high-strength steel in structural concrete · a615 and a706 a615 and a1035 a1035 grade 100 high...
TRANSCRIPT
High-Strength Steelin Structural Concrete:
From Research to Building Codes
Project Sponsors
• American Concrete Institute (ACI)ACI Foundation – Concrete Research Council
• Charles Pankow Foundation• Concrete Reinforcing Steel Institute (CRSI)
Commercial Metals CompanyMMFX Technologies CorporationNucor Corporation
• Electric Power Research Institute (EPRI)
Project Participants
• Collaborators• David Darwin• Matt O’Reilly
• Graduate Students• Ali Ajaam• Abdal Al-Sabawy• Shahedreen Ameen• Krishna Ghimire• Eduardo Guillen
• Muna Hano• Sajed Huq• Yun Shao• Jayne Sperry• Don Spradling
• Alex Weber-Kamin• Samir Yasso
• Many others…
Overview
• High-Strength Steel (HSS) as Reinforcement• HSS in slender structural walls• HSS in coupling beams• HSS as shear reinforcement• Bond and development of HSS• Summary
Overview
• High-Strength Steel (HSS) as Reinforcement• HSS in slender structural walls• HSS in coupling beams• HSS as shear reinforcement• Bond and development of HSS• Summary
Motivation
Authors’ Collection
Motivation
Photo courtesy of E. A. Burgos
Motivation
Authors’ Collection
Motivation
Photo courtesy of K. K. Mow
Motivation
Authors’collection
Potential Benefits of High-Strength Steel
• Cost savings• Reduced material quantities• Reduced reinforcement congestion• Improved quality of construction
High-Strength Steel
Usually defined as having a yield strength fy of 80 ksi or greater.
Code Limitations
0
20
40
60
80
100
1956 1963 1971
Desig
n f y
,ksi
ACI 318 Version
Shear/Torsion
Flexure/Axial
0
20
40
60
80
100
1956 1963 1971 2008
Desig
n f y
,ksi
ACI 318 Version
Shear/Torsion
Flexure/Axial
Confinement
Seismic
High Strength
Background
ACI ITG-6, 2010 ATC 98, 2014 ATC 115, 2014
0
40
80
120
160
200
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18
Stre
ss (k
si)
Strain
Conventional vs. High-Strength Steels
Grade 60
Grade 120
Grade 80
Grade 100
0.2% offset
TY
su Uniform
A615 and A706
A615 and A706A615 and A1035
A1035
ddddddGGGGGGGGGGGGGGrrrrrrraaaaaaaaddddddddddeeeeeeee 111111111000000000000000000000GGGGGGGGGGGGGrrrrrrrraaaaaaaaadddddddddeeeeeeee 1111111100000000000000000000GGGGGGGGrrrraaaaaddddddeeeee 11111100000000000High Strength
Key Parameters
Yield strength
Tensile strength
Tensile-to-yield strength ratio
Uniform elongation
Fracture elongation su
sf
fy
ft
T/Y
Main Concerns in DesignGravity Design• Crack control• Deflection control• Development length• Moment redistribution• Shear strength
Seismic Design• Strength, stiffness, and
deformation capacity• Spacing of hoops to
delay bar buckling• Bond stresses
KU selected for testsof walls and coupling beams
ATC 115
Overview
• High-Strength Steel (HSS) as Reinforcement• HSS in slender structural walls• HSS in coupling beams• HSS as shear reinforcement• Bond and development of HSS• Summary
Objectives
(1) Evaluate the effect of tensile-to-yield strength ratio (T/Y) of steel bars on wall deformation capacity
(2) Determine the minimum uniform elongation ( ) required of bars used in earthquake-resistant construction
T-shaped Wall Specimens
Id. fy (ksi) T / Y f’c (ksi)T1 60 1.25
8T2 100 1.1T3 100 1.2 1.3T4 100 1.3
Similar fyhw/ w = 3
P
Tensile-to-Yield Strength Ratio (T/Y)
0
20
40
60
80
100
120
140
0.00 0.01 0.02 0.03 0.04 0.05 0.06
Stre
ss,k
si
Strain
T/Y = 1.1 A706 Grade 60 T/Y = 1.4 0.2% Offset
fy = 100 ksi su < 10%
fy = 60 ksi su > 10%
T/Y = 1.25
Cross Section
T1 T2, T3, T4*
Stem Boundary Element
T1 T2, T3, T4
T-Shaped Wall Specimens8'-4"
34"
CLR
, TY
P
#4 @ 15", HORIZ
5'-0
"2'
-6"
10"
1'-3"
10"
1'-3"
(14) #6, VERTS
#4 @ 15", HORIZ
(14) #4, VERTS
(6) #6, VERTS
Neutral axis
Construction
Test Setup
T3
T1
Drift Ratio
= ×
Loading Protocol
-6
-4
-2
0
2
4
6
Drif
t Ra
tio, %
1 Step
1 Cycle
Stepa Drift, %1 0.20
2 0.30
3 0.50
4 0.75
5 1.00
6 1.50
7 2.00
8 3.00
9 4.00a Two cycles per step
Shear vs. Drift Ratio
T1 – 60 ksi T3 – 100 ksi
18 -12 -6 0 6 12 18
-6 -4 -2 0 2 4 6
Top Displacement, in.
