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CURVED HSS MEMBERSBO DOWSWELL, P.E., PH.D.
SDS CONSULTING
1 PDH
HSS HOLLOW STRUCTURAL SECTIONS
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§ This webinar accredits the registered attendee one PDH for the hour. Please indicate your request for a completion certificate during the poll at the end. You must complete the poll to receive a certificate.
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WELCOME
HSS HOLLOW STRUCTURAL SECTIONS
§ For those registered in FL, MD, NC, and/or NY, you are required to complete a quiz. The link will be emailed to you at the conclusion of the webinar and must be completed within 24 hours of the completion of the webinar.
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WELCOME
HSS HOLLOW STRUCTURAL SECTIONS
Local Distortion of Curved Members
HSS HOLLOW STRUCTURAL SECTIONS
LOCAL DISTORTION OF CURVED MEMBERS
§ Vertically-Curved Members
5Photograph courtesy of the AISC Bender/Roller Committee
HSS HOLLOW STRUCTURAL SECTIONS
LOCAL DISTORTION OF CURVED MEMBERS
§ Horizontally-Curved Members
6Photograph courtesy of the AISC Bender/Roller Committee
HSS HOLLOW STRUCTURAL SECTIONS
OUTLINE
• Curve geometries• Bending processes• Initial distortion• Service-load distortion• Example
HSS HOLLOW STRUCTURAL SECTIONS
Curve Geometries
HSS HOLLOW STRUCTURAL SECTIONS
CURVE GEOMETRIES
Standard Bends• Single-radius curvature• Curvature about a principal
or geometric axis
9
Photograph courtesy of the AISC Bender/Roller Committee
HSS HOLLOW STRUCTURAL SECTIONS
CURVE GEOMETRIES
Off-Axis Bends§ Bending about a non-principal or non-geometric axis
10Photographs courtesy of the AISC Bender/Roller Committee
HSS HOLLOW STRUCTURAL SECTIONS
CURVE GEOMETRIES
Compound and Reverse-Compound Bends§ Multiple arcs in the same plane
11Photographs courtesy of the AISC Bender/Roller Committee
HSS HOLLOW STRUCTURAL SECTIONS
CURVE GEOMETRIES
Multi-Axis bends§ Curvature about more
than one axis
12
Photograph courtesy of the AISC Bender/Roller Committee
HSS HOLLOW STRUCTURAL SECTIONS
CURVE GEOMETRIES
Variable-Radius Bends§ Any non-circular bend (parabolic, elliptical, etc.)
13Photographs courtesy of the AISC Bender/Roller Committee
HSS HOLLOW STRUCTURAL SECTIONS
CURVE GEOMETRIES
Spiral Bends§ Helical curve
14Photographs courtesy of the AISC Bender/Roller Committee
HSS HOLLOW STRUCTURAL SECTIONS
Bending Process
HSS HOLLOW STRUCTURAL SECTIONS
BENDING PROCESSES
• Pyramid roll bending• Incremental step bending• Induction bending• Other processes
16
Photograph courtesy of the AISC Bender/Roller Committee
HSS HOLLOW STRUCTURAL SECTIONS
BENDING PROCESSES
Pyramid Roll Bending • Member is repeatedly passed through a set of rolls
17
HSS HOLLOW STRUCTURAL SECTIONS
BENDING PROCESSES
• Contoured rolls can be used to provide support during the bending operation
18Photograph courtesy of the AISC Bender/Roller Committee
HSS HOLLOW STRUCTURAL SECTIONS
BENDING PROCESSES
Incremental Step Bending • Forces applied at several
discrete locations along the member length
19
Photograph courtesy of the AISC Bender/Roller Committee
HSS HOLLOW STRUCTURAL SECTIONS
BENDING PROCESSES
Induction Bending• Electric induction coil heats
a narrow band around the member circumference
20
Photograph courtesy of the AISC Bender/Roller Committee
HSS HOLLOW STRUCTURAL SECTIONS
BENDING PROCESSES
• After heating, the member is curved by force
21
Photographs courtesy of the AISC Bender/Roller Committee
HSS HOLLOW STRUCTURAL SECTIONS
BENDING PROCESSES
