Analysis steel frames subjected to wind load by using three design codes

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Research Report

Analysis steel frames subjected to wind load by using three design codes

Ahya Malekpour

Department of Civil Engineering, College of Engineering, Bandar Abbas Branch,

Islamic Azad university, Bandar Abbas, Iran.

ABSTRACT

Wind forces are complex. The effect of wind on a building depends on the interaction of many variables. Natural variables include wind speed, wind height, ground surface features, and the properties of the air. Building variables include the shape, location, and physical properties of structures. Together, these variables create differences in pressure that push and pull on the exterior surfaces of buildings.Globalization of the construction industry and the development of international codes and standards has intensified the need of better understanding the underlying differences betweenstandards of the wind loading. This paper, discuses the linear analysis of steelframes2,4,7,10,15and20floors, explained by using three design codes,ANSI 7-10, NBCC 05andIranian building code ,part 6. In this paper we discuss the criteria that include: the Overturning moment and drift. Note that the drift is a good indicator of structural and non-structural damage so in comparison, so the structural safety based is defined onthecriteria .One result ofthisstudyindicates thatthe proposedrulesin thebylaws ofAmerica, CanadaandIranfor the building that are analyses in a linear static way, insome cases, do notprovideadequate safetyagainst wind.

According to sixth topic the basic cutting is less than the cutting according to NBCC regulations and the cutting according to NBCC regulation is less than ANSI regulation.

According to the sixth issue in every frame the overturning moment is less than the NBCC regulation and the overturning moment in NBCC regulation is less than the ANSI regulation.

Keywords: Steel frame, moment, ANSI ,NBCC , Iranian building code ,part 6.

INTRODUCTION

Based on the Wind Load Revisions in 2010 National Building Code of Canada and Future Research-Based Submissions, Ted Stathopoulos, Ioannis Zisis[1], In this paper, Extreme wind events account for a significant number of casualties and damages all across the globe. Wind standards and building codes of practice aim to reduce the impact of such events on structures. Revisions and updates on major wind provisions take place continuously. By the end of 2010 the National Research Council of Canada will release the new edition of National Building of Canada, which will include minor revisions related to wind load recommendations. These changes are discussed along with some recent wind

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the coast of North Carolina. In the first part of this study, attention is devoted to the characterization of the wind field around an instrumented house; a comprehensive investigation on wind velocity and turbulence characteristics during the passage of three tropical storms and other significant events is summarized. In the second part, results associated with the meteorological studies are used to assist the interpretation of pressure time histories related to such extreme events. Analyses associated with the derivation of normalized pressure coefficients were concentrated on the identification of direction-dependent pressure characteristics, correlation among consecutive taps and potential effects of the wind unsteadiness on the maximum and minimum values. Building geometry and local topography effects had an important and direct influence on these analyses. The other two direct techniques, namely the wind pressure modeltest technique and high-frequency force balance technique, especially the latter, arenow the main tools for obtaining across-wind loads of tall buildings for practicalpurpose. Marukawa et al. [3], Kareem [4] and Katagiri et al. [5] have providedvaluable testing results of across-wind loads of tall buildings with typical cross sections. In this paper, 15tall building models of typical cross-sections and different aspectratios are tested in TJ-1 Boundary Layer Wind Tunnel in Tongji University to obtainthe across-wind aerodynamic loads of these buildings. New formulas for the acrosswind force spectra, the coefficients of base moment and shear force are given. A SDOF aeroelastic model of a square tall building with an aspect ratio of 6 is selected from the buildings for the across-wind load test to investigate its across-wind dynamic response and across-wind aerodynamic damping, and to further verify the reliability of the across-wind aerodynamic forces obtained from the present test. Based on the Comparison of wind loads calculated by fifteen different codes and standards,for low, medium and high-rise buildings, John Holmes, Yukio Tamura, Prem Krishna[6], The paper

describes a comparison of wind load calculations on three buildings using fifteen different wind loading codes and standards from the Asia-Pacific Region. The low-rise building is a typical steel portal-framed industrial warehouse building assumed to be located in a ruralarea. The medium-height building is a 48-metre high office building located in urban terrain.The high-rise building is 183 metres high, also located in urban terrain. The design wind speeds at the top of each building, and other wind properties such as turbulence intensity were prescribed. The comparisons showed varying degrees of agreement. Comments on the differences are given.

2. Programinput

Although these standards determine wind loading in the along-wind direction using a random-vibration-based gust factor approach, the parameters are defined differently. These parameters are re-written in a consistent format and compared with each other. Some of the difficulties in using international standards is the use of different terminology and the incorporation of factors within other terms, making it hard for designers to work in a global environment. Rewriting the basic equations in a general format will help designers decipher the nuances of the different codes/standards and understand the resulting differences in the response. Note that the scope of this analysis is limited to dynamically sensitive buildings of regular shape. All the standards recommend that extremely tall and irregular shaped structures be designed using wind tunnels. Along wind Loads In all three standards, the along wind loads are determined by multiplying the windpressure by the tributary area of the building. The general expression for pressures on abuilding for all the standards can be expressed as

p =q.G.Cp(1).

