Proceedings of the 7th Interna�onal Conference on

Coupled Instabilities in Metal Structures Baltimore, Maryland, November 7-8, 2016

Investigation on Structural Response of New Zealand Light Steel

Frame and its Equivalent Truss Frame under Lateral Loading Rashideddin Cheraghi1, Hamidreza Mohammadzadeh2

Abstract A very popular type of light steel frames is New Zealand Light Steel Frame (NZ LSF) which is produced by CAD/CAM technology. All structural elements of this frame including studs, braces and bridges have the same channel cross section and connect together by self-drilling screws. All structural connections in NZ LSF are hinged. This issue leads the mind to the point that the frame may be equivalent to a truss frame. However, an important distinction exists between the NZ LSF with a truss frame and that is the eccentricity of braces in joints. In this research, a numerical analysis is conducted to compare the response of an exact NZ LSF vs. its equivalent truss frame (ETF) under lateral loading. The results demonstrate that generally maximum axial forces in studs and braces in the ETF model are evaluated less in comparison to the exact model but the maximum axial forces in bridges are evaluated almost the same in comparison to the exact model. Furthermore, the considerable shear forces in bridges are neglected in the ETF model. The simplified model is very close in its response to exact model. Nevertheless, the braces’ eccentricity imposes considerable shear forces and bending moments in bridges. 1. Introduction A very common type of light steel frames is New Zealand light steel frame (NZ LSF) and it has become widespread in IRAN, too. The popularity of this frame is for following reasons:

1- Computer software is used both in design and manufacturing of this frame. Therefore, a LSF building designed and modeled by the computer, go to the product line to precisely manufacture that building. This procedure is known as Computer-aided Design and Computer-aided Manufacturing, CAD/CAM. This procedure elevates the speed and accuracy of construction.

2- This frame does not need any extra connection’s pieces in comparison to light steel frame which are more popular in North America and all elements connect together with self-drilling screws. Although this issue highly accelerates the process of construction, it makes some limitations on the design of high-rise buildings with the NZ LSF.

3- Reaching a rigid frame is easy in this frame without any need to attach a few of claddings.

1PhD Candidate, Khajeh Nasir Toosi University of Technology, <rashidcheraghi@email.kntu.ac.ir>

1Technical Office Manager, Panah Saz Alvand Engineering Company

2 PhD Candidate, Islamic Azad University, South Tehran Branch, <mohammadzadeh@atpeco.ir>

2 CEO, Aria Tadbir Paj Engineering Company

2

We mentioned this frame as the New Zealand Light Steel Frame, because this frame for the building of low-rise residential structures is popular in New Zealand. Moreover, there is a standard for this type of light steel frame by that country (Nash Standard 2010). A New Zealand Light Steel Frame was demonstrated in the Fig. 1.

Figure 1: A New Zealand Light Steel Frame (V.P. Paton-Cole 2010)

The response of NZ LSF under gravity and wind loads was more under investigation by researchers, but the seismic response of this frame was not under attention as much. On the other hand, we saw that for easy modeling, many designers ignore the braces’ eccentricity in their numerical modeling. Therefore, in this research, an exact NZ LSF was selected and its response was compared with its equivalent truss frame (ETF) under the seismic load to demonstrate that what are the pros and cons of simplifying the exact model as the ETF model. 2. Case Studies 2.1 Material and Cross Sec�on The Material used in this research are men�oned in the table 1 and the cross section of elements including studs, bridges and braces is all the C-shape profile with following geometrical characteristics as drawn in Fig. 2. Table 1: Cold-formed Steel Material Characteristics

ST230 Fy Fu E (Mpa) (Mpa) (Gpa)

230 310 200 Figure 2: C-shape Cross Section

89

mm

41 mm

10

mm

t= 1mm

3

60.00

120.00

120.00

120.00

120.00

120.00

120.00

120.00

120.00

120.00

60.00

XY XZ

YZ

2.2 Modeling and Seismic Loading In this research, we tried to choose a practical frame which previously designed and constructed in Tehran, Iran. Therefore, a 1-story building with the dimensions of 4*6 m2 in plan and 3 meters in height is selected and one of its load bearing frame is picked for this study (Fig. 3 and Fig. 4). The floor system of this frame is truss joist with concrete slab; the dead load of the floor system is 300 kgf/m2 and the live load is assumed 200 kgf/m2. The seismic load of each load bearing frame is about 1200 kgf and because this load is transferring to the frame at intersection of the track and studs, it is assigned to the frame as the point loads and illustrated in Fig. 4. The space of studs is 600 millimeters and the space of bridges is 1000 millimeters apart.

Figure 3: Selected Plan and Frame

Figure 4: Seismic Loading of the Target Light Steel Frame (force unit: kgf)

6 m

4 m

4

2.3 Numerical Simula�on It is tried to model the connections of NZ LSF precisely. In Fig. 5, the details of brace to bridge, bridge to stud and stud to track connection are illustrated. It is worthwhile to mention that at the connection of bridge to stud, the bridge is unable to resist moment, but at the connection of track to stud, the track is able to resist moment and its behavior is like a multi-support continuous beam. This issue is spotlighted in the fig. 5. Be concerned with aforementioned points, the numerical model was developed in the Fig. 6. The numerical model of equivalent truss frame was built exactly like NZ LSF except for ignoring the braces’ eccentricity and it is shown in the Fig. 7.

