FE modeling of elastic buckling of stud walls

42 Pages·2008·3.13 MB·English

FE modeling of elastic buckling of stud walls

September 2008 version

O. Iuorio*, B.W. Schafer

*This report was prepared while O. Iuorio was a Visiting Scholar
with B.W. Schafer’s Thin-walled Structures Group at JHU.

Summary:

The following represents work in progress on the modeling of elastic buckling (and later
collapse) of CFS stud walls with dis-similar sheathing.
1

4.4.3 ELASTIC BUCKLING OF SHEATHED STUD WALL.
Aim of this analysis is to study the behavior of walls sheathed with oriented
strand board (OSB) and gypsum board (GWB) panels when the wall is
subjected to vertical loads. It is well recognized that the strength of stud
wall can be improved by using sheathing material and that the connections
are key-points for the strength transmission. Hence, a parametric analysis
has been developed to study the wall behavior varying the screw spacing
and the sheathing material (OSB and GWB). In Table1 the parametric
analysis planning is summarized and geometrical and mechanical
components properties are defined in Table2.

Parametric analysis planning
symbol (mm) (inches)
Wall height h 2400 96
Stud 362S162-68 0,0713
Stud spacing d 300 12
50 2
75 3
100 4
150 6
Screw spacing s
200 8
304.8 12
609.6 24
1219 48
Table1. Parametric Analysis Planning

thickness E E G υ=υ
x y x y

(inches) (ksi) (ksi) (ksi)
362S162-68 0.0713 29500 29500 11346.15 0.3
OSB// 0.35 638.2 754.2 203 0.3
2
GWB 0.5 384 384 108 0.3
Table2. Geometrical and Mechanical properties
The structure has been studied with Finite Strip Method (FSM) and the
Finite Element Method (FEM) and the results of CUFSM and Abaqus have
been compared.
In particular, in the finite element analysis, the components have been
modelled with isoparametric shell finite elements (S9R5) and a reference
stress equal to 1 has been considered placed at each node of the end stud
sections, whilst the panel has been considered totally unloaded.
In order to model the connections, three different conditions have been
analyzed:
1) connections with stiffness equal to zero
2) connections with infinite stiffness (rigid connections)
3) connections characterized by stiffness obtained by experimental
tests.
1) Connection with stiffness equal to 0 – (single stud)
In the first case, the wall can be identified as a system of two studs and two
panels without any connections. Hence, it corresponds to study a single
compressed stud. The buckling curve of the first model (96in length
member without panel) obtained with CUFSM is shown in Figure 1, whilst
Figure 2 shows the deformed shape corresponding to the first mode
obtained in Abaqus. The comparison between results of finite strip analysis
and finite element analysis show that the stud is subjected to global flexural
torsional buckling, as first mode, and the load factors obtained in CUFSM
and Abaqus are very closed (load factor = 11.125 CUFSM vs load factor =
11.325 with Abaqus).
Parametric analysis 3

Figure1. CUFSM buckling curve for the model without panel

Figure2. Global buckling of a single 362S162-68 stud ( model1) - FEM result
Moreover, the occurrence of the other buckling modes has been
investigated.
Table 3 compares the CFSM and Abaqus results for any buckling mode
and Figures from 3 to 5 show the deformed shape for each buckling mode.

Local Global Global
Dist buckl Dist buckl
Model buckling flex flex-tors
Wall CUFSM 60.33 73.63 150.5 11.13 11.61
sheathed
with GWB
Abaqus 59.46 76.35 166.26 11.33 11.80
panel
Table3. Comparison between CUFSM and Abaqus results for the first model
4

Figure3. Local buckling of a single 362S162-68 stud

Figure4. Global Flexural buckling of a single 362S162-68 stud

Figure5. Global Flexural torsional buckling of a single 362S162-68 stud
Parametric analysis 5
Figure6. Distortional buckling (1) of a single 362S162-68 stud

Figure7. Distortional buckling (2) of a single 362S162-68 stud
2) Connections with infinite stiffness
2.1 General constraints in all directions.
The analysis continued considering rigid connections (second case,
connections with springs with infinite stiffness). In this case, the
compression loads acting on the studs are transferred to the panels by the
connections that have been modeled with general constraints. In particular,
in a first case general constraints acting in all direction have been
considered and the buckling curves obtained in CUFSM are shown in
Figure 8 and 9.