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pressures, acting in the windward and leeward directions. The loads are then determined by combining the pressures acting on a wall and the corresponding tributary area. Moments are determined by multiplying the load at a given height by the corresponding height. Base shear forces and moments are then determined by the sum of the loads and moments at each level .The velocity pressure can be expressed as:

q =V0.Cexposure.Cterrain.C direction.C importance.C other

(2) where฀฀air density; V0= basic wind velocity; Cexposure =velocity profile or exposure factor; Cterrain=terrain and topography factor; C direction= directionality factor; C importance=building importance factor; and C other = a factor accounting for other things such as hurricanezone, shielding, or mean recurrence interval. The effects of terrain, directionality, building importance, and other factors are not considered in this study. However, the definitions of velocity profile are analyzed in detail and compared between the standards. However, the definitions of velocity profile are analyzed in detail and compared between the standards.

Averaging times for wind velocity vary between the standards and within the standards. the reference height at which the gust factor and other parameters are calculated is different between the codes/standards, as summarized in Table 1. These differences between averaging time and reference heights affect the intermediary parameters and resulting responses, making a simple comparison between the standards challenging. Throughout this analysis, the effect of differing averaging times has been minimized as much as possible. Table 1: Averaging Times and Reference Heights The wind velocity in each code is described by a profile law, either power or

logarithmic. The velocity profiles are dependent on the exposure category. Each standard uses three to five exposures categories, and can be described by six general exposure categories.

3. Program out put 3.1.drift

Drift need to relative movement to control groups with decrease mobility both on the same floor as the relative displacement obtained

Chart1-Comparison drift according to ANSI and Sixth topic,2 floors.

Visibility Factor The base pressure The average wind speed in the Regulations

region of inteterest

2 5dan/m^2 100Km/h Iranian building code,part6

2 0.5KN/m^2 27.78m/s NBCC

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Chart3-Comparison drift according to ANSI and Sixth topic,7 floors.

Chart5-Comparison drifts according to ANSI and Sixth topic, 15floors.

Chart7-Comparison drifts according to NBCC and Sixth topic, 2floors.

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Chart9-Comparison drifts according to NBCC and Sixth topic, 7 floors.

Chart 10-Comparison drift according to NBCC and Sixth topic,10 floors.

Chart13-Comparison max drift according to Sixth topic, 2 floors.

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Chart15-Comparison max drift according to Sixth topic,7 floors.

Chart19-Comparison max drift according to ANSI,2 floors.

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Chart25-Comparison max drift according to NBCC,2 floors.

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Chart27-Comparison max drift according to NBCC,7 floors.

3.2. Over turning moment frames

Table5: Over turning moment frames

max M (1span,3span,5span)

2story Iranian 11.16

2story Nbcc 13.14

2 story ANSI 14.77

4story NBCC 58.64

4story ANSI 71.27

7story Iranian 180

7 story NBCC 206.2

7 story ANSI 257.5

10 story NBCC 455.1

10 story ANSI 579.1

15 story NBCC 1099

15 story ANSI 1445

20story Iranian 1657

20 story NBCC 2068

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4. THE RESULTS

-According to the sixth topic issue in every frame the overturning moment is less than the NBCC regulation and the overturning moment in NBCC regulation is less than the ANSI regulation. -According to all 3 regulations ,the overturning moment increasing with increasing the altitude. -according to three regulations, in 2,4,7,10 and 15 floor frames, the drift is less than the authorized limit the amount of the drift in 15 floor frames is extremely increased in comparison to other frames.

-according to three regulation, in 2,4,7,10 and 20 floor frames, maximum drift in one span frames equals or is more than three spans frames. also, maximum drift in three span frames equals or is more than five spans frames.

-according to the sixth topic maximum drift in three spans frame is more than one span and five spans drift in 15 floor frames.

-as you can see, there is no simulation in the there regulation at maximum drift in 15 floor frames.

REFERENCES

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for Numerical Methods in Engineering, 63 (55), pp. 760–788.

2. Atluri, S.N. and Shen, S. (2002),“The Meshless Local Petrov–Galerkin (MLPG) Method”, Tech Science Press, USA.

3. Udwadia, F. E. and Trifunac, M. D. (1973),“Ambient Vibration Test of Full Scale Structures,” Proc. of the 5th World Conf. On Earthquake Engineering, Rome, pp. 135-142. 4. Aghayani-pour, K. and Hakemifar, J. (2003),

“Theoretical and Experimental Study of the Strength of Piezoelectric Structures Under Pressure Forces,” Proc. of the 1stConference on Dynamics of Advanced Structures, Mechanics Research Center, Isfahan, pp. 23-25, (in Persian).

5. Trifunac, M. D. (1970), “Wind and Microtremor Induced Vibration of a 22 Story Steel Frame Building,” Earthquake Engineering Research Lab., Report EERL 70-01, California Institute of Technology, Pasadena California.

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marching methods,”