Prestressed Concrete Slab Design Example
Stephaine Zitzow
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TRB's National Cooperative Highway Research Program (NCHRP) Report 517: Extending Span Ranges of Precast Prestressed Concrete Girders contains the findings of research performed to develop recommended load and resistance factor design procedures for achieving longer spans using precast prestressed concrete bridge girders.
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Reinforced concrete and prestressed concrete are both reinforced with longitudinal and transverse steel bars, also known as rebar. The main function of the reinforcement is to strengthen concrete when it undergoes tensile stress.
Reinforced concrete, or RC, is a composite material used in construction. The low tensile strength and ductility of the concrete are fortified by the addition of reinforcing steel bars having higher tensile strength and ductility. During construction, steel bars are placed inside formwork before concrete is poured. Rebar can also be wired together into a steel cage arrangement beforehand. Concrete is then poured into the formwork and vibrated to remove air voids in the fresh concrete and ensure consolidation of the aggregates within the concrete mixture. It is imperative that the concrete completely surrounds each bar to ensure a strong bond.
Reinforced Concrete is widely used due to its workability, strength, and availability of its raw materials. It is mainly used as the main members of a particular structure such as columns, piers, piles, beams, slabs, and footings for buildings, houses, dams, bridges, and other similar structures. Reinforced concrete is easily configured to unconventional shapes because it fills the container that it is supporting. This leads to extravagant architectural structures that would otherwise be difficult to build with other materials such as steel and wood. Reinforced concrete is also typically used in public works construction of highway paving and sidewalks. Reinforcing the concrete with steel bars gives the composite section tensile strength that allows for a robust and useful composite building material.
Simply put, it is concrete formed under stress. Reinforcement bars are placed in a form and stressed by the stretching of the bars at each end, inducing tension in the bar. Concrete is poured into the form and all around the bars while they are still being stretched. When they are released, the steel tries to resume its original, shorter, length, and adds a compressive force to the concrete laterally, giving it the strength to span longer distances than normal reinforced concrete.
SkyCiv offers an easy-to-use reinforced concrete design software to help analyze and design reinforced concrete members. Using SkyCiv Beam Software, you can analyze the loads on the member, then design your concrete member using our Reinforced Concrete Design Software.
The CPCI 5th Edition Precast Concrete Design Manual is the ultimate publication covering the design, manufacture and installation of precast reinforced and prestressed concrete. It is an essential resource for every precast concrete project.
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CPCI launched its first ever webinar series to present each chapter of the CPCI Fifth Edition Design Manual. The webinars were presented by the Chapter Editors to CPCI members, engineers, owners/developers, designers and AEC professionals, from May to October, 2018, and are still available for viewing.
CHAPTER 3 - DESIGN OF ELEMENTS
The chapter on element design has required revisions due to changes in A23.3 and NBCC 2015. Sections dealing with slabs, hollowcore and both prestressed and non-prestressed beams have been examined to include recent revisions in A23.3. A new example on partial prestressing has been added, and all examples dealing with shear and torsion have been evaluated in the context of the new A23.3 code. Fourth edition errata have been included in this edition, and design examples, graphs and figures have been updated throughout.
CHAPTER 4 - DESIGN OF CONNECTIONS
Design examples and calculations have been updated throughout. The example design of the cazaly hanger has been extensively updated from the last edition. Two new design examples are included; A design example for the baseplate and anchor bolt sizing of a column connection has been added; A design example for strength analysis of a weld group has also been added.
CHAPTER 5 - ARCHITECTURAL PRECAST CONCRETE
The design of architectural precast concrete has been updated to reflect current industry practices. Important new publication references include CPCI Architectural Precast Concrete Walls: Best Practice Guide (2017) and two new precast building envelope guides by RDH Building Science Inc., Maintenance and Inspection Manual for Precast Concrete Building Enclosures (2016), and Meeting and Exceeding Building Code Thermal Performance Requirements (2017).
Design of prestressed Concrete flat slabs.pdfThe South African Institution of Civil EngineeringPostnet- Suite 81Private bag X65Halfway House 1685South AfricaThis Report is intended to serve as a manual of good practice for the design of prestressed concrete flat slabs..In addition to the recommended procedures, other methods are described for the sake of completeness and to comparedifferent methods of design.The Report was produced by a sub-committee of the Joint Structural Division of the South African Institution of CivilEngineers, and the Institution of Structural Engineers.PRESTRESSED CONCRETE FLAT SLABS1.0 IntroductionIn 1989 the Structural Division of the South African Institution of Civil Engineers created a sub-committee to examinethe design of prestressed concrete flat slabs. It was found that a certain amount of poor design was prevalent, and thecommittee decided to produce a booklet of recommendations for good practice.The matter was considered especially important because the South African Loading Code was changed with effect from1990, and the required factor on D.L. is now 1.2, whereas it was previously 1.4. This has the effect of reducingreinforcement areas, and cracking and deflection require more attention. To make allowance for this, SABS 0100 wasrevised, and among other changes, the allowable concrete shear stress was reduced by 10 percent, to lessen the probabilityof brittle shear failures.1.1 Flat SlabsFlat slabs were originally invented in the USA at the beginning of this century, and there were a number of patentedsystems.The early reinforced concrete flat slabs all had drops, and columns with capitals, and were considered to be the structureof choice for warehouse construction and heavy loads. Because of the columns capitals and drops, shear was not really aproblem.Design was based on tests on stresses in reinforcement at working loads, and the early codes required a total moment ina span of WL2/11.It was realized that statically a total moment of about WL2/8 was required for equilibrium, (If the column diameter is D,the statically required moment is (very closely) W(L-2D/3)2/8 where L-2D/3 is the effective span. The difference betweenWL2/11 and WL2/8 was attributed to a mystical '2 way action'. In fact it was due partly to tensile stresses in the concreteand partly to arching effects reducing the measured stress in the reinforcement.The philosophy, and the empirical coefficients, persisted until the 1950's when the allowable stresses in reinforcementwere increased, limit state design was introduced, and the statically required moment of WL2/8 was introduced into thecodes. This was because it was felt that it was not safe to rely on arching or tensile strength of the concrete. In additionto the changed moment coefficients, the frame method of analysis was required in certain cases.1.2 Flat PlatesFlat plates were subsequently developed, with no drops and no column capitals/Read less
The dynamic load allowance replaces the effect of impact used in AASHTO Standard Specifications. It accounts for wheel load impact from moving vehicles. For slabs, the static effect of the vehicle live load shall be increased by the percentage specified in Table below.
LRFD Table A13.2-1 (2020 specifications) is not up-to-date with the latest MASH 2016 criteria. NCHRP Project 20-7, Task 395 (TTI Project 607141), MASH Equivalency of NCHRP Report 350-Approved Bridge Railings released the following table of updated loads. This table may not reflect completely the values that will get implemented in the AASHTO LRFD Bridge Design Specifications. For example, further testing has shown that the Rail Height, H, for TL-3 may be 30 inches. There is also ongoing research that will effectively increase the capacity of overhangs in collision events.
Until both the new loads and new resistances are implemented in LRFD, the standard top transverse reinforcement scheme shown in EPG 751.10.1.7 Standard Bridge Deck Details is considered adequate for collision loads in new bridge decks. The top transverse reinforcement scheme is also considered adequate for collision loads for redecks where the effective depth to the top transverse bar is not less than 4 3/8 inches.
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