Vietnam Institute for Building Science and Technology (IBST) Building Code Requirements for Structural Concrete (ACI 318M-11) Overview of ACI 318M Design of Prestressed Concrete Evaluation of Existing Structures David Darwin Vietnam Institute for Building Science and Technology (IBST) Hanoi and Ho Chi Minh City December 12-16, 2011

This morning Overview of ACI 318M-11 Design of Prestressed Concrete (Chapter 18) Strength Evaluation of Existing Structures (Chapter 20)

This afternoon Analysis and design of Flexure Shear Torsion Axial load

Design of slender columns Design of wall structures Tomorrow morning Design of slender columns Design of wall structures High-strength concrete 4

Overview of ACI 318M-11 Legal standing Scope Approach to Design Loads and Load Cases Strength Reduction Factors

Legal standing Serves as the legal structural concrete building code in the U.S. because it is adopted by the general building code (IBC).

Scope ACI 318M consists of 22 chapters and 6 appendices that cover all aspects of building design

Chapters 1. GENERAL REQUIREMENTS 2. NOTATION AND DEFINITIONS Scope, Contract Documents, Inspection, Approval of Special Systems 2. NOTATION AND DEFINITIONS

Chapters 3. MATERIALS Cementitious Materials, Water, Aggregates, Admixtures, Reinforcing Materials

4. DURABILITY REQUIREMENTS Freezing and Thawing, Sulfates, Permeability, Corrosion

5. CONCRETE QUALITY, MIXING, AND PLACING 6 5. CONCRETE QUALITY, MIXING, AND PLACING 6. FORMWORK, EMBEDMENTS, AND CONSTRUCTION JOINTS

7. DETAILS OF REINFORCEMENT Hooks and Bends, Surface Condition, Tolerances, Spacing, Concrete Cover, Columns, Flexural Members, Shrinkage and Temperature Steel, Structural Integrity Ronan Point gas explosion Precast elements not tied together March 11, 1968 22 stories

8. ANALYSIS AND DESIGN — GENERAL CONSIDERATIONS Design Methods; Loading, including Arrangement of Load; Methods of Analysis; Redistribution of Moments; Selected Concrete Properties; Requirements for Modeling Structures (Spans, T-beams, Joists...)

9. STRENGTH AND SERVICEABILITY REQUIREMENTS Load Combinations, Strength Reduction Factors, Deflection Control 10. FLEXURE AND AXIAL LOADS Beams and One-way Slabs, Columns, Deep Beams, Bearing

11. SHEAR AND TORSION 12. DEVELOPMENT AND SPLICES OF REINFORCEMENT

13. TWO-WAY SLAB SYSTEMS 14. WALLS

15. FOOTINGS 16. PRECAST CONCRETE

17. COMPOSITE CONCRETE FLEXURAL MEMBERS 18. PRESTRESSED CONCRETE

19. SHELLS AND FOLDED PLATE MEMBERS Kresge Auditorium Massachusetts Institute of Technology, shown from the rear, roof being replaced, designed by Eero Saarinen (1955)

20. STRENGTH EVALUATION OF EXISTING STRUCTURES 21 20. STRENGTH EVALUATION OF EXISTING STRUCTURES 21. EARTHQUAKE- RESISTANT STRUCTURES 22. STRUCTURAL PLAIN CONCRETE

Appendices A. STRUT-AND-TIE MODELS* B. ALTERNATIVE PROVISIONS FOR REINFORCED AND PRESTRESSED CONCRETE FLEXURAL AND COMPRESSION MEMBERS C. ALTERNATIVE LOAD AND STRENGTH REDUCTION FACTORS

D. ANCHORING TO CONCRETE. E. STEEL REINFORCEMENT INFORMATION F D. ANCHORING TO CONCRETE* E. STEEL REINFORCEMENT INFORMATION F. EQUIVALENCE BETWEEN SI-METRIC, MKS-METRIC, AND U.S. CUSTOMARY UNITS OF NONHOMOGENOUS EQUATIONS IN THE CODE

Approach to design Qd = design loads Sn = nominal strength Sd = design strength M = safety margin

