. Appendix A: AISI standard updates
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Recent code development and design standards for cold-formed steel structures
Designation
Title
Published editions
AISI S212
North American Cold-Formed Steel
FramingdHeader Design
Reafﬁrmed in 2012 and
integrated into AISI
S240-15
AISI S213
North American Cold-Formed Steel
FramingdLateral Design
Reafﬁrmed in 2012 and
integrated into AISI
S240-15
AISI S214
North American Cold-Formed Steel
FramingdTruss Design
Updated in 2012 and
integrated into AISI
S240-15
AISI S220
North American Cold-Formed Steel
FramingdNonstructural Members
Published in 2011 and
updated in 2015
AISI S230
North American Cold-Formed Steel
FramingdPrescriptive Method
Reafﬁrmed in 2012 and
updated in 2015
AISI S240
North American Standard for Cold-Formed
Steel Structural Framing
Published in 2015
AISI S400
North American Standard for Seismic Design
of Cold-Formed Steel Structural Systems
Published in 2015
51
AISI design procedures and
practical examples for
cold-formed steel structures
3
W. Zhang 1 , C. Yu 2
1
Tongji University, Shanghai, China; 2University of North Texas, Denton, TX, United States
3.1
Introduction
In 1949 the American Iron and Steel Institute (AISI) published the ﬁrst edition of its
“Light Gage Steel Design Manual,” which was intended to supplement the design
speciﬁcation and facilitate its application to ordinary design problems (Kaehler and
Seaburg, 1996). Since then AISI has been developing and updating the manual, which
was later renamed the “Cold-Formed Steel Design Manual” (AISI, 1968). Today, the
manual provides the latest design information and examples for conformance with
S100 North American Speciﬁcation for the Design of Cold-Formed Steel Structural
Members (AISI, 2012). The 2013 edition of the “Cold-Formed Steel Design Manual”
(AISI, 2013) consisted of two volumes with eight parts.
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Part I: Dimensions and Properties. Contains information regarding the availability and properties of steels referenced in the speciﬁcation; tables of section properties; and formulas and
examples of calculations of section properties.
Part II: Beam Design. Contains tables and charts to aid in beam design, and beam design
example problems.
Part III: Column Design. Contains tables to aid in column design, and column design
example problems.
Part IV: Connections. Contains tables to aid in connection design, and connection example
problems.
Part V: Supplementary Information. Contains a table of speciﬁcation cross-references to the
examples provided in the design manual; design procedures of a speciﬁcation nature which
are not included in the speciﬁcation itself, either because they are infrequently used or are
regarded as too complex for routine design; and other information intended to assist users
of cold-formed steel (CFS).
Part VI: Test Procedures. Contains a bibliography of other pertinent test methods, and test
calibration example problems.
Part VII: North American Speciﬁcation. Contains the 2012 edition of the North American
Speciﬁcation for the Design of Cold-Formed Steel Structural Members.
Part VIII: Commentary on the North American Speciﬁcation. Contains the 2012 edition of
the Commentary on the North American Speciﬁcation for the Design of Cold-Formed Steel
Structural Members.
Recent Trends in Cold-Formed Steel Construction. http://dx.doi.org/10.1016/B978-0-08-100160-8.00003-7
Copyright © 2016 Elsevier Ltd. All rights reserved.
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Recent Trends in Cold-Formed Steel Construction
Up-to-date design examples were added in the AISI Design Manual (2013) to illustrate new design provisions in AISI S100 (2012).
Round and rectangular tubular section member design in Parts II and III.
C-section members subjected to combined bending and torsional loading in Part II.
Distortional buckling of C-section members in Parts II and III.
Sigma-shaped ﬂexural and compression member design by the direct strength method in
Parts II and III.
5. Web crippling strength of beam webs with bearing stiffeners in Part II.
6. Frame design with consideration of second-order analysis in Part III.
1.
2.
3.
4.
AISI’s “Cold-Formed Steel Design Manual” is focused on member or connection
design. For framing or system design, the Cold-Formed Steel Engineers Institute
(CFSEI) has published a series of technical notes to address speciﬁc topics related
to CFS framing for residential and commercial construction. Currently the CFSEI technical notes series consists of 10 categories.
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•
•
Durability and corrosion protection.
Fasteners and connection hardware.
Component assemblies (trusses and wall panels).
General topics.
Floor and joist systems.
Wall systems.
Roof and ceiling systems.
Thermal, ﬁre, and acoustic.
Lateral systems.
Other technical documents.
In addition to the technical notes, the CFSEI also periodically releases research reports, design guides, and technical papers to promote the use of CFS in the construction industry.