§ Summary• Each bending method has advantages and
disadvantages• The capabilities of each bender/roller vary significantly
22
HSS HOLLOW STRUCTURAL SECTIONS
Initial Distortion
HSS HOLLOW STRUCTURAL SECTIONS
INITIAL DISTORTION
§ What is Initial Distortion?• A deviation from the original cross-sectional shape• Occurs in every curved member to some degree• A single half-wave or a series of wrinkles along the entire
bend length• AKA waving or wrinkling
HSS HOLLOW STRUCTURAL SECTIONS
INITIAL DISTORTION
• Occurs in both open and closed shapes
HSS HOLLOW STRUCTURAL SECTIONS
CAUSES OF INITIAL DISTORTION
Causes of Initial Distortion
HSS HOLLOW STRUCTURAL SECTIONS
• Contact forces from the bending machine
Photograph courtesy of the AISC Bender/Roller Committee
INITIAL DISTORTION: CAUSES
HSS HOLLOW STRUCTURAL SECTIONS
• Flexural compression− Flange local buckling− Web local buckling
• Shear− Web shear buckling
INITIAL DISTORTION: CAUSES
HSS HOLLOW STRUCTURAL SECTIONS
• Radial forces
INITIAL DISTORTION: CAUSES
HSS HOLLOW STRUCTURAL SECTIONS
TYPES OF INITIAL DISTORTION
Types of Initial Distortion
HSS HOLLOW STRUCTURAL SECTIONS
I-Shaped Members• Flange bending
• Web buckling
INITIAL DISTORTION: TYPES
HSS HOLLOW STRUCTURAL SECTIONS
Pyramid roll bending with no mandrel(REF: CIDECT Report 11C-88/14E)
Square and Rectangular HSS• Concave compression flange• Outward bowing of the web
INITIAL DISTORTION: TYPES
HSS HOLLOW STRUCTURAL SECTIONS
-= 1ρwb BB
=ρ feD
INITIAL DISTORTION: TYPES
HSS HOLLOW STRUCTURAL SECTIONS
Round HSS§ Ovalization
Dn = nominal outside diameter
-=ρ max min
n
D DD
INITIAL DISTORTION: TYPES
HSS HOLLOW STRUCTURAL SECTIONS
Distortion Tolerances
HSS HOLLOW STRUCTURAL SECTIONS
§ Mill Tolerances• Cross-sectional tolerances specified in ASTM A6, A53 and
A500 are mill tolerances• Mill geometric imperfections are amplified during the
bending process
INITIAL DISTORTION: TOLERANCES
HSS HOLLOW STRUCTURAL SECTIONS
§ Contract Documents• The 2016 AISC Code of Standard Practice (COSP) has no
cross-sectional distortion tolerance for curved members• Tolerances that are not addressed in the COSP should be
mutually agreed upon by the contractor and the owner• Preferably, distortion tolerances should be specified in the
contract documents
INITIAL DISTORTION: TOLERANCES
HSS HOLLOW STRUCTURAL SECTIONS
§ Tolerance Selection• Should be based on:
− The potential effect on structural performance− Any aesthetic requirements for AESS members
• Should be established with input from the bender/roller• Reasonable cross-sectional distortions can be tolerated
without a reduction in strength• AESS requirements are usually more stringent than
strength requirements
INITIAL DISTORTION: TOLERANCES
HSS HOLLOW STRUCTURAL SECTIONS
Distortion Tolerance: Rectangular HSS§ Post-bending wall flatness tolerances (rw and rf) between
1% and 2% are common
INITIAL DISTORTION: TOLERANCES
HSS HOLLOW STRUCTURAL SECTIONS
Distortion Tolerance: Round HSS§ A post-bending ovality tolerance (r) of 5% is common
INITIAL DISTORTION: TOLERANCES
HSS HOLLOW STRUCTURAL SECTIONS
§ For AESS, a 5% tolerance will likely result in imperceptible ovalization distortions
r = 5% r = 8% r = 12%
INITIAL DISTORTION: TOLERANCES
HSS HOLLOW STRUCTURAL SECTIONS
• Practical considerations may dictate tolerances• Example: when two segments are connected with
circumferential butt welds, proper wall alignment is essential
INITIAL DISTORTION: TOLERANCES
HSS HOLLOW STRUCTURAL SECTIONS
MINIMUM RADIUS
Minimum Radius
HSS HOLLOW STRUCTURAL SECTIONS
What is the Minimum Cold-Bending Radius?
