ACI 318 - LA Civil Engineering Conference & Show

Report
Serviceability Design of Concrete
Structures in Marine Environments
Carlos E. Ospina, PhD, PE, FACI
BergerABAM Inc., Houston, TX
2014 Herbert J. Roussel Lecture
Organization
• Introduction
• Problem Statement
• Serviceability Design of Marine Concrete
Structures
– Flexural Crack Control
– Deflection Control
• Durability Considerations
• Conclusions and Recommendations
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
Problem Statement
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
Excessive Deformations
Excessive Deformations
Problem Statement
Problem Statement
Problem Statement
Problem Statement
Problem Statement
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?
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
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.
Crack Control Provisions
• Gergely-Lutz (1967) (Statistical in nature)
w max  0 . 000011  f s 3 d c A
(w in mm, fs in MPa)
• z-factor (ACI 318 prior to 1999 & AASHTO LRFD
prior to 2004)
z  fs 3 dc A
< 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
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.
Crack Control
• Frosch (1999)
– Direct Control:
w max  2
fs

Es
d
2
c
s
 
2
– Indirect Control:
s max  2
 w Es

2f 
s





2
 dc
2
2
Crack Control
• ACI 318-08:
– Based on Frosch’s model. Abandoned exposure
conditions.
s max
 280 
 280
  2 . 5 c c  300 
 380 

 f
f
 s 
 s




(fs in MPa), cc in mm)
• AASHTO LRFD 2007 (DeStefano et al, 2003):
– Adopted Frosch’s but distanced from ACI 318.
– Preserved exposure condition effect (ge)
122 , 600 g e
s
 fs
 2d c
Crack Control in ACI 318-08 & ASSHTO LRFD 2007
500
Frosch, w = 0.44 mm
Bar Spacing, s (mm)
400
Frosch, w = 0.58 mm
300
AASHTO LRFD 2007
200
ACI 318-08
100
0
0
50
100
Concrete Cover, dc (mm)
150
200
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.
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.
Limiting Crack Width
Exposure Condition
Maximum
Allowable
Crack Width
(mm)
ACI 318-95 and earlier versions
Interior Exposure
Exterior Exposure
0.41
0.33
ACI 224R-01*
Dry air or protective membrane
Humidity, moist air, soil
Deicing chemicals
Seawater and seawater spray,
wetting and drying
Water-retaining structures †
0.41
0.30
0.18
0.15
0.10
CEB/FIP MC90**
• w = 0.25 mm typically used in
marine beam construction
Reinforced Concrete Members
Exposure Classes 2 to 4
0.30 ‡
Exposure Class 1
See note ¥
De-icing agents on top of tension
See note ¤
zones of RC members
BS 8110-97
Appearance
Aggressive environments
0.30
0.30
Proposed Improvement to ACI 318
• For indirect crack control, the limiting “w”
should be shown explicitly in smax equation.
• Direct control:
w max 
fs
240 , 000
s  2 .5 c c  
• Indirect control:
 240 , 000  w
s max 
(w in mm, fs in MPa)
fs
 2 .5 c c 
s fs
190 , 000
(190 , 000 ) w
fs
Proposed Improvement to AASHTO LRFD
• Similar concept.
• Direct control:
w max  0 . 0000036  f s  s  2 d c

(w in mm, fs in MPa)
• Indirect control:
ge 
w
0 . 44
s max 
278 , 600 w
 fs
 2d c
Proposed Crack Control Equations
400
f s = 280 MPa (40 ksi)
Bar Spacing, s (mm)
Modified
Eq. 20 AASHTO LRFD
Eq.
20
Modified
300
ACI 318
200
w=0.2 mm
w=0.3 mm
w=0.4 mm
100
0
0
50
100
150
Concrete Cover, dc (mm)
dc 
200 , 000 w
2 f s
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.
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.
Direct Deflection Control
• Direct calculation in terms of Ie
Dm
2

M
L
 5 
m
 K1 


 48   E c I e




K 1  1 .2  0 .2
M
o
M
m
• Branson (1965):
 M cr
Ie  
 M
a

3

 Ig



 1 


 M cr

 M
a





3

I  I
g
 cr

• Bischoff (2005) improved Branson’s equation
• Integration of curvatures (Ghali, 1994)
Indirect Deflection Control
• Limiting curvature concept
Indirect Deflection Control
• Indirect control in terms of curvature:
Dm
 5 
 K1 

 48 
m
L
2

m

 sm
d 1  k m

• Leads to indirect control through max
span/depth ratio:
48   1  k m


h
5 K 1   s , m
L
 Dm

 L


 cm   sm
d
Max Deflections in ACI 318
Pile-supported Container Yard
• Appropriate Δ/L here?
1/750 ~ 1/1000
“Span Length” in Pile-supported Beams
RTG Crane
RTG Crane
Runway
Lo
D
Leff
Pile
Note: Deflected shape exaggerated for clarity
Indirect Deflection Control
• Minimum Thickness
ACI 318-08 Table 9.5(a)
Member
Solid one-way
slabs
Beams or ribbed
one-way slabs
Minimum Thickness, h
Simply
One end
Both ends
Cantilever
supported
continuous
continuous
Members not supporting or attached to partitions or other
construction likely to be damaged by large deflections
L/20
L/24
L/28
L/10
L/16
L/18.5
L/21
L/8
s and D/L Effect on Span/Depth Ratio
80
k m = 0.254
rr Er = 1200 MPa
70
 sm = 0.0006
 = 0.9
60
Simple Span
50
L
h
Dm/L = 1/240
Interior Span
Edge Span
(Pinned/Continuous)
40
Dm/L = 1/1000
30
 sm = 0.0013
20
 sm = 0.0006
10
 sm = 0.0013
0
1.0
1.5
2.0
Static-to-Midspan Moment Ratio ,
2.5
Mo
Mm
3.0
Staged Construction
Staged Construction
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
Durability Considerations
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
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.
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.
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.
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.
[email protected]

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