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Gravitational action generates these loads and their direction is towards earth (i.e. downwards). In Australian standard

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This is the author’s version of a work that was submitted/accepted for publication in the following source: Fatima, T., Fawzia, S., & Nasir, A. (2012) Lateral movements in composite high-rise buildings under seismic action. In Proceedings of the Australasian Structural Engineering Conference, Engineers Australia, Perth, Western Australia, pp. 1-8. This file was downloaded from: https://eprints.qut.edu.au/53828/

c Copyright 2012 [please consult the authors]

Notice: Changes introduced as a result of publishing processes such as copy-editing and formatting may not be reflected in this document. For a definitive version of this work, please refer to the published source: http://asec2012.conference.net.au/papers/123.pdf

Lateral movements in composite high-rise buildings under Seismic Action T. Fatima*, S. Fawzia* and A. Nasir** * Department of Civil Engineering, Queensland University of Technology , 2 George Street, Brisbane, QLD 4000 , 3810, Australia. (Email: [email protected] [email protected]; ) ** Safe Australia Consulting Engineers, P. O. Box 5081, QLD 4115, Australia, (E-mail: [email protected]). ABSTRACT Daring human nature has already led to the construction of high-rise buildings in naturally challenging geological regions and in worse environments of the world. However; literature review divulges that there is a lag in research of certain generic principles and rules for the prediction of lateral movement in multistorey construction. The present competitive trend orders the best possible used of available construction material and resources. Hence; the mixed used of reinforced concrete with structural steel is gaining prevalence day by day. This paper investigates the effects of Seismic load on composite multistorey building provided with core wall and trusses through FEM modelling. The results showed that increased rigidity corresponds to lower period of vibration and hence higher seismic forces. Since Seismic action is a function of mass and response acceleration, therefore; mass increment generate higher earthquake load and thus cause higher impact base shear and overturning movement. Whereas; wind force depends on building exposed, larger the plan dimension greater is the wind impact. Nonetheless; outriggers trusses noticeably contribute, in improving the serviceability of structure subjected to wind and earthquake forces. KEYWORDS Seismic load, storey drift, lateral force, deflection, base shear. INTRODUCTION Background: Human art of creating dwellings is prehistoric. Concept of shelter evolved as man started taking sanctuary in caves. With advent and innovations, the shelter has changed to necessity, comfort and luxury and ultimately became a demand of livelihood. Construction materials are also evolved with time; once dwellings made up of tree branches and straw have now been transformed into hundreds of meter high establishments, made up of concrete and steel or combination of both. These changes are brought into existence due to higher housing demands, cost competitiveness, ease of construction and time restrains etc. To attain these evolutions, man has to work against natural fury and devastations such as cyclones, storms, tides and earthquakes. The ingenuity of superior-being has lead to the construction of Burj Khalifa, Petronas towers, Jing Mao Building, Taipei 101 and many more. Wind forces and seismic agitation are two major natural constraints that an engineer has to counteract to achieve his goal. This paper however focuses on seismic effect on tall composite building within Australia. Though; greater parts of researches are concentrated on the post-elastic behaviour of structures subjected to seismic and wind actions. Certain experimental and analytical techniques have also been employed on determining the effects of seismic load on various structural members, sub-assemblages and systems. Griffith et al (2003) has created a retrofit method to recuperate the drift capacity of soft storey structures. 1

