CONTINUOUS SLAB BRIDGE COMPARITIVE STUDY

Transcription

1 CONTINUOUS SLAB BRIDGE COMPARITIVE STUDY LRFD vs. Standard Specifications By: Teddy Antonios & Matt Blythe Ohio Department of Transportation

2 Overview LRFD Changes the following significantly in the design of continuous slab superstructures: Load Factors, Modifiers & Combinations Live Load Cases Live Load Extreme Effects Equivalent Deck Strip Widths Impact Shrink./Temp. Steel Max./Min. Reinforcing Limits Crack Control Fatigue Edge Beam Design

3 Code Comparison γ Std. Spec. Basics ( Σβ ( )) DL DL + Σβ LL LL + I φru ΣDL + ΣLL Ru F.S. γ = Load Factor for All Loads β DL = Dead Load Combination Effect β LL = Live Load Combination Effect DL = Dead Loads (LL+I) = Live Loads + Impact φ = Reduction Factor R u = Ultimate Resistance or Strength LRFD Basics Ση γ Q φr i i i η i = Load Modifier γ i = Load Factor Q i = Load Effect φ = Resistance Factor R n = Nominal Resistance n

4 Live Load - Std. Spec OR OR

5 Live Load - LRFD OR +

6 Live - Load LRFD HL-93 is the design load. The extreme force effect shall be taken as the following: Design Truck combined with the Design Lane Load or Design Tandem combined with the Design Lane Load or For negative moment, 90% of 2 Design Trucks combined with 90% of the Design Lane Load. or For negative moment, 2 Design Tandems combined with the Design Lane Load. At the Discretion of the District.

7 90% of 2 Design Trucks

8 Double Tandem (C )

9 LRFD : Extreme Effects Axles that do not contribute to the extreme force effect under consideration shall be neglected. Remove design truck/tandem axles from analysis that reduce response under consideration. INFLUENCE LINE MAXIMUM POSITIVE SPAN #1 INFLUENCE LINE MAXIMUM NEGATIVE SPAN #1

10 Extreme Effects (cont d) LRFD C : Only those areas or parts of areas that contribute to the same extreme being sought should be loaded. Remove sections of design lane from analysis that reduce response under consideration. INFLUENCE LINE MAXIMUM POSITIVE SPAN #1 INFLUENCE LINE MAXIMUM NEGATIVE SPAN #1

11 Dynamic Allowance (Impact) Standard Spec IM = 50/(L+125) Maximum of 30% For All Limit States LRFD IM = 33% for design truck & tandem. For All Limit States except Fatigue as per Table IM FAT = 15% IM FAT IM = 0% for lane loading. For All Limit States as per

12 DL = DC & DW Dead Load DC = Dead load of component. DW = Dead load of wearing surface/utilities.

13 Structural Analyses & Evaluation AASHTO Std. Spec. AASHTO LRFD

14 Computing # Of Design Lanes LRFD Same as Std. Specs N L = Integer {(Bridge Width)/12} Bridges 20 to 24 feet wide shall be designed for 2 traffic lanes, each half the roadway width. In cases where the traffic lanes are less than 12 feet wide, N L is taken as the number of traffic lanes & the width of the design lane = the width of the traffic lane

15 Equivalent Strip Widths for Slab-Type Bridges (E) Std. Spec (LFD) E = (4+0.06S) < 7.0 for Wheel loads 2E < 14 for Lane loads

16 Equivalent Strip Widths (cont d) LRFD For Single Lane loaded: E = LW For more than one lane loaded 1 1 E = LW1 1 12W N L

17 Minimum Slab Depth for a Continuous Span Same as Standard Specifications LRFD Table Std. Spec. Table ( S + 10) '

18 Calculate Live Load Bending Moment per Foot of Slab. Design Truck/Tandem: M IM Design Lane: M IM = ( ) Bending Moment = + E 100 ( ) Bending Moment E 1 IM

19 Flexural Resistance LRFD

20 Distribution Reinforcement Same as Standard Specifications LRFD Std. Spec % L

21 Shrinkage and Temperature Reinforcement Std. Spec LRFD A s = 0.125in / ft A s 1.3 2( b A + g h) F y 0.11 A s 0.60

22 Maximum Reinforcement Std. Specs ρ ρ b 4 LRFD Interim Specs eliminated this provision.

23 Minimum Reinforcement Std. Specs LRFD M M S c cr cr = S I g = y t M c f r r = φm n Same as Standard Specs with a modification to the definition of y t illustrated on next slide & f r. (LRFD ) f r = 0.37 f ' c

24 Minimum Reinforcement (cont d.) Std. Spec. LRFD y t = H/2 Neglects the reinforcement y t = Dist. from NA to the tensile face Composite Section

25 Service Limit State / Crack Control Std. Spec f act. < f s Z fs = 0. 6F 3 d A w = c 0.76βf 3 s dc A Z = 130 kips severe exposure y LRFD e s 2 β s 700 γ β f s = 1+ act dc 0.7( h d d c c )

26 Fatigue Limit State Std. Spec β (L+I) = 1.0 Table A Impact = 30% LRFD γ LL = 0.75 Table IM 15% Table E Wheel ( ) < 7. 0 = S E Lane = L* W Check fatigue in regions of compressive stress if: < 2 f f DL + ( LL I )

27 Edge Beam Design Std. Spec The edge beam of continuous slabs should be designed to resist a live load moment of: ( M ) = 0.8( 0.1P S ) u LL 20 P 20 = 16 kip

28 Edge Beam Design Bridge Standard Drawing CS-1-03 Edge beam designed to resist required Std. Spec. moment.

29 Edge Beam Design (cont d) LRFD Design an edge beam unless deck s primary direction is transverse or there is a continuous, composite barrier. LRFD b Edge beams support 1 line of wheel loads and a portion of lane load if important.

30 Edge Beam Design (cont d.) Equivalent Edge Beam Strip Width LRFD b E EB E E = d EB + 12" +, or "

31 Shear LRFD Indicates that shear does not need to be checked in slab bridges. Same as in Std. Specs.

32 Conclusions LRFD Changes the following significantly in the design of continuous slab superstructures: Load Factors, Modifiers & Combinations Live Load Cases Live Load Extreme Effects Equivalent Deck Strip Widths Impact Shrink./Temp. Steel Max./Min. Reinforcing Limits Crack Control Fatigue Edge Beam Design

33 CS-1-07

34 CS-1-07

35 CS-1-07

36 Why Standardize?

37 References AASHTO LRFD Bridge Design Specifications, 2004 Including up to the 2006 Interims Standard Specifications for Highway Bridges, 2002 Ohio Department of Transportation Bridge Design Manual Including up to the 2006 Third Quarter Revisions

38 Questions? Thank you for your time.