Design Strength  Required Strength Sd = Sn  Qd Sd = design strength = Sn  = strength reduction factor  = load factors Qd = design loads  and  in Chapter 9 of ACI 318M

Loads Qd specified in ASCE 7, Minimum Design Loads for Buildings and Other Structures American Society of Civil Engineers (ASCE) Reston, Virginia, USA

Loads Dead loads (D)* Live loads (L)* Roof live loads (Lr)* Wind loads (W)  full load Earthquake loads (E)  full load Rain loads (R)* Snow loads (S)* * Service-level loads

Loads Impact – include in L Self-straining effects (temperature, creep, shrinkage, differential settlement, and shrinkage compensating concrete) (T) Fluid loads (F) Lateral soil pressure (H) Factored Load = U = Qd

Load cases and load factors by ASCE 7 and ACI 318M U = 1.4D U = 1.2D + 1.6L + + 0.5(Lr or S or R) U = 1.2D + 1.6(Lr or S or R) + (1.0L or 0.5W) U = 1.2D + 1.0W + 1.0L + 0.5(Lr or S or R) U = 1.2D + 1.0E + 1.0L + 0.2S

Load cases and load factors by ASCE 7 and ACI 318M U = 0.9D + 1.0W U = 0.9D + 1.0E

Load factors by ACI 318M If W based on service-level forces, use 1.6W place of 1.0W If E based on service-level forces, use 1.4E in place of 1.0E Details of other cases covered in the Code

Strength reduction () factors Tension-controlled sections 0.90 Compression-controlled sections Members with spiral reinforcement 0.75 Other members 0.65 Shear and torsion 0.75 Bearing 0.65 Post-tensioning anchorages 0.85 Other cases 0.60 – 0.90

Tension-controlled and compression- controlled sections

T-beam d h b hf bw As dt

Strain through depth of beam

Design Strength ( x nominal strength) must exceed the Required Strength (factored load) Bending Mn  Mu Axial load Pn  Pu Shear Vn  Vu Torsion Tn  Tu

Load distributions and modeling requirements

Chicago place

Structure may be analyzed as elastic using properties of gross sections Ig = moment of inertia of gross (uncracked) cross section Beams: Ib = ½ Ig  Iweb = Columns: Ic = Ig =

Analysis by subframes 1. The live load applied only to the floor or roof under consideration, and the far ends of columns built integrally with the structure considered fixed

2. The arrangement of load may be limited to combinations of factored dead load on all spans with full factored live load on alternate spans, and factored dead load on all spans with full factored live load on two adjacent spans

(a) (b) (c)

Moment and shear envelopes

Columns designed to resist axial forces from factored loads on all floors or roof and maximum moment from factored live loads on a single adjacent span of the floor or roof under consideration loading condition giving maximum ratio of moment to axial load More on columns

For frames or continuous construction, consider effect of unbalanced floor or roof loads on both exterior and interior columns and of eccentric loading due to other causes For gravity load, far ends of columns built integrally with the structure may be considered fixed At any floor or roof level, distribute the moment between columns immediately above and below that floor in proportion to the relative column stiffness

Simplified loading criteria

ln Beams, two or more spans Beams, two spans only Slabs, spans ≤ 3 m Beams,  col stiffnesses ≥ 8  beam stiffnesses

Max –ve left Max +ve Composite Max –ve right

Allowable adjustment in maximum moments for t  0.0075

Design of prestressed concrete (Chapter 18)

Behavior of reinforced concrete

Reinforced concrete under service loads

Theory of prestressed concrete Stresses

Methods of prestressing concrete members Pretensioning Post-Tensioning Pretensioned concrete is typically cast in a stressing bed where the tendons are pulled prior to placing the concrete. Post-tensioned concrete is stressed after the concrete has hardened.