In recent years CFS framing has become an attractive and competitive structural
system for midrise residential and ofﬁce buildings due to its light weight, fast installation, low waste and noise during construction, and noncombustibility. Among the
three sheathing materials for shear walls approved in AISI S213 North American
Standard for Cold-Formed Steel FramingdLateral Design (AISI, 2012) and the International Building Code (IBC, 2015), ﬂat steel is the only noncombustible material for Types I and II construction and other building types such as medical care
facilities, etc., which require noncombustible materials for their load-bearing structures. Fig. 3.1 shows the failure mechanism of CFS shear walls sheathed by ﬂat steel
sheet: the tension ﬁeld action dominated the load-bearing behavior of these shear
walls.
The existing design methods in AISI S213 and the IBC for CFS-sheathed shear
walls are solely based on experimental data with limited options in sheathing and
framing conﬁgurations. To improve the situation, an analytical model and closedform design equations for steel-sheet-sheathed CFS shear walls were developed by
Yanagi and Yu (2014). Named the effective strip method, it offers an alternative
AISI design procedures and practical examples for cold-formed steel structures
AR = 4
AR = 2
55
AR = 1.33
Figure 3.1 Failure mechanism of steel-sheet-sheathed shear walls.
Yanagi, N., Yu, C., 2013. Effective strip model for cold-formed steel shear wall using steel sheet
sheathing. In: Proceedings of the 21st International Specialty Conference on Cold-Formed Steel
Structures, St. Louis, MO, April 2013.
approach for engineers to predict the normal strength of CFS shear walls. The method
was approved by the AISI Committee on Steel Framing and will be included in AISI
S240 and AISI S400. A design example for the effective strip method is provided in
this chapter.
In any structural system, the connection method and its details are always the critical elements to achieve the desired structural behavior and ensure the minimum safety
level deﬁned by building codes and design speciﬁcations. The connections in CFS
structures are no different to those in any other structural system, and are even more
critical as fasteners are most commonly used and sometimes have dual responsibilities:
load bearing and energy dissipation. Among the various connections and connectors, a
thin-walled CFS clip angle is a common method used in CFS framing. Fig. 3.2 shows
applications of CFS clip angles to connect the stud to the track in CFS framing, and
attach the curtain wall framing member to the main structure. As illustrated in
Fig. 3.2, a CFS clip angle can be subjected to shear, axial, bending, and a combination
of those three. However, CFS clip angle design has not been developed in the major
design speciﬁcationsdAISI S100, the Australian/New Zealand Standard AS/NZS
4600 Cold-Formed Steel Structures (AS/NZS 4600, 2005), and Eurocode 3 Design
of Steel Structures (2006). To address the knowledge gap in the behavior and strength
of CFS clip angles, a research project was conducted by Yu et al. (2015) at the
University of North Texas to develop design methods for clip angles subjected to
shear, compression, and tension forces. This chapter gives examples of shear and
compression designs of CFS clip angles.
56
Recent Trends in Cold-Formed Steel Construction
(a)
(b)
Figure 3.2 Examples of applications of CFS clip angles: (a) CFS framing; (b) curtain wall
framing.
3.2
Sheet steel shear wall designdeffective
strip method
3.2.1
The effective strip method
The effective strip method for determining the nominal shear strength (resistance) for
Type I shear walls with steel sheet sheathing is based on research by Yanagi and Yu
(2014). The method assumes a sheathing strip carries the lateral load via a tension ﬁeld
action, as illustrated in Fig. 3.3. The shear strength of the shear wall is controlled by the
tensile strength of the effective sheathing strip, which is determined as the lesser of
the fasteners’ tensile strength and the yield strength of the effective sheathing strip.
The statistical analysis in Yanagi and Yu (2014) yielded a load and resistance factor
design (LFRD) resistance factor of 0.79 for the effective strip method. To be consistent
with the resistance factors (0.60 for LRFD) speciﬁed in AISI S213 (2012) Section
E2.3.2, the original design equation in Yanagi and Yu (2014) was adjusted accordingly.
The nominal shear strength (resistance) per unit length for a Type I shear wall with
steel sheet sheathing can be determined in accordance with the effective strip method as:
Vn ¼ minimumð1:33Pn cos a; 1:33We tFy cos aị
[3.1]
where
Pn ẳ nominal shear strength (resistance) of screw connections within the effective strip
width, We, on the steel sheet sheathing
a ẳ Arctanh=wị
h ẳ shear wall height
w ẳ shear wall length
t ¼ design thickness of steel sheet sheathing
Fy ¼ yield stress of steel sheet sheathing
[3.2]
AISI design procedures and practical examples for cold-formed steel structures
57
We
Va
α
T
1W
e
2
h
1W
e
2
α
W
Figure 3.3 Effective strip model for steel sheet sheathing.