Drift Ratio, %
T
C
T
C
-18 -12 -6 0 6 12 18
-320
-160
0
160
320
-6 -4 -2 0 2 4 6
Top Displacement, in.
Shea
r, k
Drift Ratio, %
T
T
C
C
Bar fracture
Mn
Test Results
Bar buckling occurred during the 2% drift cycle in both T1 and T3
T3 – 100 ksi
Test Results
T1 – 60 ksi T3 – 100 ksiBar buckling
Test Results
T3 – 100 ksiT1 – 60 ksi4% Drift Ratio
Barfracture
Test Results
Test Results
T2 (Grade 100, su = 5.5%, T/Y = 1.10)2% Drift Ratio
T2 – 100 ksi
#4#6
Mn
Lug base radiusr < 0.25 h
Fractured No. 4 Bar from T2
Special Attributes of No. 4 Bars in T2(1) Sharp edges in deformation pattern of longitudinal bars.
(Reported poor performance in low-cycle fatigue tests)(2) Lowest uniform elongation su.
(Measured su was 5.5%)(3) Lowest T/Y ratio.
(Measured T/Y was 1.10)(4) Strain gauges on No. 4 bars at the wall-base interface.
(Potentially creating a weak plane)(5) Zero percent Vanadium.
(Affecting low-cycle fatigue and strain aging)
Wall Tests
0 2 4 6 8 10 12 14 16
Uniform elongation, %
0
1
2
3
4
5
6D
rift
ratio
, %
T1
T2
T3
T4
T5
T6
Wall T/Y 1/0.85 (or 1.18)
6% (uniform)10% (fracture)
T/Y = 1.10
1.33
1.18
Main Findings
• Walls designed using Grade 60 or Grade 100 reinforcement, 1.18, 6%, and 10% had similar strength
%).
Overview
• High-Strength Steel (HSS) as Reinforcement• HSS in slender structural walls• HSS in coupling beams• HSS as shear reinforcement• Bond and development of HSS• Summary
Coupling Beams in Practice
Coupling Beams in Practice
Courtesy of: Pete Heeringa
Coupling Beam Demands
• Structural walls are a common lateral bracing system
• Often pierced to allow doors and windows
• Short “coupling beams” restore some of the stiffness and strength
Single Uncoupled Coupled
Coupling Beam Demands
• Wall deformations impose large deformation demands on coupling beams
• Beams are often short with span-to-depth ratios < 4
• High strength, stiffness and ductility requirements to maintain coupling
Park and Paulay, 1975
Coupling Beams in Practice
Why so much reinforcement?
L-Street apartment building in Anchorage, Alaska after 1964 earthquake
Courtesy of: EERI and IAEE, World Housing Encyclopedia, retrieved from http://db.world-housing.net/pdf_view/111/, February 2018.
