Use of Models to Teach the Behavior of Structural Concrete Some Model Slides Courtesy of James Hanson, PhD, PE Rose-Hulman Institute of Technology Lawrence Novak, SE, SECB, F.ACI, F.SEI, LEED AP

2 Outline Flexural Behavior Shear Behavior Lab Tests Rebar Development Creep & Shrinkage Seismic Behavior Engineering Judgment

3 Characteristics of Concrete Factor of 10!! Basic Concept Strong in Compression Weak in Tension

4 Load-Deformation Characteristics for Concrete

5 Styrofoam Beam - Loaded Tension Compression

6 Flexural Behavior Where’s the tension? Closing Opening

7 Flexural Behavior (Cont) Careful: Tension not always on bottom! Closing Opening

8 How To Build It 2” Thick Insulating Board 6’ long, 6” deep Cuts every 4” ~ 2¼”

9 Unreinforced Flexural Behavior

10 Plain Concrete Beam (no rebar)

11 Plain Concrete Beam

12 Reinforced Concrete Beam

13 Load-Deformation Characteristics for Reinforcing Steel

14 Why Steel? Strong in tension Does not react with concrete Compatible thermal expansion and contraction Ductile Bond strength

15 Reinforced Concrete Beam

16 Load-Deformation for Plain and Reinforced Beams

17 Case Study Stairway Landing Pair of Precast Ledger Beams If you look underneath….

18 Case Study Remember, this is from underneath!

19 How To Build It

20 How To Build It (Cont)

21 How To Build It (Cont) Mix Design: (5 day cure) 15 lbs 4 lbs 3 lbs Sand Cement (Type I ) Water For 4 day cure, hold ¼ lb water For 3 day cure, hold ½ lb water [use judgment for workability] Makes approx 4 beams Mix design by PCA

22 How To Build It (Cont) 4x4 Plywood Base 1” PVC 18” Height based on ½” clearance between weights and base

23 How To Build It (Cont) Better to have small gap

24 Diagonal Tension Due to Shear

25 Shear and Stirrups Crack (Saw Cut) Stirrup (Bungee Cord Thru Drilled Hole in Beam)

26 Shear and Stirrups After Concrete Cracks Shear Load Supported by Stirrups (stretching bungee cord) Held Together by “Stirrup”

27 How To Build It (Cont) Rubber cord Hinge (over “crack”) Top View

28 Diagonal Tension Due to Shear

29 Rebar Development ~ 1 3/8” #11 9/8” #9 3/8” #3

30 Rebar Development Why the ribs?

31 Bond and Development Length Mechanical Bond Adhesion Bond

32 Mechanical Bond

Headed Bars in Tension 33 Headed Bars Can Reduce Rebar Congestion Trump Tower Chicago Courtesy ERICO

34 Creep and Shrinkage “The Rocket Science of Structural Concrete” Short-term Elastic Behavior Superimposed over Long-term Inelastic Behavior

35 Creep and Shrinkage Determine the Important Parameters Use Analogy of Inflatable Mattress

36 Creep and Shrinkage Short-term Loads = Elastic

37 Creep and Shrinkage Long-term Loads = Inelastic

38 Creep and Shrinkage Important Parameters Scale of Element

39 Creep and Shrinkage Important Parameters Scale of Element = V/S Ratio (Volume to Surface Ratio)

40 Creep and Shrinkage Volume to Surface Ratio Slab or Wall; V/S = t / 2 Square Column; V/S = s / 4 Circular Column; V/S = r / 2

41 Creep and Shrinkage Important Parameters Size of “Openings”

42 Creep and Shrinkage Important Parameters Size of “Openings” = Age and Strength of Concrete

43 Creep and Shrinkage Important Parameters Pressure Differential

44 Creep and Shrinkage Important Parameters Pressure Differential = Relative Humidity

Data represent: 6 x 12 in. cylinders moist-cured 7 days, then drying at 73 deg. F and 50% RH of ambient air Concrete Drying Concrete Internal Relative Humidity, % 45

Shrinkage Stresses Can lead to cracks starting at exposed surface Curing enables strength gain before drying 46

Shrinkage Shrinkage Strain Time to in./in. 47

Shrinkage vs. Size 48

Shrinkage Stresses Lead to Surface Cracks Restrained concrete can crack as a result of shrinkage –Can cause unsightly or harmful cracks Reinforcement controls shrinkage crack size 49

50 Models Assist in Developing Engineering Judgment

51 Engineering Judgment What is it? How is it Used in Practice? Can it be Learned? How is it Acquired?

52

53 Engineering Judgment is a Journey not a Destination

We all Know Which Way is UP RightWrong Engineering Judgment

55 Engineering Judgment Load Path: Completing the Triangles

56 Engineering Judgment Load Path: Completing the Triangles

57 Engineering Judgment Load Path: Strut and Tie Modeling

58 “Engineering does not tell men what they should want or why they want it. Rather it recognizes a need and tries to meet it.” Hardy Cross, 1952