Rule of Thumb?
44
INITIAL DISTORTION: MINIMUM RADIUS
HSS HOLLOW STRUCTURAL SECTIONS
Rigid Guidelines Are Not Available§ Minimum radius is dependent on:• Bending axis• Shape of the cross-section• Slenderness of the cross-sectional elements• Bending method and equipment• Level of acceptable cross-sectional distortion• Level of acceptable cold-working of the material
45
INITIAL DISTORTION: MINIMUM RADIUS
HSS HOLLOW STRUCTURAL SECTIONS
§ Bending requirements should be discussed with the bender-roller who will provide the service
46
INITIAL DISTORTION: MINIMUM RADIUS
HSS HOLLOW STRUCTURAL SECTIONS
REDUCTION OF INITIAL DISTORTION
Reduction of Initial Distortion
HSS HOLLOW STRUCTURAL SECTIONS
Local buckling resistance » distortion resistance
• Lower l = less distortion• l is usually dictated by
performance during the bending operation—not service-load design
b
t hl = b/t, h/tb, h = element widtht = element thickness
INITIAL DISTORTION: REDUCTION
HSS HOLLOW STRUCTURAL SECTIONS
§ Stiffened elements perform better than unstiffened elements
INITIAL DISTORTION: REDUCTION
HSS HOLLOW STRUCTURAL SECTIONS
§ Round elements perform better than rectangular elements
INITIAL DISTORTION: REDUCTION
HSS HOLLOW STRUCTURAL SECTIONS
STRUCTURAL EFFECTS
Structural Effects
HSS HOLLOW STRUCTURAL SECTIONS
§ Local Buckling• Excessive initial distortion can result in a reduction of the
local buckling strength• AISC Specification Tables B4.1a and B4.1b
INITIAL DISTORTION: STRUCTURAL EFFECTS
HSS HOLLOW STRUCTURAL SECTIONS
• For flat elements, the AISC Specification local buckling provisions are valid for out-of-flatness ratios up to (do/t)max = 0.264.
t = element thicknessdo = initial out-of-flatness
INITIAL DISTORTION: STRUCTURAL EFFECTS
HSS HOLLOW STRUCTURAL SECTIONS
• Strength ratio, Cd, as a function of do/t
INITIAL DISTORTION: STRUCTURAL EFFECTS
HSS HOLLOW STRUCTURAL SECTIONS
• When do/t > 0.264, the local buckling strength is multiplied by a reduction factor, Cd
• The effective width-to-thickness ratio is
d
ll =e C
dæ öç ÷è ø
d d 2
0.95 +0.30=1.23 - o oCt t
INITIAL DISTORTION: STRUCTURAL EFFECTS
HSS HOLLOW STRUCTURAL SECTIONS
• Replace l with le in the Specification equations• In many cases, the member strength will not be reduced
INITIAL DISTORTION: STRUCTURAL EFFECTS
HSS HOLLOW STRUCTURAL SECTIONS
§ Round HSS§ Ovalization causes an increase in sectional properties
about one axis and a decrease about the perpendicular axis
INITIAL DISTORTION: STRUCTURAL EFFECTS
HSS HOLLOW STRUCTURAL SECTIONS
Section property ratios for various ovalization parameters.Section
PropertyaEquation
r4% 5% 8% 10% 12%
Ix/I 1 + 0.760r 1.03 1.04 1.06 1.08 1.09Iy/I 1 - 0.741r 0.970 0.963 0.940 0.926 0.911Sx/S 1 + 0.242r 1.01 1.01 1.02 1.02 1.03Sy/S 1 - 0.261r 0.990 0.987 0.980 0.974 0.969Zx/Z 1 + 0.369r 1.01 1.02 1.03 1.03 1.04Zy/Z 1 - 0.313r 0.987 0.984 0.974 0.968 0.962rx/r 1 + 0.365r 1.01 1.02 1.03 1.04 1.04ry/r 1 - 0.386r 0.985 0.981 0.969 0.962 0.954
aThe x-axis is the major axis and the y-axes is the minor axis.