Prestressing steels

Strength of prestressing steels available in U.S. Seven-wire strand: fpu  1725, 1860 MPa fpy (stress at 1% extension)  85% (for stress-relieved strand) or 90% (for low-relaxation strand) of fpu fpu = ultimate strength fpy = yield strength

Strength of prestressing steels available in U.S. Prestressing wire: fpu  1620 to 1725 MPa (function of size) fpy (at 1% extension)  85% of fpu

Strength of prestressing steels available in U.S. High-strength steel bars: fpu  1035 MPa fpy  85% (for plain bars) and 80% (for deformed bars) of fpu fpy based on either 0.2% offset or 0.7% strain

Maximum permissible stresses in prestressing steel Due to prestressing steel jacking force: 0.94fpy 0.80fpu manufacturers recommendation Post-tensioning tendons, at anchorage devices and couplers, immediately after force transfer: 0.70fpu

Prestressed concrete members are designed based on both Elastic flexural analysis Strength

Elastic flexural analysis Considers stresses under both the Initial prestress force Pi and the Effective prestress force Pe Note: = concrete compressive strength = initial concrete compressive strength (value at prestress transfer)

Classes of members U – uncracked – calculated tensile stress in precompressed tensile zone at service loads = ft  T – transition between uncracked and cracked < ft  C – cracked ft >

Concrete section properties e = tendon eccentricity k1= upper kern point k2= lower kern point Ic = moment of inertia Ac = area radius of gyration: r2 = Ic/Ac section moduli: S1 = Ic/c1 S2 = Ic/c2

Bending moments Mo = self-weight moment Md = superimposed dead load moment Ml = live load moment

Concrete stresses under Pi

Concrete stresses under Pi + Mo

Concrete stresses under Pe + Mo + Md + Ml

Maximum permissible stresses in concrete at transfer Extreme fiber stress in compression, except as in (b), Extreme fiber stress in compression at ends of simply supported members Extreme fiber stress in tension at ends of simply supported members * Extreme fiber stress in tension at other locations * * Add tensile reinforcement if exceeded

Maximum permissible compressive stresses in concrete at service loads Class U and T members Extreme fiber stress in compression due to prestress plus sustained load Extreme fiber stress in compression due to prestress plus total load

Flexural strength Aps T = Apsfps

Stress-block parameter 1

Stress in prestressing steel at ultimate Members with bonded tendons: p = Aps/bdp = reinforcement ratio b = width of compression face dp = d (effective depth) of prestressing steel

Members with bonded tendons and non-prestressed bars:

Members with unbonded tendons with span/depth ratios  35: but not greater than fpy or greater than fpe + 420 MPa fpe = stress in Aps at Pe =

Members with unbonded tendons with span/depth ratios > 35: but not greater than fpy or greater than fpe + 210 MPa

Loss of prestress Prestessing steel seating at transfer Elastic shortening of concrete Creep of concrete Shrinkage of concrete Relaxation of prestressing steel Friction loss due to intended or unintended curvature of post-tensioning tendons

Limits on reinforcement in flexural members Classify as tension-controlled, transition, or compression-controlled to determine  Total amount of prestressed and nonprestressed reinforcement in members with bonded reinforcement must be able to carry 1.2  cracking load

Minimum bonded reinforcement As in members with unbonded tendons Except in two-way slabs, As = 0.004Act Act = area of that part of cross section between the flexural tension face and center of gravity of gross section Distribute As uniformly over precompressed tension zone as close as possible to extreme tensile fiber

Two-way slabs: Positive moment regions: Bonded reinforcement not required where tensile stress ft  Otherwise, use As = Nc = resultant tensile force acting on portion of concrete cross section in tension under effective prestress and service loads Distribute As uniformly over precompressed tension zone as close as possible to extreme tensile fiber

Two-way slabs: Negative moment areas at column supports: As = 0 Two-way slabs: Negative moment areas at column supports: As = 0.00075Acf Acf = larger gross cross-sectional area of slab-beam strips in two orthogonal equivalent frames intersecting at the columns Distribute As between lines 1.5h on outside opposite edges of the column support Code includes spacing and length requirements

Two-way slabs Use Equivalent Frame Design Method (Section 13.7)

Banded tendon distribution Use efficient placement of tendons Place banded tendons above uniform tendons at column locations (common error) Common construction sequence of layout Banded tendons Uniform tendons Photo courtesy of Portland Cement Association

