Module 01 Fall 2010

CIE 525 Reinforced Concrete Structures Instructor: Andrew Whittaker

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1. ADMINISTRATIVE MATTERS, COURSE CONTENT, AND INTRODUCTION

1.1 Administrative Matters

Instructor:

Andrew Whittaker 212 Ketter Hall, North Campus Email: [email protected]

Schedule:

Fall 2010 Lectures: Tu, Th; 3:30 pm to 4:50 pm; Clemens 4 Review session, Tu, Th; 12:30 pm to 1:50 pm; TBD One 80-minute midterm examination One 3-hour final examination to be scheduled during the examination period

Office Hours:

Tu, Th, 2:00 pm to 3:30 pm; 230 Ketter Hall

Grading:

Homework counts 35 points. The midterm exam counts 25 points. The final exam counts 40 points. Letter grades will depend on position in class and knowledge of subject matter.

Student Conduct:

Student conduct is governed by the rules of the University and students are expected to know and abide by the University policies on academic honesty and integrity. These policies state "...students are responsible for the honest completion and representation of their work, for the appropriate citation of sources, and the respect of other's academic endeavors. By placing their name on academic work, students certify the originality of all work not otherwise identified by appropriate acknowledgements." Violation of these policies is subject to penalties that include receiving a failing grade in the course, suspension, and dismissal.

1.2 Course Content

1.2.1 Emphasis of Class

The objective of course CIE 525 is to develop an advanced understanding of reinforced concrete structures. The primary focus will be on behavior, analysis, and design of components, elements, and systems that are common in building structures. Emphasis will be placed on seismic design.


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1.2.2 Lecture Topics

Topics to be covered should include the following:

• Design approaches

• Materials

• Moment-curvature analysis

• Component response to flexure, axial, and shear loads

• Bond and anchorage

• Yield line analysis of slabs

• Strip-method analysis of slabs

• Strut-and-tie models

• Design of buildings and frame-building components for gravity and earthquake effects

• Design of nuclear structures for earthquake effects

1.2.3 Reading

There are no assigned textbooks for this class because no textbook covers all of the material that will be addressed in CIE 525. However, students are strongly encouraged to purchase a copy of the ACI 318 Building Code and Commentary, 2008 Edition.

The textbooks listed below provide useful reference material for the class.

1. Wight, J. K. and MacGregor, J. G., 2009, Reinforced Concrete Mechanics and Design, 5th Edition

2. Priestley, M. J. N. and Paulay, T., 1992. Seismic Design of Reinforced Concrete and Masonry Buildings, John Wiley

3. Priestley, M. J. N., Seible, F., and Calvi, G. M., 1996, Seismic Design and Retrofit of Bridges, Wiley InterScience

4. ACI, 2002, Examples for the Design of Structural Concrete with Strut-and-Tie Models, ACI Special Publication 208, Farmington Hills, MI.

5. ACI, 2006, Code Requirements for Safety-Related Nuclear Structures and Commentary, Farmington Hills, MI

Other reading, including journal papers and conference proceedings, will be assigned on a topic-by-topic basis.


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Lecture Module Topic

1-8

1 Course outline, design approaches

2 Reinforced concrete materials

3 Confined reinforced concrete

4 Moment-curvature analysis

5 Response of components to flexure, and axial and shear loads

6 Bond and anchorage

8-18

7 Strut and tie models

8 Yield line analysis of slabs

8 Strip-method analysis of slabs

19 - Midterm examination

20-30

10 Seismic analysis and design: a primer

11 Design of buildings with frame and slab systems; behavior under gravity and earthquake loads

11 Design of buildings with frames; behavior and design of beams, columns, and joints

11 Design of buildings with walls; wall behavior and design

12 Design of reinforced concrete nuclear structures


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1.3 Design of Structural Framing Systems

1.3.1 Procedures for Component Evaluation

Below is an introduction to procedures that are used for proportioning reinforced concrete cross sections for gravity and lateral loads. For additional information, refer to Chapter 2 of the MacGregor text.

1.3.2 Allowable Stress Design (ASD)

Allowable Stress Design (ASD), which is also known as Working Stress Design, has been used for structural engineering analysis for more than 150 years. Best estimates of maximum loads are applied to a linearly elastic model of a structure for the calculation of member stresses (for steel) or stresses in concrete and rebar (in reinforced concrete). The member stresses are required to be less than service values (e.g., 0.6Fy for a steel component) that are established for each material for different actions (axial, bending, shear, torsion). Sample information is presented in the picture to the right (courtesy of J. P. Moehle).

The ASD method has a significant number of shortcomings. First, the reliability of the design (or safety index) is unknown. Second, no account is taken of the uncertainties in the loads, that is, how accurate are the estimates of the dead and live loads. Third, member stresses provide little information on the capacity of a component and the structure to resist the applied loads. In modern reinforced concrete design, allowable stresses are rarely used: deflection calculations under service loads being one exception. We will not use the ASD procedure to proportion cross sections in CIE 525.