The overall behaviour of structural system has been investigated by certain analytical models in various studies. Such as; Lumantarna et al, (2003), determined the inelastic torsion response of buildings using a displacement based approach whereas; Edwards et al,( 2003), examined the equivalent damping ratios in reinforced concrete frame building to incorporate into structural analysis methods for seismic displacement response predictions. There is still a huge lack of academic material on overall building behaviour when subjected to seismic action. Although; there are many customised examples of structures in which bracings are used to control seismic displacements of high-rise construction for instance; Taipei 101 in Taiwan. In this structure belt trusses and mega-columns stabilize the core while internal columns carry gravity loads from limited number of floors usually to the transfer level at every 8-storey intervals. Nevertheless; these examples are the best learning objects, however; research is needed to formulate generic rules and principles that can help predict structural performance at an initial stage. This would assist engineers in achieving more promising and definite results and could help them avoid discrepancies and alterations at detailed design stage. Aims/objectives: Scarcity of literature, which encompasses a holistic composite structure subjected to earthquake excitation, lead to this investigation. Fawzia et al (2010, 2011) has employed wind loading on composite structure and extrapolated deflections for various truss combinations. However; this study is conducted to find out displacements and storey drifts of a composite office building subjected to Seismic agitation. The method used involves creating of two prototypes of varying heights through finite element modelling (FEM) in Strand7 (Release 2.3.8) programme. The height of building prototypes is selected as per Australian standard prescribed limits as well as general practice of the country. METHODS Overall procedure: The prototypes used in this paper are similar to models used in the previous study of Fawzia et al (2011). Earthquake forces are calculated according to the Dynamic Analysis procedure of Australian standard (AS 1170.4; 2007) and applied to these finite element model (FEM). Loadings As mentioned earlier any structure has to counteract against two loading types i.e. Gravity Loads and/or Lateral loads. Lateral loads are intern constituted by wind and seismic forces. Gravity load. Gravitational action generates these loads and their direction is towards earth (i.e. downwards). In Australian standard (AS/NZS 1170.0;2002, AS/NZS 1170.1; 2002) these loads are classified into three major types; i.e. Self weight of structural elements that correspond to size and material properties, Super-imposed dead loads and Live loads. Self weight of structure. Modelling of prototype is based on following characteristics of members and elements; Composite Slab. Lysaght Bondek metal sheeting (BlueScope Lysaght Manual, 2003 ed) of 1.0 BMT is chosen for 120mm Slab overall depth. Primary beam and Secondary beam. Primary and secondary beams are of Structural steel I- sections of approximate sizes as given in the Onesteel tables (Ng, 2005). Composite column. Load tributary area supported by column dictates column cross-sectional size, strength and type of material. Hence; column cross-sectional area increases as the load increases. 2

Reinforced concrete (RCC) wall. Reinforced concrete wall if treated as lateral support, then termed as bracing to the structure. The cross-sectional area must be sufficient to resist horizontal loadings and that can only be achieved through optimization. However; to generate prototype, a minimum thickness satisfying the FRL (fire rating level) criteria of Building code of Australia (BCA, edition 2011) is adopted. Super-imposed dead load (SDL). The load of permanent fixtures and fittings such as ceilings, airconditioning ducts, floor finishes, partitions etc. is taken as; 1.5 kN/m2. Live load (LL). Live load mainly corresponds to human load and they are highly variable. Typical office LL of 3 kN/m2 is adopted for this paper. Seismic load. This study is kept within the scope of Australian standards and country’s general practice in order to benefit engineers in their routine task. Australian standard (AS 1170.4, 2007) recommend that any such structure that is 50m or more comes under EDC III (i.e. earthquake design category III) and a dynamic analysis must carried out for such structures. Three methods are outlined in Australian standard (1170.4; 2007) as below: (a) Horizontal design response spectrum (b) Site-specific design response spectra (c) Ground-motion time histories for specific sites This paper, however; cover the first method of dynamic analysis, where “T” (i.e. Period of vibration appropriate to the mode of vibration of the structure) is obtained by solving the model for “Natural Frequency”. T = 1/f

(sec)

where: f = natural frequency of structure in “Hz”.

Base Shear. The horizontal equivalent static shear force (V) acting at the base of structure in any horizontal direction is given in equation 1: V = Cd(T 1 )Wt = [C(T 1 ) S p /µ]Wt =[ k p ZC h (T 1 )S p /µ]Wt

[1]

Where: C d (T 1 ) = Horizontal design spectrum co-efficient; C(T 1 ) = elastic site hazard spectrum C h (T 1 ) =spectral shape factor for the fundamental natural period of structure; k p = Probability Factor; Z = Hazard factor for specific Australian locations; S p =Structural Performance factor; µ = Structural ductility factor; T 1 =Translational first mode natural period; W t =Seismic weight of structure ; W t = ∑G i + ∑ψ c Q i ; G i = permanent action (self weight + SDL); ψ c = combination factor Q i = imposed action (LL). The vertical distribution of Base Shear is carried out according to equation 2 as: F i = k F,i V =

