Lecture Lecture 1 1 Introduction Introduction Dr. Hazim Dwairi - PDF document

Prestressed Concrete Hashemite University The Hashem ite University Departm ent of Civil Engineering Lecture Lecture 1 1 – – Introduction Introduction Dr. Hazim Dwairi Dr Hazim Dwairi Dr Hazim Dwairi Dr. Hazim Dwairi Dr. Hazim Dwairi Dr. Hazim Dwairi The Hashemite University The Hashemite University Prestressed Concrete Prestressed Concrete Historical Background Historical Background • First patent of prestressed concrete was in First patent of prestressed concrete was in 1872 1872 by P.H. Jackson at San Francisco by P.H. Jackson at San Francisco - by P H Jackson at San Francisco by P H Jackson at San Francisco - he used a tie he used a tie he used a tie he used a tie rod (Prestressed) to construct a beam from rod (Prestressed) to construct a beam from individual concrete block. individual concrete block. • Early attempts of prestressing failed due to the Early attempts of prestressing failed due to the loss of prestressing force with time loss of prestressing force with time – – a better a better understanding of losses was needed in addition understanding of losses was needed in addition understanding of losses was needed, in addition understanding of losses was needed, in addition to, high strength steel. to, high strength steel. • In In 1928 1928, E. , E. Freyssinet Freyssinet of France started modern of France started modern development of prestressed concrete. development of prestressed concrete. Dr. Hazim Dwairi Dr. Hazim Dwairi The Hashemite University The Hashemite University Prestressed Concrete Prestressed Concrete Dr. Hazim Dwairi 1

Prestressed Concrete Hashemite University Everyday Life Exam ples Everyday Life Exam ples • Force Force- -fitting of metal fitting of metal bands on wooden bands on wooden bands on wooden bands on wooden barrels barrels • Pre P Pre- -tensioning the tensioning the i i i i h h spokes in a bicycle spokes in a bicycle wheel wheel Dr. Hazim Dwairi Dr. Hazim Dwairi The Hashemite University The Hashemite University Prestressed Concrete Prestressed Concrete Definition Definition • Prestressed is a form of concrete in which Prestressed is a form of concrete in which internal stresses are introduced by means of a internal stresses are introduced by means of a high strength pre high strength pre strained reinforcement high strength pre high strength pre-strained reinforcement. strained reinforcement strained reinforcement. Prestressing relies on bond and/or bearing Prestressing relies on bond and/or bearing mechanisms to achieve stress transfer to mechanisms to achieve stress transfer to concrete. concrete. • Prestressing force induce internal actions of Prestressing force induce internal actions of such magnitude and distribution to counteract such magnitude and distribution to counteract such magnitude and distribution to counteract such magnitude and distribution to counteract the external loading. In Prestressed concrete the external loading. In Prestressed concrete members, steel is in tension and concrete is in members, steel is in tension and concrete is in compression, even before the application of any compression, even before the application of any external loading. external loading. Dr. Hazim Dwairi Dr. Hazim Dwairi The Hashemite University The Hashemite University Prestressed Concrete Prestressed Concrete Dr. Hazim Dwairi 2

Prestressed Concrete Hashemite University Reasons for Prestressing Reasons for Prestressing • Prestressed concrete has been developed to Prestressed concrete has been developed to overcome some of the limitations of Reinforced overcome some of the limitations of Reinforced Concrete namely: Concrete namely: Concrete, namely: Concrete, namely: � In flexure of reinforced concrete member, concrete is In flexure of reinforced concrete member, concrete is cracked and functions only to hold the reinforced bars cracked and functions only to hold the reinforced bars in place and protect them from carrion, thereby, in place and protect them from carrion, thereby, adding excess weight without additional strength. adding excess weight without additional strength. � Cracking lowers the moment of inertia of the section, Cracking lowers the moment of inertia of the section, th thereby increasing deflection. Prestressing eliminates th thereby increasing deflection. Prestressing eliminates b i b i i i d fl d fl ti ti P P t t i i li li i i t t cracks. cracks. � make use of the high strength in tension of make use of the high strength in tension of prestressing steel strands which is prestressing steel strands which is 270 270Ksi = Ksi =1860 1860MPa MPa (four to five times of that of conventional steel). (four to five times of that of conventional steel). Dr. Hazim Dwairi Dr. Hazim Dwairi The Hashemite University The Hashemite University Prestressed Concrete Prestressed Concrete Reasons for Prestressing Reasons for Prestressing � Eliminate cracking at service loading conditions. Eliminate cracking at service loading conditions. � Improve shear and Improve shear and torsional torsional strengths. strengths. � Add protection to the steel. Add protection to the steel. Dr. Hazim Dwairi Dr. Hazim Dwairi The Hashemite University The Hashemite University Prestressed Concrete Prestressed Concrete Dr. Hazim Dwairi 3