Yanagi, N., Yu, C., 2013. Effective strip model for cold-formed steel shear wall using steel sheet
sheathing. In: Proceedings of the 21st International Specialty Conference on Cold-Formed Steel
Structures, St. Louis, MO, April 2013.
We ¼ Wmax ;
¼ rWmax ;
when l
0:0819
when l > 0:0819
[3.3]
[3.4]
where
Wmax ¼ w=sin a
r¼
1 À 0:55ðl À 0:08ị0:12
l0:12
[3.5]
[3.6]
58
Recent Trends in Cold-Formed Steel Construction
l ẳ 1:736
a1 a2
b1 b2 b23 a
[3.7]
where
a1 ẳ Fush =310:3
ẳ Fush =45
for Fush in ksiị
a2 ¼ Fuf =310:3
¼ Fuf =45
b1 ¼ tsh =0:457
¼ tsh =0:018
b2 ¼ tf =0:457
¼ tf =0:018
ðfor Fush in MPaÞ
ðfor Fuf in MPaÞ
ðfor Fuf in ksiÞ
ðfor tsh in mmÞ
ðfor tsh in inchesÞ
ðfor tf in mmị
for tf in inchesị
b3 ẳ s=152:4 for s in mmị
ẳ s=6 for s in inchesị
Fush ẳ tensile strength of steel sheet sheathing
Fuf ¼ minimum tensile strength of framing materials
tsh ¼ design thickness of steel sheet sheathing
tf ¼ minimum design thicknesses of framing members
s ¼ screw spacing on the panel edges
a ¼ wall aspect ratio (h:w).
The effective strip method is valid within the following range of parameters.
1.
2.
3.
4.
5.
6.
Designation thickness of stud, track, and stud blocking: 0.838e1.37 mm (0.033e0.054 in.).
Designation thickness of steel sheet sheathing: 0.457e0.838 mm (0.018 e0.033 in.).
Screw spacing at panel edges: 50.8e152 mm (2e6 in.).
Height to length aspect ratio (h:w): 1:1e4:1.
Sheathing screw shall be minimum No. 8.
Yield stress of steel sheet sheathing shall not be greater than 345 MPa (50 ksi).
3.2.2
Design example
In this section, a design example of 1220 Â 2440 mm (4 ft Â 8 ft) CFS shear walls using 1.092 mm (43 mil) framing and 0.838 mm (33 mil) sheet steel sheathing is provided. The nominal yield stress is 230 MPa (33 ksi) for the framing and 345 MPa
AISI design procedures and practical examples for cold-formed steel structures
59
(50 ksi) for the sheathing. Sheathing-to-framing fasteners at the perimeter are No. 8
self-drilling tapping screws spaced at 76 mm (3 in.).
Step 1: Estimating the effective strip width.
Fush ¼ 448:2 MPa
Fuf ¼ 310:3 MPa
tsh ¼ 0:879 mm
tf ¼ 1:146 mm
a ¼ ð2440 mmÞ=ð1220 mmÞ ¼ 2:0
a ¼ tanÀ1 a ¼ tan1 2:0ị ẳ 63:43 degrees
Maximum effective width of the steel sheet sheathing
Wmax ẳ
W
W
1220 mm
ẳ
ẳ
ẳ 1363 mm
sin a sin63:43 degreeị sin63:43 degreeị
a1 ẳ Fush =310:3 MPaị ẳ 448:2 MPaị=310:3 MPaị ẳ 1:444
a2 ẳ Fuf =310:3 MPaị ẳ 310:3 MPaị=310:3 MPaị ẳ 1:0
b1 ẳ tsh =0:457 mmị ẳ 0:879ị=0:457 mmị ẳ 1:923
b2 ¼ tf =ð0:457 mmÞ ¼ ð1:146Þ=ð0:457 mmÞ ¼ 2:508
b3 ¼ s=152:4 mmị ẳ 76 mmị=152:4 mmị ẳ 0:499
l ẳ 1:736
a1 a2
1:444 Â 1:0
¼ 0:601
¼ 1:736 Â
2
b1 b2 b3 a
1:923 Â 2:508 0:499ị2 2:0
l ẳ 0:601 > 0:0819
Effective strip width of the steel sheet sheathing
1 À 0:55ðl À 0:08Þ0:12
Wmax
l0:12
1 0:55 0:601 0:08ị0:12
ẳ
1363 mmị ẳ 712:0 mm
0:601ị0:12
We ẳ rWmax ẳ
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Recent Trends in Cold-Formed Steel Construction
Step 2: Determining the nominal shear capacity of individual connections.