Figure 5: The Connection of Bridge to Stud (Left) and The Connection of Stud to upper Track (Right)

Figure 6: Exact Numerical Model of NZ LSF

Bridge

Track

Bridge

Track

Brace

Stud

Bridge

5

Figure 7: Equivalent Truss Frame Model of NZ LSF

3. Results

The frames were analyzed in the finite element software, SAP2000 ver14 and the internal forces including axial and shear forces and the bending moment under the 1200 kgf seismic load were calculated in both Exact and the ETF model. All forces and moments mentioned in the following figures are in the units of kgf and kgf.m. As illustrated in Fig. 8 and Fig. 9, the maximum tension and compression forces in studs in the exact model are 1183 kgf and 945 kgf, respec�vely and in the ETF model are 1074 and 835 kgf, respectively. Furthermore, the maximum tension and compression forces in Braces in the exact model are 435 kgf and 610 kgf, respec�vely and in the ETF model are 378 and 677 kgf, respectively. And at the end, the maximum tension and compression forces in Bridges in the exact model are 245 kgf and 187 kgf, respectively and in the ETF model are 235 and 210 kgf, respectively. In both frames, the distribution of axial forces is similar and close together, but generally, in the ETF model axial forces in studs and braces are evaluated less in comparison to the exact model and axial forces in bridges in both models are almost the same. As depicted in Fig. 10 and Fig. 11, the considerable shear forces and bending moments are imposed to the bridges under the seismic load. And this is the main distinction between the exact model and the ETF model. As shown in the Fig. 12 and Fig. 13, In the ETF model, the shear forces and bending moments are only observed in the upper track and their values are not considerable. Moreover, in the exact model, the maximum shear force and the maximum bending moment in bridges are 485 kgf and 22 kgf.m and in the ETF model, are 30 kgf and 10 kgf.m .

6

Figure 8: Axial Forces of the Exact Model

Figure 9: Axial Forces of the ETF Model

162

148

7

Figure 10: Shear Force Diagram of the exact model

Figure 11: Bending Moment Diagram of the exact model

0.36

9.599E-03

-0.37

0.39

-0.01

-0.38

0.39

-0.02

-0.37

0.43

-0.07

-0.36

0.43

-0.09

-0.34

0.47

-0.14

-0.33

0.47

-0.17

-0.30

0.45

-0.19

-0.26

0.40

-0.14

-0.26

0.39

-0.12

-0.28

-0.36

0.09

0.27

30

0.7

2

-59

.71

29

6.3

3

28

1.2

1

-58

.33

30

2.1

0

-27

.66

31

1.8

8

15

.03

-48

.26

31

4.7

3

18

.26

-46

.13

31

2.8

6

-5.6

0 48

4.8

3

-61

.01

43

3.0

4

-29

.45

11

.87

29

6.6

8

-26

.46

30

5.0

3

-63

.13

32

6.3

1

30

2.0

6

-60

.93

30

7.2

3

30

1.3

2

-62

.46

32

3.2

3

29

8.7

6

-60

.23

30

3.5

4

31

3.4

2

-70

.61

39

2.6

5

37

9.1

7

-84

.09 46

1.7

54

47

.91

-84

.32

39

5.3

3

40

1.1

4

-78

.51

38

3.9

8

30

1.2

8

-60

.36

30

2.3

0

30

5.5

1

-57

.15

26

5.9

9

-32

2.1

1

33

.92

8.3

1

-36

7.5

0

21

.94

9.9

4

-35

9.5

0

26

.19

-10

.41

15

.87

-36

8.1

6

51

.56 -4

80

.67

9.1

1

34

.33

-32

7.3

1

X

Z

8

Figure 12: Shear Force Diagram of the ETF model

Figure 13: Bending Moment Diagram of the ETF model

9

4. Conclusion The response of New Zealand light steel frame and its equivalent truss frame under lateral loading were investigated. All internal forces were calculated in both frames and compared together and following conclusions were obtained:

- In the ETF model, the axial forces in studs and braces were computed less in comparison to the exact model and this reduc�on is about 10%-12% for maximum forces in studs and is about 13% for the maximum tension force in braces. However, there are some exceptions that axial forces were evaluated more in comparison to the exact model.

- As the 5 cm braces’ eccentrici�es were eliminated in the ETF model, the considerable shear forces and bending moments existing in bridges in the exact model, were neglected. To other words, the maximum shear force in the exact model is 485 kgf which is a noticeable force in comparison to the maximum 30 kgf shear force in the ETF model which bridges should resist. Therefore, the ETF model will highly underestimate shear forces and bending moments in the bridges under the seismic load and moreover the distribution of these internal forces through the frame are completely different from the exact model.

References NASH Standard (2010), “Residential and Low-rise Steel Framing, Part 1: Design Criteria” available in

http://www.nashnz.org.nz/ V.P. Paton-Cole, E.F. Gad, C. Clifton, C. Davies, S. Hicks, N. Lam (2009). “Shaking Table Test Of A Full Scale Brick

Veneer Steel-framed House” a technical report available in http://www.nashnz.org.nz/ V.P. Paton-Cole, E.F. Gad, C. Clifton, D.J. Heath, C. Davies, S. Hicks, N. Lam (2009). “Seismic Performance of a Brick

Veneer Steel-Framed House” a technical report available in http://www.aees.org.au/ SAP2000 So�ware ver. 14 (2010) “Structural Analysis Program.” Computers and Structures, Inc.