Figure 8: Buckling curve of a wall sheathed with OSB panel – CUFSM result.
6

Figure9: Buckling curves for modes from 1 to 4.
The buckling curve corresponding to the first mode identifies the load
factor corresponding to the local buckling (LF = 62.20) and for a half-
wavelength equal to 96” it identifies a flexural-torsional buckling (LF =
80.36). On the other hand, that buckling curve does not present any
minimum for distortional buckling; the latter starts to appear at the third
mode (Figure9). Hence, the minimum for the distortional buckling
corresponding to the third mode has been considered and it has been
referred as dist. 2. Moreover, the half-wavelength has been fixed and the
corresponding point on the first mode curve has been considered (this value
has been considered as dist.1).

Figure10. Definition of distortional buckling 1(dist1).
Finally, the Global Flexural buckling has been defined considering an
half-wavelenght equal to wall height (96”) and the third mode. All the
results are summarized in Table4.
Parametric analysis 7

Screw Local Dist Dist Global Global

Model spacing buckling buckl1 buckl2 flex-tors flex
Wall
80.36 236.98
sheathed
118.87 236.98 mode1 mode3
with CUFSM contin 62.3
mode1 mode3 length length
OSB
96” 96”
panel
Table4. CUFSM results for the model with rigid connection acting in all directions
The FEM model has been developed in order to study the behavior
varying the screw spacing and the results have been synthesized in Table5
and Table6.

Model Screw spacing CUFSM (Load Abaqus (Load Buckling mode
factor) factor)
Without - 11.125 11.325 Global_ Flexural
connection
Wall sheathed continuous 62.38 64.18 Local _ Stud
with OSB
2” 63.577 Local _ Stud
panels
3” 62.441 Local _ Stud
4” 62.423 Local _ Stud
6” 62.342 Local _ Stud
8” 61.981 Global_ Panel
12” 29.037 Global_ Panel
24” 8.022 Global_ Panel
Wall sheathed continuous 63.07 64.486 Local _ Stud
with GWB
2” 63.627 Local _ Stud
panel
3” 62.115 Local _ Stud
4” 62.139 Local _ Stud
8
6” 62.02 Local _ Stud
8” 62.047 Local _ Stud
12” 57.564 Global_ Panel
24” 16.053 Global_ Panel
Table5. Comparison between CUFSM and Abaqus results at 1st mode varying the screw
spacing.
Screw Local Dist Dist Global

Model spacing buckling buckl1 buckl2 flex-tors
80.36
118.87 236.98 mode1
CUFSM contin 62.3
mode1 mode3 length
96”
1 64.18 64.23
Wall 2 63.58 119.03 234.22 64.17
sheathed
3 62.44 120.22 228.97 64.09
with
OSB 4 62.42 119.22 226.22 64.0
panel
Abaqus 6 62.34 123.26 222.35 63.79
8 62.37 117.05 221.56 63.54
12 62.50 119.76 224.66 63
24 62.6 112.22 197.54 62.87
48 62.98 53.74
Table6. Comparison between CUFSM and Abaqus results
Table5 shows that for screw spacing up to 6”, the local buckling of the stud
occurs as first mode (Figure11). Instead, for screw spacing between 8 and
48” the global buckling of the sheathing governs the behavior (Figure12)
and the number of sheathing waves depends on the number of connection
(8 waves for screw spacing equal to 12”, Figure12, and 4 waves for screw
spacing equal to 24” Figure13).
Parametric analysis 9

Figure11. Wall sheathed with OSB panels – first mode – Abaqus result

Figure12. Buckling behavior of wall sheathed with OSB panels and screw spacing equal
to 12” – First mode – Abaqus result

Figure13. Buckling behavior of wall sheathed with GWB panels and screw spacing equal
to 24” – First mode – Abaqus result