[ 2]

Where: k F,i = seismic distribution factor for ith level; Wi = seismic weight of structure at ith level (kN); h i = height of level i above the structural base (m); k = exponent depend on T 1; n = number of levels in structure. Framing layout 3

Main Features of Model. Figure 1a, shows L-shaped layout with walls given at right and left hands, designated as RW and LW respectively and at top left corner of building named as CW (Figure 1b). Two additional walls i.e. LSW and RSW (Figure 1b) are also provided at the later stage of analysis in 57-storey model. Storey height of 3.5m is assumed which correspond to the general practice for office dwelling in Australia. Stair-wells and lift-shafts are provided to satisfy the minimum access and egress requirement for office building classification “Class 5” of building code of Australia (BCA 2011). Secondary beams are supported by primary beams provided along X-axis of plan. Typical column spacing is 10 m centre to centre.

Figure 1a. Model layout

Figure 1b. Shear walls designation

According to definition of Australian Standard (AS 4100:1998), Simple construction is selected for modelling. Therefore; frame moment releases are provided for primary and secondary beams (Figure 2a). Lateral load path consist of Core/Shear walls, outrigger and belt trusses. Figure 2b, illustrate the “fixed” bases of column and core/shear walls. The lateral load attracted by the vertical element depends on its cross-sectional area and rigidity, therefore; it can be stipulated that core/shear wall will attract maximum of the lateral force. Floor load constituted by dead and live load remains same throughout the building height on each floor level. For simplicity transfer level load (mechanical and electrical equipments) is not considered. Therefore; sizes of slab and beams are same on each level.

Figure 2a. Moment Release

Figure 2b. Fixed supports

Column grouping. Columns are grouped for each five stories according to the vertical loads. Figure 3, shows elevations of model where different colours of a column correspond to property change along the building heigh. Australian code for Concrete Structures (AS 3600; 2009) is utilized for cross-sectional area calculation of vertical element. UC (universal column) and WC (welded column) sections (ASI, 2009) are used as steel component of composite section. The transformed properties of composite columns are given as: 4

Transformed Elastic Modulus Transformed Density A c E c + A ST E s = A g E T A c γ c + A ST γ s = A g γ T Where: A g = gross area of section; A c = area of concrete; A ST = area of steel; E c = elastic modulus of concrete; Es = elastic Modulus of steel; E T = elastic modulus of transformed section; γ c = density of concrete; γ s = density of steel; γ T = density of transformed section. Modelling Optimization: Properties of slab, beams, columns and walls are Input in Strand7 (Release 2.3.8) for two different model heights. These models are run several times and results are compared to select optimum models as Jayachandran (2009) outlines that; overall optimization of tall building frame is complex and time consuming. Modelling Arrangements: Representative model arrangements in Table 1, are inspired by the previous work of Fawzia et al. (2009) and Wu et al (2003), in which the optimum location of outriggers is studied under trapezoidal horizontal loads.

Figure 3. Model Elevation

Models are tested with various outrigger levels and incremental core wall thickness to obtained the most viable element arrangement that readily satisfy the serviceability requirements of Australian standards. Core wall thicknesses are changed every ten levels designated in Table 1 as L1-L10 (i.e. from level1 to level10) and so on. Table 1. Representative Models Belt truss and outrigger levels 42-Storey Model Without truss 21st * 21st & 42nd* 21st & 42nd * 21st & 42nd * 20th , 21st & 41st , 42nd ** 18th , 30th & 42nd * 57-Storey Model Without truss 57th & 34th * 57th & 34th * 57th ,56th & 34th, 33th ** 57th , 40th & 24th * 57th,56th - 40th, 39th & 24th, 23rd ** 57th ,56th - 40th, 39th & 24th , 23rd **

