DESIGN ANALYSIS OF UNBRACED FRAME USING ANSYS

FARHANA BINTI ABDUL HALIM

Thesis submitted in partial fulfillment of the

fe4üifefiient föf the a.VâfdS of the degro of

Bachelor of Civil Engineering

Faculty Of Civil Engineering And Earth Resources

Uniiiersiti Mâiaysiä PähätIg

NOVEMBER 2010

ABSTRACT

Recently, the application of unbraced steel frame in building construction

arises rapidly nowadays, various researches has been carried out to study the behavior

of this type of frame. The tested frame structure was designed using available

recommendation in the existing literature. Finite Element Analysis is devejoped to

stimulate unbraced frame's deflection behavior. The result is compared with the range

of value of several goodness-of-fit measures which are provided from random output

parameter in each response surface set. Polynomial mathematical model generated

using various combination parameters using multiple regression analysis were found

to be statistically significant. Parametric study using response surface methodology

through finite element analysis may form efficient approximation to immediate

deflection. It shows a comparable result and thus, this program is expected applicable.

VI

ABSTRAK

Dewasa mi, aplikasi kerangka tidak dirembat sebagai struktur utama bangunan

semakin pesat membangun dari semasa ke semasa, pelbagai kajian telah dilakukan

bagi mengkaji sifat-sifat kerangkajenis mi. Ujian terhadap struktur kerangka mi telah

diambil dari struktur yang direka bentuk oleh kajian literasi sebelum ini Kaedah

Elemen Unsur Terhingga telah dibangunkan untuk meransang sifat lenturan kerangka

tidak dirembat. Keputusannya akan dibandingkan dengan anggaran nilai dan beberapa

ukuran 'goodness-of-fit' yang mana terhasil dari hasil keluaran parameter yang rawak

di setiap set tindakbalas permukaan. Model matematik polinoniial telah dihasilkan

oleh pelbagai gabungan parameter menggunakan analisis kemerosotan pelbagai yang

dijumpai dan menjadi statistik pelting. Kajian parametrik menggunakan kaedah

tindakbalas permukaan melalui elemen unsure terhingga boleh menghasilkan

pesongan yang hampir benar. mi menunjukkan perbandingan antara hasil keputusan

dan program mi dianggap boleh ditenima guna.