Development of prestressing strand development length = transfer length

Shear for prestressed concrete members is similar to that for reinforced concrete members, but it takes advantage of presence of prestressing force

Post-tensioned tendon anchorage zone design Load factor = 1.2  Ppu = 1.2Pj Pj = maximum jacking force  = 0.85 Strut and tie model

Strength evaluation of existing structures (Chapter 20)

Strength evaluation of existing structures (Chapter 20) When it is required When we use analysis and when perform a load test When core testing is sufficient Load testing

A strength evaluation is required when there is a doubt if a part or all of a structure meets safety requirements of the Code If the effect of the strength deficiency is well understood and if it is feasible to measure the dimensions and material properties required for analysis, analytical evaluations of strength based on those measurements can be used

If the effect of the strength deficiency is not well understood or if it is not feasible to establish the required dimensions and material properties by measurement, a load test is required if the structure is to remain in service

Establishing dimensions and material properties Dimensions established at critical sections Reinforcement locations established by measurement (can use drawings if spot checks confirm information in drawings) Use cylinder and core tests to estimate

Core testing

If the deficiency involves only the compressive strength of the concrete based on cylinder tests Strength is considered satisfactory if: Three cores are taken for each low-strength test The average of the three cores  No individual core has a strength <

Steel Reinforcing and prestressing steel may be evaluated based on representative material

If analysis is used, values of  may be increased Tension-controlled 0.90  1.0 Compression controlled 0.75 and 0.65  0.90 and 0.80 Shear and torsion 0.75  0.80 Bearing 0.65  0.80

Load test procedure Load arrangement: Select number and arrangement of spans or panels loaded to maximize the deflection and stresses in the critical regions Use more than one arrangement if needed (deflection, rotation, stress)

Load intensity Total test load = larger of (a) 1.15D + 1.5L + 0.4(Lr or S or R) (b) 1.15D + 0.9L + 1.5(Lr or S or R) (c) 1.3D In (b), load factor for L may be reduced to 0.45, except for garages, places of assembly, and where L > 4.8 kN/m2 L may be reduced as permitted by general building code

Age at time of loading  56 days

Loading criteria Obtain initial measurements (deflection, rotation, strain, slip, crack widths) not more than 1 hour before application of the first load increment Take readings where maximum response is expected Use at least four load increments Ensure uniform load is uniform – no arching

Take measurements after each load increment and after the total load has been applied for at least 24 hours Remove total test load immediately after all response measurements are made Take a set of final measurements 24 hours after the test load is removed

Acceptance criteria No signs of failure – no crushing or spalling of concrete No cracks indicating a shear failure is imminent In regions without transverse reinforcement, evaluate any inclined cracks with horizontal projection > depth of member Evaluate cracks along the line of reinforcement in regions of anchorage and lap splices

Acceptance criteria Measured deflections At maximum load: 24 hours after load removed:

Acceptance criteria If deflection criteria not met, may repeat the test (at least 72 hours after first test) Satisfactory if:

Provision for lower loading If the structure does not satisfy conditions or criteria based on analysis, deflection, or shear, it may be permitted for use at a lower load rating based on the results of the load test or analysis, if approved by the building official

Case study 1905 building Chicago, Illinois USA Cinder concrete floors Load capacity OK for use as an office building? Cinder = ashes of coal

Safety shoring

Deflection measurement devices CET=cable extension transducers – sometimes also called “string potentiometers”

Load through window

Moving lead ingots through the window

Load stage 14

Findings Floor could carry uniform load of 2.4 kN/m2 Building satisfactory for both apartments (1.9 kN/m2) and offices (2.4 kN/m2)

Summary Overview Prestressed concrete Strength evaluation of existing structures

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The University of Kansas David Darwin, Ph.D., P.E. Deane E. Ackers Distinguished Professor Director, Structural Engineering & Materials Laboratory Dept. of Civil, Environmental & Architectural Engineering 2142 Learned Hall Lawrence, Kansas, 66045-7609 (785) 864-3827 Fax: (785) 864-5631 daved@ku.edu