Core/Shear wall thickness (mm) RW= LW=CW

Model Name

L1-L10 = 500, L11-L20= 450, L21-L30 = 400, L31L40 = 350, L41-L42 = 300

42S1 42S2 42S3

L1-L10 = 550, L11-L20= 500 , L21-L30 = 450, L31L40 = 400, L41-L42 = 350

42S4

L1-L10 = 600, L11-L20= 550, L21-L30 = 500, L31L40 = 450, L41-L42 = 400

42S5 42S6 42S7

L1-L10 = 700, L11-L20= 650, L21-L30 = 600, L31L40= 550, L41-L50 = 500, L51-L57 = 450, L1-L10 = 800, L11-L20= 750, L21-L30 = 700, L31L40= 650, L41-L50 = 600, L51-L57 = 550,

57S1 57S2 57S3 57S4 57S5 57S6

L1-L10 = 800, L11-L20= 750, L21-L30 = 700, L31L40= 650, L41-L50 = 600, L51-L57 = 550, LSW=RSW: L1-L10 = 550 L11-L20 =500, L21-L30 =

57S7 5

450, L31- L57 = 350 ** Double level of trusses;

* One level of trusses;

Modelling Validation: Model validation is very imperative and essential part of computer analysis. In this instance; vertical load of typical interior and exterior columns, structural self weight and base shear of model as generated by the programme, are checked with manually calculated values. Checks are performed repeatedly and the difference of 5% to 10% between manual and computer generated results are accepted. RESULTS AND DISCUSSION Results 42-storey model. The rigorous analysis shows that with increase of frequency value (i.e. decrease in period) gives lower deflection (Table 2). Table 2. Results for 42-storey model Model name

Period (sec)

42S1 42S2 42S3 42S4 42S5 42S6 42S7

3.781 3.566 3.450 3.372 3.305 3.104 3.196

Max. Deflection (mm) 490.21 444.51 404.68 373.49 346.91 298.24 320.00

Max. Storey drift (mm) 15.48 14.1 15.03 13.93 12.99 11.31 10.00

500

drift ratio 0.442% 0.403% 0.429% 0.398% 0.371% 0.323% 0.286% Storey Drifts

0.450%

Deflections Storey drifts (mm)

Deflection (mm)

450 400 350 300

0.400% 0.350% 0.300%

42S7

0.250% 42S1 42S2 42S3 42S4 42S5 42S6 42S7 Models

Figure 4a. Deflection of 42-storey models

Figure 4b. storey drift of 42-storey models

250 42S1

42S2

42S3

42S4 42S5 Models

42S6

The deflection curve in Figure 4a is somewhat regular except the trend changes from 42S6 to 42S7. This is due to the change in rigidity from two double outrigger levels to three single outrigger levels respectively. The drift ratio in Figure 4b, is decreased at 42S2 most probably due to the introduction of outriggers and requires further investigation. 57-storey model. Figure 5a and 5b, illustrate similar graph trends for displacements and storey drifts. Model 52S4 and 52S5 show similar trend as 42S6 and 42S7. Table 3. Results for 57-storey model Model name

Period (sec)

Max. Deflection (mm)

Max. Storey drift (mm)

drift ratio 6

6.053 5.471 5.337 5.003 5.168 4.741 4.562

57S1 57S2 57S3 57S4 57S5 57S6 57S7

307.836 253.656 226.461 195.591 213.892 177.506 137.855

7.101 6.288 5.524 5.149 5.451 4.86 3.54

350

0.215% Deflections

Storey Drifts

0.195% Storey drifts (mm)