INITIAL DISTORTION: STRUCTURAL EFFECTS
HSS HOLLOW STRUCTURAL SECTIONS
Service-Load Distortion
HSS HOLLOW STRUCTURAL SECTIONS
§ Cross-sectional distortion is caused by radial forces in curved members subjected to flexure
SERVICE-LOAD DISTORTION
HSS HOLLOW STRUCTURAL SECTIONS
§ Element Local Bending
• Flat elements: flange out-of-plane bending
• Round HSS: ovalization
SERVICE-LOAD DISTORTION
HSS HOLLOW STRUCTURAL SECTIONS
§ Simplified Method• Reduced flexural properties account for local bending
• Applicable only to I-Shaped, Rectangular and Round members
SERVICE-LOAD DISTORTION
HSS HOLLOW STRUCTURAL SECTIONS
§ See Design Guide Sections 6.7 and 7.8 for further information on both the general and simplified methods
SERVICE-LOAD DISTORTION
HSS HOLLOW STRUCTURAL SECTIONS
Example
HSS HOLLOW STRUCTURAL SECTIONS
§ Problem Statement• Vertically-curved member• HSS 20´8´3/8• ASTM A500 Gr. C• Bent the hard way
EXAMPLE
HSS HOLLOW STRUCTURAL SECTIONS
• Determine if a 3/8 in. initial distortion in the 8 in. wall reduces the strong-axis flexural strength
EXAMPLE
HSS HOLLOW STRUCTURAL SECTIONS
§ Material properties of ASTM A500 Gr. C§ (AISC Manual Table 2-4)
Fy = 50 ksi
§ Dimensions of HSS 20´8´3/8§ (AISC Manual Table 1-11)§
§ t = 0.349 in.
EXAMPLE
HSS HOLLOW STRUCTURAL SECTIONS
= >d 3 8in.= 1.07 0.264
0.349in.o
t
( ) ( )
d
d dæ öç ÷è ø
2
2
=1.23 - 0.95 +0.30
=1.23 - 0.95 1.07 +0.30 1.07= 0.556
o oCt t
EXAMPLE
HSS HOLLOW STRUCTURAL SECTIONS
§ The width-to-thickness ratio of the 8 in. wall is
§ The effective width-to-thickness ratio of the 8 in. wall is
d
= =ll = 19.9 26.7
0.556e C
( )( )-l = = =
8 3 0.34919.9
0.349bt
in. in.in.
EXAMPLE
HSS HOLLOW STRUCTURAL SECTIONS
§ The limiting width-to-thickness ratio is
=
l =
=
>
29,000ksi50ksi
1.12
1.12
27.0 26.7
py
EF
o.k.
Therefore, a 3/8 in. initial flange distortion has no effect on the available member strength
(AISC Specification Table B4.1b)
EXAMPLE
HSS HOLLOW STRUCTURAL SECTIONS
Conclusions
HSS HOLLOW STRUCTURAL SECTIONS
§ Curve Geometries• Standard bends• Specialty bends
Photograph courtesy of the AISC Bender/Roller Committee
CONCLUSIONS
HSS HOLLOW STRUCTURAL SECTIONS
§ Bending Processes• Each bending method has
advantages and disadvantages• The capabilities of each
bender/roller vary significantly• Involve a bender/roller early in
the design process
CONCLUSIONS
HSS HOLLOW STRUCTURAL SECTIONS
§ Initial Distortion• Increases with element slenderness (b/t, h/t, D/t)• Reasonable distortions can be tolerated without a
reduction in strength• AESS requirements are usually more stringent than
strength requirements• Tolerances should be specified in the contract documents
CONCLUSIONS
HSS HOLLOW STRUCTURAL SECTIONS
QUESTIONS?
HSS HOLLOW STRUCTURAL SECTIONS
POLL QUESTION 1
Which of the following should be considered when establishing a distortion tolerance?
a. Contact a bender-roller company for inputb. Use the cross-sectional tolerance from an ASTM standardc. Measure the actual distortion after the member has been bentd. Based on the visual appearance of AESS memberse. Based on the potential local buckling strength reduction
HSS HOLLOW STRUCTURAL SECTIONS
POLL QUESTION 2
Do you want a PDH Certificate for this webinar?
CURVED HSS MEMBERSBO DOWSWELL, P.E., PH.D.
SDS CONSULTING
1 PDH