Prestressed Concrete Hashemite University Full & Partial Prestressing Full & Partial Prestressing • Full prestressing: sufficient precompression to Full prestressing: sufficient precompression to ensure “crack free” at full design load. Freyssinet ensure “crack free” at full design load. g Freyssinet y 1930 1930. . • Partial prestressing: precompression is not Partial prestressing: precompression is not enough to prevent cracks under full design load. enough to prevent cracks under full design load. Thus, the member will normally contain some Thus, the member will normally contain some conventional steel bars. conventional steel bars. ti ti l t l t l b l b • In many cases, partial prestressing improves the In many cases, partial prestressing improves the structural performance and is commonly used. structural performance and is commonly used. Dr. Hazim Dwairi Dr. Hazim Dwairi The Hashemite University The Hashemite University Prestressed Concrete Prestressed Concrete Reinforced Concrete Mem bers Reinforced Concrete Mem bers C c h d jd A s T 1 1. “jd” almost constant under increasing load “jd” almost constant under increasing load 2. “T & C” increase proportionally to applied load Dr. Hazim Dwairi Dr. Hazim Dwairi The Hashemite University The Hashemite University Prestressed Concrete Prestressed Concrete Dr. Hazim Dwairi 4

Prestressed Concrete Hashemite University Prestressed Concrete Mem bers Prestressed Concrete Mem bers C c t h h C a 2 c b a 1 A ps T C Dead + Live No Load Dead Load M =T a 2 = C a 2 M =T a 1 = C a 1 1. Internal lever arm “a” increases under the applied load. 2. T&C remain virtually constant under working load conditions Dr. Dr. Hazim Hazim Dwairi Dwairi The Hashemite University The Hashemite University Prestressed Concrete Prestressed Concrete Plain Concrete Plain Concrete h f t b 2 bh = = M M f t r t 6 6 = ' 2 ( 0 . 1 f )( 0 . 617 bh ) c = 2 ' 0 . 0167 bh f c Dr. Hazim Dwairi Dr. Hazim Dwairi The Hashemite University The Hashemite University Prestressed Concrete Prestressed Concrete Dr. Hazim Dwairi 5

Prestressed Concrete Hashemite University Reinforced Concrete (Service Reinforced Concrete (Service lim it state) lim it state) C (0.9-0.12) h 0.9h h h =0.78h 0.78h T b 1 = ' C ( 0 . 45 f )( b )( 0 . 36 ) c 2 = = 2 ' 0 0 . 081 081 bh bh f f c = = 2 ' ( 0 . 78 ) 0 . 0632 M C h bh f 2 c M is 3 . 78 times l arg er 2 ⎧ 2 ' 0 . 0617 bh f due to D.L moment = 2 ' ⎨ c 0 . 0632 M bh f 2 c 2 ' ⎩ 0 . 0465 bh f due to L.L moment c Dr. Hazim Dwairi Dr. Hazim Dwairi The Hashemite University The Hashemite University Prestressed Concrete Prestressed Concrete Prestressed Prestressed concrete (working concrete (working lim it state) lim it state) ' 0 . 45 f c C 0.9h 0.56h 0.56h C C 0.66h T 0.23h ' 0 . 45 f c PS+DL+SI+LL PS+DL+SI 3 = = = ( 0 . 56 ) ' ' M C h C 0 . 5 ( 0 . 45 f ) bh 0 . 225 bhf c c = ' ( ( 0 0 . 225 225 bhf bhf )( )( 0 0 . 56 56 h h ) ) with i h superimpos i ed d DL DL : c = = 2 ' a 0.23h ( 0 . 126 bh f ) c + = with superimpos ed DL LL : 2 M ( RC element) 2 = = a 0.56h 7 . 5 M ( Plain concrete element) 1 Dr. Hazim Dwairi Dr. Hazim Dwairi The Hashemite University The Hashemite University Prestressed Concrete Prestressed Concrete Dr. Hazim Dwairi 6