300 Deflection (mm)

0.203% 0.180% 0.158% 0.147% 0.156% 0.139% 0.101%

250 200 150

0.175% 0.155% 0.135% 0.115% 0.095%

100 57S1

57S2

57S3

57S4 57S5 Models

57S6

0.075% 57S1

57S7

Figure 5a. Deflection of 57-storey models

57S2

57S3

57S4 57S5 Models

57S6

57S7

Figure 5b. storey drift of 57-storey models

Discussion: Base shear “V” as given by equation 1 is equal to C d (T 1 ) and W t , where; C d (T 1 ) = C h (T 1 ), is given as 1.874/T2 in table 6.4 (for site sub-class C e , T>1.5) of Australian Standard (AS 1170.4-2007). This implies that base shear is inversely proportion to the square of period. Time period decreases with increased rigidity or mass as seen in Table 2 and Table 3 (for instance in Table 2, 42S2 with one level outrigger has “T” equal to 3.566 sec and 42S6 with two double level of outrigger has “T” equal to 3.104 sec). Therefore; it implies that stocky structures are more likely to attract higher seismic forces. Conversely, wind load depend on structural plan area and do not rely on structural rigidity. With increased plan dimensions, more area is exposed to wind gust and therefore attract more wind load. As seen in Figure 6a and 6b, for same modelling arrangements, deflection trend is similar under wind and seismic loading, however; deflections (in number) are considerably higher when building is under wind load. 1600 1400

Wind

600

Seismic

500

Wind

1000 800 600 400

Deflection (mm)

Deflection (mm)

1200

Seismic

400 300 200 100

200 0 57M1 57M2 57M3 57M4 57M5 57M6 57M7 Models

Figure 6a. Deflection of 57-storey models

0 42M1 42M2 42M3 42M4 42M5 42M6 42M7 Models

Figure 6b. Storey drift of 42-storey models 7

CONCLUSIONS This study provides an insight into the behaviour of building under seismic load. The results showed similar displacement trends with the difference in deflection values as seen in figure 6a and 6b taken from previous work of Fawzia et al (2011). This difference is owing to the difference in horizontal force values which are considerably higher in case of wind. Composite structures consist of steel sections, composite slabs and columns of lesser mass, hence; attract less seismic forces if compared with the conventional reinforced concrete structure. However; provision of outriggers trusses markedly reduce displacement and hence their efficacy is equally advantageous for both type of lateral load impact. REFERENCES Australian Standards, Structural design Actions, Part 4: Earth actions in Australia, AS 1170.4-2007, Part 0: General Principal, AS/NZS 1170.0:2002 and Part 1: Permanent, Imposed and other actions, AS/NZS 1170.1:2002. Australian Steel Institute (ASI), Design Capacity Tables. Volume 1, Fourth edition 2009. BlueScope Lysaght Manual, Using Bondek- design and construction guide 2003 edition, BlueScope Steel limited, Australia. Building Code of Australia (BCA). Volume 1 and Edition 2011. Edwards, M., Wilson, J. L. & Lam, N. T. K. 2003, “Seismic displacement response predictions using a calibrated substitute structure approach”, Proceedings 6th Pacific Conference of Earth Engineering, University of Canterbury, Christchurch, New Zealand, March, Paper No. 110. Fawzia S. and T. Fatima, Deflection Control in composite building by using Belt truss and Outrigger System. Proceedings of the 2010 World Academy of Science, Engineering and Technology conference, pp. 25-27 August 2010, Singapore. Fawzia S. , Nasir A. and Fatima T., Study of effectiveness of outrigger system for high-rise composite building for cyclonic region. Proceedings of the 2011 World Academy of Science, Engineering and Technology conference, pp. 937-945, Issue 60, December 2011, Thailand. Griffi th, M. C., Wu, Y. & Oehlers, D. 2003, “Steel plated seismic retrofi t for RC columns in soft storey frames”, Proceedings Australian Earth Engineering Society Conference, Melbourne, November, Paper No. 9. Jayachandran P., Design of tall Buildings - Preliminary Design and Optimization. National Workshop on High-rise and Tall buildings, University of Hyderabad, India, May 2009. Lumantarna, E., Lam, N. T. K. & Wilson, J. L. 2003, “A displacement approach to the analysis for seismically induced torsion in buildings”, Proceedings Australian Earth Engineering Society Conference, Melbourne, November, Paper No. 15. Ng A. and Yum G., Span Tables for Simply supported Composite Beams. Onesteel Market Mills Design Note DN3. Ed1.1, Nov 2005. Strand7 Pty Ltd. Strand7, Finite Element Analysis System. User’s Manual 2005, Sydney, Australia. Wu J. R. and Li Q. S., Structural Performance of Multi-Outrigger-Brace Tall Buildings. Struct. Design Tall Spec. Build. vol. 12, pp. 155–176 (2003).

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