1.3.3 Strength Design (SD) or Load and Resistance Factor Design (LRFD)

Strength Design (SD) or LRFD is routinely used for the design of reinforced concrete structures and is used by many engineers for the design of steel structures (although ASD persists in many parts of the US). Loads are factored to calculate an ultimate load, where the load factors are based on a statistical interpretation of measured conditions and thus reflect plausible variations in the loads (i.e., maximum values) from the mean estimates of the loads. Load factors are greater for live loads than dead loads for example. The ultimate load is

Allowable Stress Designw

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M

V = wl/2

M = wl2/12

l

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s

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fs

fs’

allfI

Mcf

Linear Structural Analysis

Linear Analysis of Cross Section

Verification:

no load factors

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M

Vu = wul/2

Mu = wul2/12

l

c

s

s’

fs

fs’

nu

nu

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Nonlinear Analysis of Cross Section

Verification:

factored loads

Strength Design


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then applied to a linearly elastic model of the structure to calculate component actions. Component capacities (i.e., axial, flexure, shear) are calculated assuming some inelastic behavior of the cross section. Sample information is presented in the above picture (courtesy of J. P. Moehle). Note the use of a non-linear stress block (although the shape is simplified to facilitate calculation of the strength of the cross section).

The SD procedure is more rational than the ASD procedure. Uncertainties in the loads are considered through the use of load factors and load combinations. Load factors from ACI-318-05 are presented in the box to the right. Contrast these combinations with those of ASD. The consequences of failure can be considered more directly through the use of capacity reduction (phi) factors, with small values of phi assigned to undesirable failure modes (e.g., 0.9 for flexure and 0.75 for shear). Note however that the analysis assumes linearly elastic response but that component capacities are calculated at the strength level, which implies some measure of inelastic response in the cross section.

Moment redistribution in beams as an example

1.3.4 Capacity Design

Capacity design is used to prevent undesirable failure mechanisms, for example, a beam failing in shear (a brittle mode of failure) before it fails in flexure (a ductile mode of failure), and a column failing in flexure (compromising the gravity load system) before the beams framing into the column fail in flexure. Many have attributed capacity design to expert engineers in New Zealand in the 1970s but such an approach was first proposed, to my knowledge, by Blume, Newmark, Corning, and Sozen in the late 1950s (see Design of Multistory Reinforced Concrete Buildings for Earthquake Motions published in 1961).

The figure to the right (courtesy of J. P. Moehle) provides summary information on capacity design. The example is for a cantilever reinforced concrete beam where the objective is to prevent shear failure of the beam. Key steps in the procedure are as follows:

1. Select the desired failure mechanism, which is usually flexure in reinforced concrete construction. <Why?>

5. Determine resulting forces

6. Design to avoid failures otherthan selected mechanism

M

curvature

Mn

1. Flexural yield mode2. Design for flexure

3. Detail for ductile response

4. Estimate overstrength.

Capacity Design

Mu

Vu Vp

Mn Mu

Mp

Mp

Vp


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2. Proportion the component (beam) for that failure mechanism using strength design for the factored loads and detail the component for ductile response. (We will discuss how to do this later in the semester.)

3. Determine the probable strength of the cross section by analysis accounting for actual sizes and selected rebar, which may be larger than that required to resist the effects of factored loads. (We will learn how to do this in Module 3.) In the figure above, the probable strength is Mp that is substantially greater than the design strength.

4. Determine the applied load required to produce the probable strength and design the remainder of the component (i.e., for shear in the sample problem) so that the nominal (shear) strength exceeds the actions associated with this back-calculated applied load.

1.3.5 Plastic Design

Plastic design is merely strength design using plastic analysis rather than linearly elastic analysis. Plastic analysis is covered in detail in other classes, but we will use such methods for the yield-line analysis of slab systems. For plastic analysis, a mechanism is proposed and the plastic hinges are detailed for inelastic response. See the picture to the right (courtesy of J. P. Moehle) for sample information. Component strengths are calculated using SD. Undesirable failure modes are then avoided using capacity design.

1.3.6 Recent Developments in Component Evaluation

The 1990s saw remarkable innovation in the practice of earthquake engineering. Force based design procedures that had been used almost exclusively for 70 years started to give way to displacement-based procedures that had been developed in principle by Sozen, Moehle, and others in the 1970s and 1980s. It had long been recognized that code-compliant buildings and bridges would undergo substantial inelastic deformation in a design earthquake. Given this knowledge and the clear understanding that damage was related directly to deformations and not forces (see the drawing to the right, courtesy of J. P. Moehle), expert structural engineers have moved towards analysis, design (proportioning), and evaluation based on estimates of displacements. Displacement-based design (DBD) cannot be used alone as a design tool.

wu

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M

Vu = wul/2

Mo = wul2/8

l

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s

s’

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fs’

no

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2


Nonlinear Analysis of Cross Section

Verification:

factored loads

Plastic Design

Mn

Mn

Force

Displacement

spalling

collapse

yielding


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Rather, a minimum level of strength must be provided for service load conditions. However, DBD has seen widespread acceptance in the past 5 years and this procedure now underpins much of the FEMA 273/274/356 and ASCE-41 documents that provide guidelines for the seismic rehabilitation of structures.

Documents

Module 01 Fall 2010