serviceability design of concrete structures in marine environments carlos e. ospina, phd, pe, faci...
TRANSCRIPT
![Page 1: Serviceability Design of Concrete Structures in Marine Environments Carlos E. Ospina, PhD, PE, FACI BergerABAM Inc., Houston, TX 2014 Herbert J. Roussel](https://reader035.vdocuments.mx/reader035/viewer/2022062421/56649cf15503460f949c060c/html5/thumbnails/1.jpg)
Serviceability Design of Concrete Structures in Marine Environments
Carlos E. Ospina, PhD, PE, FACIBergerABAM Inc., Houston, TX
2014 Herbert J. Roussel Lecture
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Organization• Introduction• Problem Statement• Serviceability Design of Marine Concrete
Structures– Flexural Crack Control– Deflection Control
• Durability Considerations• Conclusions and Recommendations
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Intro - Serviceability Design• “Strength is essential…and otherwise unimportant” (Hardy Cross)
• Objectives of serviceability design in concrete structures:– Avoid excessive deformations and vibrations under service loads– Preserve structural functionality and visual appearance– Prevent corrosion of steel reinforcement– Maintain durability
• Most common serviceability limit states (SLS) in design of concrete structures– Cracking– Deflections
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Problem Statement
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Challenges in Serviceability Design of Marine Concrete Structures
• Heavy loads• Equipment and operations sensitive to
structural deformations• Harsh environment• Staged construction common• Specialized design guidelines/standards scarce
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Problem Statement
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Problem Statement
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Problem Statement
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Problem Statement
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Problem Statement
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Problem Statement• Serviceability design provisions abound in
building/bridge design codes. • Specialized design standards for marine
concrete structures are scarce.• How to interpret these for the serviceability
design of marine concrete structures?• What are their limitations and shortcomings?
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Problem Statement• How to control cracking and deflections in
marine concrete structures in a practical manner?
• This presentation focuses on serviceability design provisions in ACI 318 and AASHTO LRFD and how they can be used for serviceability design of marine concrete structures
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Flexural Crack control in RC Members
• Cracking can be controlled directly (crack width, w) or indirectly (smax).
• Marine terminal engineers seem particularly keen at requesting direct crack width calculations.
• Two schools of thought:– Flexural cracks induced by bond between rebars and
concrete– Flexural cracks NOT induced by bond. Crack spacing
affected by clear cover.
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Crack Control Provisions• Gergely-Lutz (1967) (Statistical in nature)
• z-factor (ACI 318 prior to 1999 & AASHTO LRFD prior to 2004)
< 25,000 N/mm (interior exposure) < 30,000 N/mm (exterior exposure)
• Counterintuitive. Increase in dc leads to increase in z• Test beams had very small concrete covers
3max 000011.0 Adfw cs
3 Adfz cs
(w in mm, fs in MPa)
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Revised Approach (Frosch 1999)• Bond is not a major parameter controlling crack
widths. Crack spacing depends mainly on concrete cover (Broms 1965).
• Max crack spacing is twice the minimum.
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Crack Control• Frosch (1999)
– Direct Control:
– Indirect Control:
22
max 22
s
dE
fw c
s
s
2
2
max 22 c
s
s df
Ews
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Crack Control• ACI 318-08:
– Based on Frosch’s model. Abandoned exposure conditions.
• AASHTO LRFD 2007 (DeStefano et al, 2003):– Adopted Frosch’s but distanced from ACI 318.– Preserved exposure condition effect (ge)
sc
s fc
fs
2803005.2
280380max
cs
e df
s 2600,122
(fs in MPa), cc in mm)
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Crack Control in ACI 318-08 & ASSHTO LRFD 2007
0
100
200
300
400
500
0 50 100 150 200
Ba
r S
pa
cin
g, s
(mm
)
Concrete Cover, dc (mm)
Frosch, w = 0.44 mm
Frosch, w = 0.58 mm
ACI 318-08
AASHTO LRFD 2007
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Observations to ACI 318 crack control rules
• ACI 318-08 does not report which limiting crack width it complies with.
• Apply smax eq. with judgment if cracking limits or regime differ from those aimed by ACI 318.
• Smax evaluated at fs = 0.67 fy. This is slightly higher than 0.6 fy used til 1999. The 0.67 stems from higher load factors adopted in the 2002 code. Increased stress should be tied to a larger (0.67/0.6 x w) limiting crack width.
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Observations to ACI 318 crack control rules
• For the sake of fairness, ACI 318 warns designers to be cautious when using the crack control provisions when dealing with aggressive environments.
• This seems to imply exposure conditions do matter.
