Building Code Provisions for Precast/Prestressed Concrete: a Brief History
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HISTORICAL-TECHNICAL SERIES Building Code Provisions for Precast/Prestressed Concrete: A Brief History Thomas J. DArcy, RE., FPC This article traces the evolution of building code Consulting Engineer The Consulting Engineers Group, Inc. provisions for precast/prestressed concrete in the San Antonio, Texas United States. The first part presents the influence of European practices, then discusses American developments, PCI initiatives in writing code provisions and the role of the AC! Building Code. The latter part discusses the emergence of the mode! building code provisions with particular emphasis on seismic design issues. George D. Nasser, RE. ack in 1949-1950, when the Walnut Lane Memorial Editor Emeritus Bridge was being constructed in Philadelphia, Penn Precast/Prestressed Concrete Institute sylvania, prestressed concrete was not recognized by Chicago, Illinois B the ACT Building Code nor by any other official jurisdic tion in the United States. (It is generally recognized that it was the excitement and publicity generated by the Walnut Lane Bridge, the first major prestressed concrete structure in North America, that gave birth to the precastlprestressed concrete industry in the United States.) But before we di gress any further, let’s go back to the origins of prestressed concrete. European Influence In 1936, the French pioneer Eugene Freyssinet, generally S.K. Ghosh, Ph.D., FPCI regarded as the “father” of prestressed concrete, announced President at a special meeting before the British Institution of Struc S.K. Ghosh Associates, Inc. in London that by combining concrete with Northbrook, Illinois tural Engineers high strength prestressing steel he had discovered a com pletely new material possessing properties very different 2 This new from those of ordinary reinforced concrete.” “revolutionary” material would always be in compression 116 PCI JOURNAL for CriterIa for Prestressed Concrete Bridges Criteria DESIGN Temporary stresses Temporary stresses before creep and shrinkage shall not exceed the following: I Concrete: ipretensioned extreme 0. 6Sf’,, Compression in fiberlt-.nsiofled 0.5Sf’,, Tension 0. 05f’, Prcstresoing steel: Tension 0. 8Sf’. Stress under dead, live, or impact load - Stress after creep and shrinkage under dead, live, or impact load, or any corn Bridges bination of these forces, shall not exceed the following: Concrete: Compression in extreme fiber 0.1f’, Tension is extreme fiber 0 Where the computations show tension in the extreme fiber, unprestressed reinforcement may be used, and designed to take the total tensile stresses, provided that the computed tension in the concrete before the unprestressed steel is added ‘toes not exceed 0.081’,. Prestressing steel 0. 6f’. or 0. 8f’.,,, whichever is ieee. Creep, shriuleage, and elastic deformation Decrease In prestress in steel due to creep, shrinkage, and elastic deformation shall be assumed to be as follows: Pretensioncd coecrete 6,000+16f,.+0.04f.,. Pool-tensioned concrete 3,000+llf,.+0.04f.s In these criteria the efficiency of the anchorage has been assumed to be 100 percent. The designer should add to the figure given for creep and shrinkage as amount sufficient to allow for the anchorage efficiency, as determined by test. Light-weight aggregate: An amount to be determined by tests. Decrease in prestress due to friction Where the prestressing steel is ‘draped’ and wherever miuor irregularities occur in the alinement of the ducts, the stress in the interior of the beam will be somewhat less than that at the jack, due to friction. This loss shall be estimated DEPARTMENT OF COMMERCE and verified in the field as given in the section on construction under the heading U. S. “Post-tensioning method (p. 5). A guide to the estimation of the loss will be found in the discussion. BUREAU OF PUBLIC ROADS A eotniloe eppeus on Poe, vi. WASHINGTON’ 1954 Fig 1. U.S. Bureau of Public Roads Criteria for Prestressed Concrete Bridges (1954). and thus would not allow tensile stresses or cracking under crete, i.e., members reinforced by a combination of pre any service loads. [It should be appreciated that Freyssinet’s stressing steel and mild steel reinforcement, that allowed concept (including some applications) of prestressed con some tension under service load, could perform very well crete occurred much earlier than 1936, which was inspired even in a cracked state.35 His tests showed that partially pre in connection with his work on time-dependent deforma stressed concrete beams could withstand tensile stresses as tions of reinforced concrete arch bridges. However, his Lon high as 750 psi (5 MPa) under service loads. don lecture was the first time that the English-speaking This concept was further reinforced when a partially pre world became fully aware of the significance of his work on stressed concrete beam was built on the roof of a London the potential of prestressed concrete.] train station. This beam was