Connection shear limited by tilting and bearing
t1 ¼ 0:879 mm
t2 ¼ 1:146 mm
d ¼ 4:166 mm
Fu1 ¼ 448:2 MPa
Fu2 ¼ 310:3 MPa
t2 =t1 ¼ ð1:146 mmÞ=ð0:879 mmÞ ¼ 1:304
1:0 < t2 =t1 < 2:5
For t2/t1
Pns
1.0,
8
n
o1=2
3 1=2
>
>
Fu2 ẳ 4:2 1:146 mmị3 4:166 mmị
310:3 MPaị ẳ 3:263 kN
> 4:2 t2 d
<
ẳ min
2:7t1 dFu1 ẳ 2:7 0:879 mmị 4:166 mmị 448:2 MPaị ẳ 4:431 kN
>
>
>
:
2:7t2 dFu2 ẳ 2:7 1:146 mmị 4:166 mmị 310:3 MPaị ẳ 4:000 kN
For t2/t1 ! 2.5,
(
Pns ¼ min
2:7t1 dFu1 ¼ 2:7 Â 0:879 mmị 4:166 mmị 448:2 MPaị ẳ 4:431 kN
2:7t2 dFu2 ẳ 2:7 1:146 mmị 4:166 mmị 310:3 MPaị ẳ 4:000 kN
By linear interpolation of the smallest of the above two cases
Pns ¼ 3:412 kN
Connection shear limited by end distance
t ¼ 0:879 mm
a ¼ tanÀ1 a ¼ tanÀ1 ð2Þ ¼ 63:4 degrees
wf ¼ flange width of stud ¼ 41:2 mm
wf
41:2 mm
e ¼ 2 cos
a ¼ 2Âðcos 63:4 degreesị ẳ 46:0 mm (assume the screws are installed at the
center of the ﬂange of the outer stud)
AISI design procedures and practical examples for cold-formed steel structures
61
Fu ¼ 448:2 MPa
Pns ẳ teFu ẳ 0:879 mmị 46:0 mmị 448:2 MPaị ẳ 18:114 kN
Connection shear limited by shear in screw.
The nominal shear strength of the screw is provided by the manufacturer. It is
assumed that a No. 8e18 Phillips truss head screw by HILTI is used. The screw shear
strength can be found in a HILTI self-drilling screws report (ESR-2196, 2013).
Pss ¼ 5:204 kN
Pns;s ¼ minf3:412 kN; 18:114 kN; 5:204 kNg ¼ 3:412 kN
Similarly,
Pns;t ¼ 3:412 kN ðsheathing to track connectionÞ
Pns;ts ¼ 3:412 kN ðsheathing to track and stud connectionÞ
Step 3: Determining the nominal shear strength of the shear wall.
tsh ¼ 0:879 mm
Fy ẳ 344:7 MPa
&
Vn ẳ minimum
'
We
We
Pns;t ỵ
Pns;s ỵ Pns;ts cos a; We tFy cos a
2s sin a
2s cos a
Limited by connection capacity
We
We
Pns;t ỵ
Pns;s ỵ Pns;ts cos a
2s sin a
2s cos a
ẳ
712:0 mm
3:412 kNị
2 76 mmị sin63:43 degreeị
ỵ
712:0 mm
3:412 kNị ỵ 3:412 kN
2 76 mmị cos63:43 degreesị
cos63:43 degreeị
ẳ 25:50 kN
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Recent Trends in Cold-Formed Steel Construction
Limited by sheathing yield capacity
À
Á
We tsh Fy cos a ¼ ð712:0 mmÞ Â ð0:879 mmÞ Â 0:3447 kN mm2
Â cos63:43 degreesị
ẳ 96:49 kN
Vn ẳ minimumf25:50 kN; 96:49 kNg ẳ 25:50 kN
Nominal shear strength of the shear wall
Vn ¼ 25:50 kNð20:90 kN=mÞ
3.3
Shear design of load-bearing clip angle connectors
3.3.1
Shear strength without consideration of clip
angle deformation
The nominal shear strength (resistance), Vn, of the cantilevered leg of a clip angle is
calculated as follows:
Vn ¼ 0:17lÀ0:8 Fy Bt
0:35Fy Bt
[3.8]
where
rﬃﬃﬃﬃﬃﬃ
Fy
l¼
Fcr
[3.9]
2
kp2 E
t
Fcr ¼
2
12ð1 À m Þ B
[3.10]
À2:202
L
k ¼ 2:569
B
[3.11]
B ¼ width of cantilevered leg measured parallel to the applied shear force
L ¼ ﬂat length of cantilevered leg measured from the center of the ﬁrst line of fasteners to the
bend line.
These equations are valid within the following range of parameters and boundary
conditions.
Clip angle design thickness: 0.838e2.464 mm (0.033e0.097 in.).
Clip angle design yield strength: 228e345 MPa (33e50 ksi.).
L/B ratio: 0.18e1.40.
The fastener pattern allows full engagement of the cantilevered leg in bearing the shear load.