VII

TABLE OF CONTENT

CHAPTER ITEM PAGE

TITLE j

DECLARATION ii

DEDICATION iv

ACKNOWLEDGEMENT v

ABSTRACT vi

ABSTRAK vii

TABLE OF CONTENT viii

LIST OF TABLES xiii

LIST OF FIGURES xiv

1 INTRODUCTION 1

1.1 General i 1.2 Problem Statement 2

VIII

Ix

1.3 Objective 3

1.4 Scope Of Work

3

2 LITERATURE REVIEW 4

2.1 Introduction 4

2.2 Frame 5

2.2.1 Braced Frame 5

2.2.2 Steel Frame 6

2.3 Buckling Mode 7

2.3.1 Buckling Mode For Unbraced Frame 7

2.4 Loads On Structure JCS 8

2.4.1 Static Force

9

2.4.2 Wind Loads 9

2.5 Finite Element Analysis 10

2.5.1 History Of Finite Eletñeñt Analysis 11

How Does Finite EJ pAt Analysis Vprk

12

2.5.3 Meshing 13

2.5.4 Advantages Of Finite Element Analysis 14

2.6 Ansys Niametñc Design Language 14

2..l Benefit Of Parametric Design Analysis 15

16

17

18

18

18

19

20

22

22

x

3 METHODOLOGY

3.1 Introduction

3.2 Geometry

3.3 Preprocessing

3.3.1 Introduction

3.3.2 MOklIIij

3.3.3 Define The Element Type

3.3.4 Define Real Constant

3.3.5 Deflti Mäteiiâl P Opthiá

3.3.6 Define Section

3.3.7 Meshing

3.4 Solution

3.4.1 Introduction

3.4.2 Analysis Type

3.4.3 Dèfiiie FeCônstraints

3.4.4 Define Frame Load

3.4.5 Apply The Solution

3.5 Post-Pessing

3.5.1 Introduction

3.5.2 Reaction Forces

3.5.3 M ei Forces Aiid tEesses

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24

26

2

26

26

27

30

30

30

30

31

A

4

3.5.4 Member Displacement And Rotational 31

RESULT AND ANALYSIS 34

4.1 Introduction 34

4.2 Unbràced Frame Stture 35

4.3 Probabilistic Model Information 36

4.3.1 Random Input Variables 36

4.3.2 Young Modulus 37

4.3.3 Poisson's Ratio 38

4.3.4 Pressure 39

4.3.5 Läd 40

4.3.6 Real Constant 41

4.4 Probabilistic Analysis Summary 42

4.5 Ressiii iii iysis &àiiiT.ài 44

4.6 Probabilistic Analysis Results 46

4.6.1 Sample History Plots 48

5 CONCLUSION AND RECOMMENDATION 49

5.1 Conclusion 50

5.2 Re w. -diradn&ti 51

XII

REFERENCES 52

APPENDIX 54

LIST OF TABLES

NO. TITLE PAGE

3.1 Keypoint data 19

3.2 Real constant data 21

3.3 Section definitions 23

3.4 Line attributes assignments 25

4.1 Random Input Variables Specifications 37

4.2 Specifications for Response Surface Method Analysis 43

4.3 Specifications for the Design of Experiment Levels 44

4.4 Listing of the Results for Solution Set 45

4.5 Goodness-Of-Fit Measure 48

XII,

NO TITLE

2.1 Critical buckling mode for an unbraced frame

2.2 Typical types of loading condition

2.3 Finite Element Analysis on Slabs,

Beams and Columns

2.4 Mesh diagram on 2D model

3.1 Frame structure

3.2 Library of element type

3.3 Real constant for BEAM3

3.4 Beam tools

3.5 Line attributes

3.6 Apply U, ROT on keypoint

3.7 Apply pressure on beams selection window

3.8 ANSYS workspace window after the top

of the frame is selected for application

of a distributed load

3.9 Apply pressure on beams window

PAGE

7

8

11

13

17

20

21

23

25

27

28

29

29

LIST OF FIGURES

xiv

xv

3.10 Reaction solution window 31

3.11 List nodal solution window 32

3.12 Nodal solution table 33

4.1 Unbraced Frame Structure 35

4.2 PDF & CDF of Input Random Variable EX 38

4.2 PDF & CDF of Input Random Variable PRXY1 39

4.3 PDF & CDF of Input Random Variable PRES 1 40

4.4 PDF & CDF of Input Random Variable FX1 41

4.5 PDF & CDF of Input Random Variable RI 42

4.6 Sampled Values for Output Parameter

Maximum Deflection 49

4.7 Sampled Values for Output Parameter

Maximum VonMises Strain so

4.8 Sampled Values for Output Parameter

Maximum VonMises Stress 50

CHAPTER 1

INTRODUCTION

1.1 General

Recently, the need for structural rehabilitation of civil infrastructures all over

the world is well known and a great amount of research is going on this field. Every

time new technology will develop which are changes in the contributing factors to

structure such as increase in load requirement, corrosion deterioration to exposure to

aggressive environments, changes in functionality, potential damaged caused by

mechanical and environmental effects, increase in material strength and durability,

etc.

Many different methods are suitable for repair and strengthening, such as

additional reinforcement cover by concrete, external steel plate bonding, using of steel

material, etc. Nowadays, steel frame system with beams and columns is become the

conventional building structure in construction world. Structural design and structural

2

analysis are both of the criteria needed to create a structure that safely accomplish its

function in order to produce structures in a stability condition. In civil engineering

field, steel is widely used in building construction. Its popularity may be due to the

various sizes and the shape of steel sections to be used for various type of structures

such as small and the simple buildings as well as complicated infrastructures

construction. Generally, steel frame not is only design to sustain vertical loads but

also able to resist lateral loads.

In this research, Finite Element Method (FEM) models were used to stimulate

the behavior of the steel frame structure's ability using ANSYS 12.1 program. The

ANSYS was founded in 1970, develops and globally markets engineering simulation

software and technologies widely used by engineers and designers across a broad

spectrum of industries like civil and mechanical engineering. This program is capable

of predicting deflection and stress in concrete concepts and also includes model's

constitutive laws for concrete material, based on smeared crack concepts and for high

strength composite materials.

1.2 Problem Statement

Columns and beams which are in parts of frame structure are also known as

main components in building which have their own capability to support the load

from roof, floors and transfer it to lower level and foundation. Because of that, frame

structures must be designed well with appropriate calculation to make sure it can

support the loads so that it is safe for long period of time. In order to make the

calculation more easily but still in good design method of the structure, therefore

ANSYS Parametric Design Language is use to generate a new equation for

calculating the design of steel frame. The new equation is derived from parameters

which are depends on loads, length of span, Poisonn's ratio, pressure on structure and

so on. Therefore, this is the necessary of this research.

3

1.3 Objective

The objectives of this research are following:

• To present software analysis with analytical results.

• Generate a new equation for frame structure by proving the parameters

in ANSYS Parametric Design Language (APDL) using ANSYS

program.

