilibrium water content) at bamboo bar
· Part of bamboo bar that applied (part of foot, mid or head)
· Situation and internodes distance is each (part of joint unable to hold up to compressive force and flexural)
Tensile strength of bamboos are different at part of interior or exterior bar, bar lines (slender bar has resilience to higher level tensile force), and at part of which bar is applied.
Compression strength of bamboos are different at part of joint and part between joints of bamboo bar. At part of bar without nodes is having the power of to compressive force which 8 - 45% higher than bamboo bars with nodes.
Shear strength is strength measure in the case of ability to stand against forces that making a part of bamboo to shift from other part in nearness. Shear strength at bamboo bars are different due to wall thickness, at part of joint and at part among joint bamboo bar. Part of bar without node is having the shear force power 50% higher than bamboo bars with nodes.
Flexural strength is strength to stand against the forces that bend bamboo bar or to stand dead and life loads. Because bamboo is elastic material, displacement happened according to strength of material becomes rather height (1/20 average). That needs to concern at development of building, where construction displacement usually may not exceed 1/300 of length.
Stress strength of bamboo can be compare with steel. According to research by Morisco in the year 1994 - 1999, tensile strength of skin of ori bamboo approximate reaches 500 MPA, or twice of yield stress of steel, while tensile strength average of petung bamboo also higher from yield stress of steel (Morisco, 2006).
Figure 1. Bamboo and Steel’s Stress-strain Diagram.
2.2 Bamboo’s joint
For usage of construction material, bamboo bars must be related and connected one to another. The connector can be in the form of simple equipment like rope and nail; lamination connection by using glue and peg; and connection applies with bolt and plate.
3 STRUCTURAL ANALYSIS IdealiZaTions
3.1 Space Frame idealization
Space Frame idealization is the basic idealization in structure analysis. In this idealization, there is 6 degree of freedom that is axial force, strong and weak direction of shear, strong and weak direction of moment, and torsion.
Figure 2. Space Frame Idealization.
3.2 Space Truss idealization
Space Truss idealization is simplifications of Space Frame idealization, that only can calculate axial force.
Figure 3. Space Truss Idealization.
4 BAMBOO BUILDING’S STRUCTURAL ANALYSIS
4.1 Structural analysis utilizing software SAP2000 version 12
The process for structural analysis for bamboo building was utilizing software SAP2000 version 12 and the stepping procedures are explained on Figure 4.
Figure 4. Flow Chart for Design Process
Explanations for each steps from flow chart on Figure 4 are follows;
1. Setting Nodal/Joints Positions
Due to the complicated of structure, first thing need to do is positioning joints. Joints are placed based on CAD drawing or imitation model. Every joint then labeled to ease the investigation on next steps.
2. Defining Material and Sections Properties
Structure modeled was made of bamboo having properties: density 7,845 x 10-9 kN/mm3, modulus of elasticity 13 kN/mm2, and Poisson ratio 0,3. Members are consist of tubular bamboo and split bamboo beams. Both could be modeled using ‘section designer’ facility which is embedded on SAP2000 v.12. and checked partially by manual calculations. Split beam was assumed as rectangular beams while original tubular bamboo set to be a pipe having diameter and thickness similar to bamboo available for construction. For simplifications, all members modeled as single member even they might consist of two or more bars on design level.
3. Setting Material and Section Properties on Structural Member
Members having defined material and section properties then set on position. All foundations are assign as hinge.
4. Input Loading and Load Combinations
Load applied on structural model are dead load, live load, wind load, and earthquake and their combinations based on Indonesian Standard of Loading for Building and Housing.
· Dead load (DL):
Dead load is self weight of structures. Self weight of member, which is bamboo bars and bamboo split beam automatically involved by SAP2000 v.12 on the calculations. Dead load for floor was determined to be 50 kg/m2 and supported by beams at the edge of covering are as continues/uniform load. Dead load for roof are determined as point load as the result of transformation of uniform load 30 kg/m2 placed at defined joints.
· Live load (LL):
Live loads on floor are determined as 250 kg/m2 (based on Indonesian Standard of Loading for Building and Housing) for Heart of School and 500 kg/m2 for Suspension Bridge. Live load for roof are rain load and load from worker and their equipments. Rain load was taken to be 30 kg/m2 and load affected by worker and their equipments were 100 kg per 5 m, assumed as point load.
· Wind load (WL):
Wind load given to model structure was 35 kg/m2 working as point load at certain position, which is one weak axes of structure at horizontal direction.
· Earthquake load (E):
Earthquake load was analyzed as a response spectrums having acceleration at weak axes of structure at horizontal direction with scale factor of 1.
Combinations for that entire load base on the specifications for wooden structure are as follows:
o 1.2 DL + 1.6 LL
o 1.2 DL + 0.5 LL + 1.3 WL
o 1.2 DL + 1.6 LL + 0.8 WL
o 1.05 DL + 0.9 LL + 1.05 E
4.2 Structural analysis of Heart of School
Heart of School and Suspension Bridge are two different building that analyzed as sample analysis. Heart of School is the biggest building that build by PT. Bambu at Green School at Sibang Kaja, Bali. Heart of School is a 3 storey school. Complexity of the building that need to do deep analysis.