purposely allowed to develop Word of Freyssinet’ s concept of prestressed concrete, to cracks during service loads. These cracks were held open gether with its applications, gradually reached the outside with stainless steel razor blades. The beam was exposed to world, but its full implementation was, unfortunately, inter acidic smoke from coal-fueled locomotive trains for several rupted by the onset of World War II. However, interest in years. The end result was that the beam performed very prestressed concrete took on a new dimension after the war, well, showing no major signs of distress. especially because of the pressing need to build new bridges Practitioners also discovered that prestressed concrete and buildings due to the wartime destruction of the Euro beams, designed for compression only, were vulnerable to pean infrastructure. At the same time, there was a world excessive camber as well as long-term creep and shrinkage. wide shortage of structural steel. Thus, prestressed concrete Thus, the concept of allowable tension was born, which pre provided an efficient and economical solution to Europe’s vails in today’s concrete codes. rebuilding program. In the post-war years, several European researchers and practitioners questioned whether prestressed concrete mem American Developments bers needed to be in total compression during their service Returning now to the Walnut Lane Bridge, this structure life. A change in concept was particularly advocated by Paul was designed by Professor Gustave Magnel of Belgium. The Abeles in England. Based on research and his work with design specifications were basically European. The anchor British Railways, he showed that partially prestressed con- age hardware used was the Magnel system, a patented sys November-December 2003 117 V V STRESSES V Section 4. DESIGN V S1adazd SPECIFICATIONS V (A) PRESTRESSING STRAND ANDVijWIRE (1) Initial stresses shall not 70% of minimum ultimate strength for - V PreTedned —Vdopted the PRESTRESSED CON stress-relieved strand and/or wire. Bonded Peâtressed Concrete by V For V CRETE INSTITUTE. Octobex-7th. 1954. effective November 7, 1954. Amended MaccIt 7, 1955. VV:VV (2) 1ss in im prestreus due to creep, shge and plamic deforemtion V :VV.. V shallbeasaumednotlessthanl6%. V V : - V V Section 1 SCOPE (B)V CONCRETE - Pre (1) Maximum allowable stresses in concrete at the thne of transfer of per- (A) - .Thesc specificatktns rover the design and use of Pre-tenstoned Bonded V V shall be as follows: V these speer stressing V : - stressed Concrete, in-any structure to be erected under the provisioes of - ficadons. ;Z. - - V Compression in Bridge Members : VV’ . 0.50 ff Compression to Building Members 0 55 Tension ç•, . 0.06 f V V ‘V V Unless additional is taken by reinforcing steel. :-. Section 2 DEFINITIONS V - refers to the concrete (A) The term ‘Pr ensioned Bonded PcestrcssedConceete’ V (2) Maximum allowable stresses under fmsl dead and live load cosditions - V V haedening of V - inwltich the prcstressing strands astdjor- wire are tensioned, before the shall be as follows: - V V V V V - cnesete.- between fixed abutioents in a preotressing bed, or against strong rnoUlds. V VVV When the concrete has haidened, the connection between the strands (and/or wire), V Compression in bridge embers 0.40 f V V 0.44 f and the abutments are ieleused and the pre.kstsionett strands (and/or wires) will Compression in building members V fiber in bridge members . 0. contract and thus to create nsainly internal cmpseusive stress in concrete through : Tension in bottom V V V V V ... V . .- V f Tension in bottom fiber in building members : 0.05 bond between the strands and the concrete Tensionintopflbcr 0.04f Unless the additional is carried by reordng steel, (B) - The definitions of a11 other teemr pertaining to prestressed concreteshail V - V V more f but not than 0.08 V V V Committee V conform to the latestreportof Joint ACI-ASCE 323 - tension Diagonal V V - V V VV -, - (C) When concrete of light weight aggregate is used, data on stress losses due to Fig. 2. PCI’s first Specifications for Pretensioned Bonded creep, shrinkage, and plastic deformation should be presented and these stress losses Prestressed Concrete (1954). used instead of those listed under 4 (A) (2). V the ACT Building Code. Nevertheless, interest in prestressed concrete was evident as early as 1944 by the formation of P 1 I STBB11I1B BUI[BIO COAf the ACI-ASCE Joint Committee 323 (later 423) on Pre stressed Concrete. This committee was to play an important fOfi PflfSTf8Sf0 COIICBETE role in the formulation of provisions for prestressed concrete 14 years later (1958). Based primarily on the work of Eric L. Erickson, in PBfSfTfB fOB BfVItLU Louisiana, the U.S. Bureau of Public Roads (the precursor of the Federal Highway Administration) published in 1954 the Copies of our tentative Building Code have been distributed Criteria for Prestressed Concrete Bridges (see Fig. 1). This to our membership and to registrants at the PCI Fifth Annual a major impact on the future of pre Convention.