• To study the response and behavior of unbraced frame structure

through a series of analysis under different load and dimension case.

1.4 Scope of Work

These researches are mainly focused on the design of steel frame structures by

using British Standard and use the data to generate the new equation by proving the

parameters in ANSYS. In order to achieve the objectives of the researches, there are

few researches scope is necessary to be followed. Study the types of steel design

frames and the characteristics of the structure.

Explore the ANSYS program by learning how to use the programming by

using tutorial from internet. Practicing of tutorials can helps to solve problem when

running the real models. Model of columns are designed using ANSYS parametric

design language (APDL). Finite Element Analysis is using to calculate the parameters

which the parameters must success to get new equation as mention in objectives.

CHAPTER 2

LITERATURE REVIEW

2.1 Introduction

In recent years, numbers of studies have been made in designing the unbraced

multi-storey steel frames and the development in this type is still continued until now.

Many researchers try to present the easier and simple way for example comes out with

software such as ANSYS, Visual Basic and so on for determination of the section

sizes which meet the principals and limitations as stated in BS 5950-1:2000.

Apart from that, several methods were introduced to fulfill the criteria which

are Wind-Moment Method and Merchant-Rankine Method. However, the

effectiveness of these methods is still in question because the second order effect due

to sway does not taking into account though sway deflection give the major effect in

5

the construction of frames. It is irrelevant to attempt final design that exceeds the

sway limitation because sway does not affect the frame structure. Therefore, a

simplified method which meets the sway deflection criterion is introduced and the

method is called as Direct Design Method.

This method does not stand alone. A supporting computer program should be

established to help the researchers reducing the difficulty when designing the frame

structure. The new software among students in Malaysia that can be used together to

compared with manual calculation is ANSYS 12.1. The establishment of this

computer program actually can help us to modify the structure's design and more easy

if it does not meet the specification as stated in BS5950-1:2000.

2.2 Frame

Frame is a structure made up from columns and beams that connected to each

others at a joints. It can be classified into two types which are braced frame and

unbraced frame. The behavior of these two frames is different to each other when

subjected to the lateral loadings.

2.2.1 Braced frame

Braced frame is a frame that has been supported by a bracing system to

prevent it from sway when subjected by lateral loading to meet the requirement. The

installation of bracing system had provided more stability to the system because it can

resist the lateral loadings from winds and earthquake or in other words, lateral

loadings did not taken by the column and beam (Schodek, 2004). For this type of

6

frame, it only took vertical loadings from dead load and live load. Figure 2.1 shows

the bracing system used in steel frame construction.

2.2.2 Steel frame

The complexity of analyzing and designing of steel frames have decreased the

use of steel material in building construction. However, many approaches have been

made to study the feasibility of steel frames structure since 1856 by Besemer's

process. Nowadays, many tall building is used the steel frame structure.

Frame structure can be classified into two types, braced and unbraced frames.

Braced frame which consist of bracing system that will provide lateral stability to the

structure, so, it will be resistance to the lateral effect. On the other hand, unbraced

frame does not provided with the bracing system and will easily sway if the lateral

loads act on it.

The lateral loading normally cause by the wind forces or earthquake. When

these lateral forces acting against the structure, it will induce overturning moment and

should be balanced by the internal moment developed by the members. Therefore, the

frames should be design properly to ensure that it will be able to carry both lateral and

vertical loads.

8

2.4 Loads On Structure

There are many types of loading that can affect the behavior of the structures.

When designing a building, those loadings should be taken into consideration because

if it is miscalculate, absolute failure will occur in the structure. Figure below

illustrates and describing the types of loads that must be considered (Atiliana, 2009).

Loads

Static force Dynamic force

Live Dead loads Force due to continuous Impact (eg: ioads(movable) (fixed) settlements, etc. blast)

ckcupazicy Self-weight of Internal forces structure

EJlrironmentaj g' Wind forces L elements

Figure 2.2 Typical types of loading condition

9

2.4.1 Static force

The static forces consist of live load, dead load and forces due to settlement or

thermal effect. These types of load are typically being considered when designing

every structure.

Live loads are forces that act vertically downward onto the structure but it is

not fixed in character. The value can change anytime and movable. Occupancy and

environmental can be described as live loads. Occupancy loads include furniture,

stored material, human or any other similar material.

On the other hand, dead loads are non-movable loads and fixed in behavior.

There are several items that can be marks as dead loads and those are self-weight of

the structure, slab, bricks, mechanical equipment or any other building.