Figure 5. Heart of School Model in SAP2000.
4.3 Structural analysis of Suspension Bridge
Suspension Bridge is a bamboo bridge structure with 22 m length. Suspension Bridge is also one of building which is made PT. Bamboo in Green School in Sibang Kaja, Bali. Suspension Bridge is built above river and destined for pedestrian. In form and structural, Suspension Bridge is alike truss bridge, where burden at bridge deck hold by its structure system above.
Figure 6. Suspension Bridge Model in SAP2000.
4.4 Space Frame and Space Truss Idealizations in SAP2000 version 12
Both of the building structures (Heart of School and Suspension Bridge) each analysed as Space Frame and Space Truss. As Space Frame, structure is modelled like ordinary building structure. While as Space Truss, done moment release at certain bars, like at bar that is not continuously. The difference modelling of idealisation Space Frame and Space Truss is by choosing type DOF at option Analysis Options.
Figure 7. Space Frame Idealization in SAP2000.
Figure 8. Space Truss Idealization in SAP2000.
Figure 9. Analysis Options in SAP2000.
5 ANALYSIS RESULT
Comparison between usages of Space Frame and Space Truss idealizations at Heart of School and Suspension Bridge are as follows:
Table 1. Heart of School Displacement
Table 2 Heart of School Member Forces.
Table 3. Suspension Bridge Displacement.
Table 4. Suspension Bridge Member Forces.
Table 5. Natural frequency and period at structures .
From Tables 1. and 3. seen that displacement of both structures, analysis by using Space Truss idealization are smaller than Space Frame idealization. Displacements of U1, U2 and U3 direction are displacement in global axis, where U1 is displacement at x direction, U2 at y direction and U3 is displacement at z direction. While displacement at R1, R2 and R3 directions is gyrations of x, y and z direction in radians. Space Truss Idealization R1, R2 and R3 = 0 because of gyrations is not activated in Analysis Options at SAP2000.
For check member forces, Heart of School divided become 5 groups; that is 2nd floor beams, 3rd floor beams, roof beams, column and core. Column and core group is differentiated due to core is a unity of separate structure consisted of 10 columns; each column are consist of 2 bamboos. Suspension Bridge is divided to become 3 groups; that is floor beams, roof frame and purlin. At Tables 2. and 4. seen that as a whole member forces value at analysis with Space Truss idealization smaller than analysis with Space Frame idealization. Torsion value at Space Truss idealization = 0 because Space Truss idealization doesn't permit existence of torsion.
At Tables 5. presented natural frequency and period at each structures. It is seen that analysis with Space Frame idealization have natural frequencies value is smaller than Space Truss idealization, and period of Space Frame idealization are bigger than Space Truss idealization. So that can be said that structures with of Space Frame idealization are stiffer than structures with Space Truss idealization.
6 Summary and SUGGESTION
6.1 Summary
From result of structural analysis by using software SAP2000, that analysis that applies Space Truss idealization produces displacements and member forces which smaller than at using Space Frame idealization. This thing happened because structure with of Space Frame idealization is stiffer than structures with Space Truss idealization, that’s seen from value of natural frequency Space Frame idealization is smaller than Space Truss idealization.
6.2 Suggestion
The election of idealizations at structural analysis actually bases on type joint applied at the structure. For that purpose needs further study to each joints type at bamboo structure.
7 ACKNOWLEDGEMENTS
Both structures that utilized as analysis samples (Heart of School and Suspension Bridge) are two buildings at Green School in Sibang Kaja, Bali; which are courtesy of PT. Bamboo.
REFERENCES
Dewobroto, W., 2007, Aplikasi Rekayasa Konstruksi dengan SAP2000 Edisi Baru, PT Elex Media Komputindo, Jakarta
Frick, H., 2004, Ilmu Konstruksi Bangunan Bambu, Penerbit Kanisius, Yogyakarta.
Morisco, dkk, 2008, Final Report on Structural Analysis of The Mepantigan, Suspension Bridge, and Kitchen of The Green School, Bali, Laboratory of Structural Engineering Department of Civil and Environmental Engineering Gadjah Mada University, Yogyakarta.
Morisco, dkk, 2008, Project Report Structural Analysis and Design for The Heart of School, Laboratory of Structural Engineering Department of Civil and Environmental Engineering Gadjah Mada University, Yogyakarta.
Morisco, 2000, Sambungan Bambu dengan Celah dan Pengisi, Forum Teknik jilid 247, No. 1, Maret 2000, Yogyakarta.
Morisco, 2006, Teknologi Bambu, Jurusan Teknik Sipil, Fakultas Teknik, Universitas Gadjah Mada, Yogyakarta.