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Exposure Condition
Maximum Allowable
Crack Width (mm)
ACI 318-95 and earlier versions
Interior Exposure 0.41 Exterior Exposure 0.33
ACI 224R-01*
Dry air or protective membrane 0.41
Humidity, moist air, soil 0.30 Deicing chemicals 0.18
Seawater and seawater spray, wetting and drying
0.15
Water-retaining structures † 0.10
CEB/FIP MC90**
Reinforced Concrete Members Exposure Classes 2 to 4 0.30 ‡
Exposure Class 1 See note ¥ De-icing agents on top of tension
zones of RC members See note ¤
BS 8110-97
Appearance 0.30
Aggressive environments 0.30
Limiting Crack Width
• w = 0.25 mm typically used in marine beam construction
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Proposed Improvement to ACI 318• For indirect crack control, the limiting “w”
should be shown explicitly in smax equation.• Direct control:
• Indirect control:
000,190
5.2000,240max
sc
s fscs
fw
s
cs f
wc
f
ws
)000,190(5.2
000,240max
(w in mm, fs in MPa)
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Proposed Improvement to AASHTO LRFD
• Similar concept.• Direct control:
• Indirect control:
cs dsfw 20000036.0max
cs
df
ws 2
600,278max
44.0
we
(w in mm, fs in MPa)
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Proposed Crack Control Equations
0
100
200
300
400
0 50 100 150
Ba
r S
pa
cin
g, s
(mm
)
Concrete Cover, dc (mm)
fs = 280 MPa (40 ksi)
w=0.4 mmw=0.3 mmw=0.2 mm
Eq. 20
fs = 280 MPa (40 ksi)
w=0.4 mmw=0.3 mmw=0.2 mm
Eq. 20
sc f
wd
2
000,200
Modified ACI 318
Modified AASHTO LRFD
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Crack Control in FRP-reinforced Concrete Elements
• Frosch’s model applicable.• Need to account for variable elastic modulus
and bond characteristics of FRP bars.• Larger w values allowed because of superior
corrosion resistance of FRP reinforcement.• Refer to ACI 440 standards. ACI 318 not valid
for FRP-reinforced concrete.
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Deflection Control• Deflections can be controlled directly (through
direct D calculation) or indirectly (by specifying max span/depth ratio or min member thickness)
• Philosophy in ACI 318 is to waive direct deflection calculation if max span/depth ratio or min member thickness are complied with.
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Direct Deflection Control• Direct calculation in terms of Ie
• Branson (1965):
• Bischoff (2005) improved Branson’s equation• Integration of curvatures (Ghali, 1994)
ec
mm IE
LMK
2
1 48
5
m
o
M
MK 2.02.11
gcra
crg
a
cre II
M
MI
M
MI
33
1
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Indirect Deflection Control• Limiting curvature concept
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Indirect Deflection Control• Indirect control in terms of curvature:
• Leads to indirect control through max span/depth ratio:
21 48
5LK mm
L
k
Kh
L m
ms
m
,1
1
5
48
dkdsmcm
m
smm
1
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Max Deflections in ACI 318
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Pile-supported Container Yard
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• Appropriate Δ/L here?1/750 ~ 1/1000
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“Span Length” in Pile-supported Beams
Pile
RTG Crane
Lo
Lef f
RTG Crane Runway
Note: Def lected shape exaggerated for clarity
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Indirect Deflection Control• Minimum Thickness
ACI 318-08 Table 9.5(a)
Minimum Thickness, h Simply
supported One end
continuous Both ends continuous
Cantilever
Member Members not supporting or attached to partitions or other construction likely to be damaged by large deflections
Solid one-way slabs
L/20 L/24 L/28 L/10
Beams or ribbed one-way slabs
L/16 L/18.5 L/21 L/8
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es and D/L Effect on Span/Depth Ratio
0
10
20
30
40
50
60
70
80
1.0 1.5 2.0 2.5 3.0
Static-to-Midspan Moment Ratio ,
m
o
M
M
sm = 0.0006
km = 0.254
rr Er = 1200 MPa = 0.9
Interior Span
Edge Span(Pinned/Continuous)
Simple Span
sm = 0.0013
h
L
sm = 0.0006
sm = 0.0013
m/L = 1/240
m/L = 1/1000
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Staged Construction
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Staged Construction
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Staged Construction
• Book-keeping of bending moment growth is key to calculate crack widths, rebar stresses and concrete stresses as staged construction progresses
• Shored vs Non-shored construction also important
≠
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Durability Considerations
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Precast Concrete Elements• Construction per PCI MNL 116• 42 MPa Concrete Strength at 28 days• ASTM C150 Type II cement with C3A between 6 and 10% • Water-cement ratio ≤ 0.40• Minimum cement content of 400 kg/m3
• Max chloride ion content = 0.06% by weight of cement• Add Calcium Nitrate as needed (piles)• 75 mm clear cover at soffits• Aggregate needs to be innocuous (alkali-silica reaction)• Silica fume: max 8% cement replacement
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Conclusions and Recommendations (I)• Crack and deflection control in concrete
structures can be done directly or indirectly. • Indirect control techniques require Designers to
know what limiting crack width/deflection is being controlled.
• Indirect (smax) crack control equations in ACI 318 and AASHTO could be more transparent. Need more explicit dependence on wmax.
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Conclusions and Recommendations (II)
• The proposed modification to the ACI 318 crack control equation provides a means for controlling very narrow crack widths (narrower than the limits adopted by ACI 318 for buildings).
• ACI 318 crack control equation only works for steel-reinforced concrete. For FRP-reinforced concrete, refer to ACI 440.
• Frosch’s generalized equation for smax is a good tool for crack control for given limiting w.
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Conclusions and Recommendations (III)
• Indirect deflection control typically expressed through max span/depth ratios.
• D/L limits given in ACI 318 are adequate for RC buildings but may be overly liberal for port structures with equipment sensitive to deflections.
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Conclusions and Recommendations (IV)
• Indirect deflection control checks are appropriate for preliminary member sizing followed by detailed direct deflection calcs.
• Crack control in concrete elements cast in stages requires evaluation of crack widths and rebar stresses at every step.
• Designers need to adopt design codes carefully, understanding scope and limitations.