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. carlos.ospina@abam.com