Solution: High-Strength Steel
Grade 60
High-Strength
Steel
Coupling Beam Tests
In Practice Test Specimen
Coupling Beam Tests
Coupling Beam Tests
Span = 2.5*depth, Grade 100, 8 ksi concrete
ResultsGrade 80 Grade 100
10 10
-10 -5 0 5 10Chord Rotation, %
-250
-125
0
125
250
Shea
r, k
ip-20
-10
0
10
20
Shea
r St
ress
/ f
cm, p
si/p
si
Results
1010
Grade 80Grade 120
Findings/Recommendations
• Use of smaller amounts of HSS (such that is maintained) provides the expected strength and similar deformation capacity to conventional coupling beams
• Higher shear stresses may be permissible (and achievable) in HSS coupling beams
• Use of Grades 80 and 100 (and possibly 120) HSS reinforcement should be permitted in coupling beams
Overview
• High-Strength Steel (HSS) as Reinforcement• HSS in slender structural walls• HSS in coupling beams• HSS as shear reinforcement• Bond and development of HSS• Summary
Shear Reinforcement – Mat Foundation
Objective
• Develop guidance for use of• Grade 80 steel as transverse reinforcement• Headed transverse reinforcement
• Headed transverse bars not engaging longitudinal reinforcement
Stirrups (hooked)
Headed (Engaged)
Headed (Not Engaged)
Scope and Test Setup
• 39 Specimens• Reinforcement:
Grades 60 and 80• Depths: 12 to 48 in.• Concrete compressive
strengths: 4 and 10 ksi
Observations
Stirrups (hooked)
Headed (Engaged)
Strength: Hooked vs. Headed
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
V T /V
n
=On average, headed shear
reinforcement led to the same strength as stirrups
SS HEHE HNEHNE
Strength: Grade 60 vs 80
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
V T /V
n
Grades 60 and 80 shear reinforcement
led to equivalent shear strength…
…when area of shear reinforcement decreased in proportion to
increase in grade
S HEHNE
= =
Crack Widths
36 in. depth, Grade 60 hooked stirrups, 4 ksi concrete
Crack Widths
Grade 60
Grade 80
Crack Widths
h = 48 in.
h < 20 in.
Findings/Recommendations
• Grade 60 and 80 stirrups provide equivalent strength and similar inclined crack widths (variables like beam depth had a much stronger influence than grade)
• Headed and hooked stirrups provide equivalent shear strength
• Both Grade 80 and headed stirrups should be permitted –with some limitations on headed stirrups (not discussed)
Overview
• High-Strength Steel (HSS) as Reinforcement• HSS in slender structural walls• HSS in coupling beams• HSS as shear reinforcement • Bond and development of HSS• Summary
Bond and Development of HSS
• Hooks:
• Heads:
• Straight:
y e c rdh b
c
fd
f
y edt b
c
fd
f
y t e sd b
c b tr
b
fd
f c Kd
y e cs odh b
c
fd
f
y e cs odt b
c
fd
f
y t e yd b
c b tr
b
fd
f c Kd
ACI 318-14 (fy ksi) ACI 318-19
Straight Bars in Tension
COV = 0.25
COV = 0.14
Straight Bars in Tension (Full Confinement)
0
10
20
30
40
50
60
70
80
0 5,000 10,000 15,000 20,000
ACI 318-14 Proposed
d/ d
b
fy = 60 ksi
0
10
20
30
40
50
60
70
80
0 5,000 10,000 15,000 20,000
d/ d
bfy = 100 ksi
Overview
• High-Strength Steel (HSS) as Reinforcement• HSS in slender structural walls• HSS in coupling beams• HSS as shear reinforcement• Bond and development of HSS• Summary
Summary
• Use of Grades 80, 100, and 120 high-strength reinforcement in smaller amounts (such that is maintained) provides the expected strength and similar deformation capacity
Summary
• Grade 80 reinforcement is likely to be permitted by the ACI Building Code (318-19) for all applications
• ACI 318-19 may permit use of Grade 100 reinforcement in select applications (e.g., walls), with more applications likely permissible in future editions
• New laboratory facilities have allowed KU to be a key player in advancing these changes to ACI 318