2.4.2 Wind loads

This type of loading is one of the most important characteristic especially for

multi-storey buildings because it can cause sway to the structure. As the wind move, it

will deflect or stop when something blocking its path. This phenomenon will cause

the kinematic energy transform into potential energy. This potential energy is called

force that induced horizontally onto the structure. Therefore, high-rise building will

be design properly to resist this force.

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2.5 Finite Element Analysis

Finite element analysis (FEA) is consists of computer model of a material or

design that is stressed and analyzed for specific results. It is used in new product design,

and existing product refinement. In other words, FEA is a numerical method to find out

an approximate solution for variables in a problem which is difficult to obtain

analytically. The calculation of potential design changes such as temperature, buckling

and deflection are usually complicated. A numerical method that is able to solve these

engineering problems id the finite element analysis. In case of structural models failure,

FEA may used to help determine the design modifications to meet the new condition.

The concept of the finite element analysis is solving a continuum by a discrete

model. It is done by dividing the problem into small several elements. Each element is in

simple geometry and this is easier to be analyzed than the actual problem or the real

structure. Each element is then applied with known physical laws. The equation which is

formed by each element or parameters then will combined to form a global equation. The

new equation can be used to solve the field variables such as displacement, buckling,

temperature and so on.

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The aeronautics, automotive, defense, and nuclear industries had started using the

finite element application since early 70's. However, this is limited to expensive

mainframe computer. Zienkiewicz and Cheung is the important person in developing the

finite element technology at that time. But later, Hinton and Crisfield carried out the

finite element into modeling and solution of nonlinear problems (Reddy, 1993).

With the development of the CAE technology, engineering drawing can be

produced. Besides, the analysis can be carried out and also the Finite element modeling

can be done. The finite element becomes more and more important today which can

simplify and solves various types of engineering problems.

2.5.2 How Does Finite Element Analysis Work

FEA uses a complex system of points called nodes which make a grid called

a mesh. This mesh is programmed to contain the material and structural properties

which define how the structure will react to certain loading conditions. Nodes are

assigned at a certain density throughout the material depending on the anticipated

stress levels of a particular area. Regions which will receive large amounts of stress

usually have a higher node density than those which experience little or no stress.

Points of interest may consist of: fracture point of previously tested material, fillets,

corners, complex detail, and high stress areas. The mesh acts like a spider web in that

from each node, there extends a mesh element to each of the adjacent nodes. This web

of vectors is what carries the material properties to the object, creating many elements

(Peter Widas, 1997).

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Figure 2.4 Mesh diagram on 2D model

The powerful design tool has significantly improved both the standard of

engineering designs and the methodology of the design process in many industrial

applications. The FES has substantially decreased the time to take products from concept

to the production line. It is primarily through improved initial increased accuracy,

enhanced design and better insight into critical design parameters, fewer hardware

prototypes, a faster and less expensive design cycle, increased productivity (Peter

Widas, 1997).

2.5.3 Meshing

The important requirement of the FEM is the need to split the solution domain

(model geometry) into simply shaped subdomains called 'finite elements'. This is a

discretization process commonly called meshing and element are called finite because of

their finite, rather than infinitesimally small size having infinite numbers of degree of

freedom. Thus the continuous model with an infinite number of degrees of freedom

(DOF) is approximately by a discretized FE model with a finite DOF. This allows the

reasonably simple polynomial functions to be used to approximate the field variables in

each element. Meshing the model geometry also discretized the original continuous

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APDL are can be known as tools to help designers perform parametric analyses in

which simulation software automatically solves for entire ranges of specified variables

and generates displays that enable users to readily spot trends and identify an optimal

design. Parametric analysis can guide the product development process to a design

configuration that can save the time compared if by individual analyses were manual

performed. The period of time to set up is required in APDL which a model can increase

significantly with the complexity of the geometry.

ANSYS Workbench platform is one of the most efficient ways of deal with

geometric parameters which enables parameters of the CAD model to be driven directly

from simulation.

2.6.1 Benefit of parametric design analysis

Parametric analysis is an excellent to get accurate information about the

influence of all parameters on the design objectives, such as system performance with

respect to stress, loading, deflection of structure and other variables. With this

information, the design team can make informed decisions throughout product

development. In addition, the design teams also can response quickly to any

modification due to external constraints for example, manufacturing (Thieffry, 2008).

CHAPTER 3

METHODOLOGY

3.1 Introduction

Nowadays, research undertaken the finite element analysis was proven to be a

successful tool in predicting the behavior of frame structures. From the previous

research, finite element analysis provides only approximate solution and the results

are not fully can be used in real design with reasonable confidence. This is because

there are various ways to perform the finite element process such as meshing of

structure which varied analysis results may be obtained. Therefore, the comparison

between manual calculations and finite element analysis is needed to determine their

efficiency and reability.