20191002 curved hss members 092819 - steel tube institute · certificate during the poll at the...

CURVED HSS MEMBERSBO DOWSWELL, P.E., PH.D.

SDS CONSULTING

1 PDH

HSS HOLLOW STRUCTURAL SECTIONS

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Local Distortion of Curved Members


LOCAL DISTORTION OF CURVED MEMBERS

§ Vertically-Curved Members

5Photograph courtesy of the AISC Bender/Roller Committee


LOCAL DISTORTION OF CURVED MEMBERS

§ Horizontally-Curved Members



OUTLINE

• Curve geometries• Bending processes• Initial distortion• Service-load distortion• Example


Curve Geometries


CURVE GEOMETRIES

Standard Bends• Single-radius curvature• Curvature about a principal

or geometric axis

9

Photograph courtesy of the AISC Bender/Roller Committee


CURVE GEOMETRIES

Off-Axis Bends§ Bending about a non-principal or non-geometric axis

10Photographs courtesy of the AISC Bender/Roller Committee


CURVE GEOMETRIES

Compound and Reverse-Compound Bends§ Multiple arcs in the same plane



CURVE GEOMETRIES

Multi-Axis bends§ Curvature about more

than one axis

12



CURVE GEOMETRIES

Variable-Radius Bends§ Any non-circular bend (parabolic, elliptical, etc.)



CURVE GEOMETRIES

Spiral Bends§ Helical curve



Bending Process


BENDING PROCESSES

• Pyramid roll bending• Incremental step bending• Induction bending• Other processes

16



BENDING PROCESSES

Pyramid Roll Bending • Member is repeatedly passed through a set of rolls

17


BENDING PROCESSES

• Contoured rolls can be used to provide support during the bending operation



BENDING PROCESSES

Incremental Step Bending • Forces applied at several

discrete locations along the member length

19



BENDING PROCESSES

Induction Bending• Electric induction coil heats

a narrow band around the member circumference

20



BENDING PROCESSES

• After heating, the member is curved by force

21

Photographs courtesy of the AISC Bender/Roller Committee


BENDING PROCESSES

§ Summary• Each bending method has advantages and

disadvantages• The capabilities of each bender/roller vary significantly

22


Initial Distortion


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


INITIAL DISTORTION

• Occurs in both open and closed shapes


CAUSES OF INITIAL DISTORTION

Causes of Initial Distortion


• Contact forces from the bending machine


INITIAL DISTORTION: CAUSES


• Flexural compression− Flange local buckling− Web local buckling

• Shear− Web shear buckling



• Radial forces



TYPES OF INITIAL DISTORTION

Types of Initial Distortion


I-Shaped Members• Flange bending

• Web buckling

INITIAL DISTORTION: TYPES


Pyramid roll bending with no mandrel(REF: CIDECT Report 11C-88/14E)

Square and Rectangular HSS• Concave compression flange• Outward bowing of the web



-= 1ρwb BB

=ρ feD



Round HSS§ Ovalization

Dn = nominal outside diameter

-=ρ max min

n

D DD



Distortion Tolerances


§ 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


§ 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



§ 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



Distortion Tolerance: Rectangular HSS§ Post-bending wall flatness tolerances (rw and rf) between

1% and 2% are common



Distortion Tolerance: Round HSS§ A post-bending ovality tolerance (r) of 5% is common



§ For AESS, a 5% tolerance will likely result in imperceptible ovalization distortions

r = 5% r = 8% r = 12%



• Practical considerations may dictate tolerances• Example: when two segments are connected with

circumferential butt welds, proper wall alignment is essential



MINIMUM RADIUS

Minimum Radius


What is the Minimum Cold-Bending Radius?

Rule of Thumb?

44

INITIAL DISTORTION: MINIMUM RADIUS


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

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§ Bending requirements should be discussed with the bender-roller who will provide the service

46



REDUCTION OF INITIAL DISTORTION

Reduction of Initial Distortion


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


§ Stiffened elements perform better than unstiffened elements



§ Round elements perform better than rectangular elements



STRUCTURAL EFFECTS

Structural Effects


§ 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


• 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



• Strength ratio, Cd, as a function of do/t



• 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



• Replace l with le in the Specification equations• In many cases, the member strength will not be reduced



§ Round HSS§ Ovalization causes an increase in sectional properties

about one axis and a decrease about the perpendicular axis



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.



Service-Load Distortion


§ Cross-sectional distortion is caused by radial forces in curved members subjected to flexure

SERVICE-LOAD DISTORTION


§ Element Local Bending

• Flat elements: flange out-of-plane bending

• Round HSS: ovalization



§ Simplified Method• Reduced flexural properties account for local bending

• Applicable only to I-Shaped, Rectangular and Round members



§ See Design Guide Sections 6.7 and 7.8 for further information on both the general and simplified methods



Example


§ Problem Statement• Vertically-curved member• HSS 20´8´3/8• ASTM A500 Gr. C• Bent the hard way

EXAMPLE


• Determine if a 3/8 in. initial distortion in the 8 in. wall reduces the strong-axis flexural strength

EXAMPLE


§ 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


= >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


§ 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


§ 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


Conclusions


§ Curve Geometries• Standard bends• Specialty bends


CONCLUSIONS


§ 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


§ 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


QUESTIONS?


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


POLL QUESTION 2

Do you want a PDH Certificate for this webinar?

CURVED HSS MEMBERSBO DOWSWELL, P.E., PH.D.

SDS CONSULTING

1 PDH