1、ACI 347.2R-05 became effective July 29, 2005.Copyright 2005, American Concrete Institute.All rights reserved including rights of reproduction and use in any form or by anymeans, including the making of copies by any photo process, or by electronic ormechanical device, printed, written, or oral, or r
2、ecording for sound or visual reproductionor for use in any knowledge or retrieval system or device, unless permission in writingis obtained from the copyright proprietors.ACI Committee Reports, Guides, Standard Practices, andCommentaries are intended for guidance in planning,designing, executing, an
3、d inspecting construction. Thisdocument is intended for the use of individuals who arecompetent to evaluate the significance and limitations of itscontent and recommendations and who will acceptresponsibility for the application of the material it contains.The American Concrete Institute disclaims a
4、ny and allresponsibility for the stated principles. The Institute shall notbe liable for any loss or damage arising therefrom.Reference to this document shall not be made in contractdocuments. If items found in this document are desired by theArchitect/Engineer to be a part of the contract documents
5、, theyshall be restated in mandatory language for incorporation bythe Architect/Engineer.347.2R-1Guide for Shoring/Reshoring of ConcreteMultistory BuildingsReported by ACI Committee 347ACI 347.2R-05This guide presents information and design criteria for shoring/reshoringoperations during the constru
6、ction of reinforced and post-tensioned multi-story buildings. It provides methods for developing safe constructionschedules and provides design examples. It is written for the use of form-work engineer/contractors and engineer/architects.Keywords: construction loads; falsework; form removal, formwor
7、k; post-tensioning; reshoring; shoring.CONTENTSChapter 1Introduction, p. 347.2R-2Chapter 2Shoring/reshoring construction needs, p. 347.2R-22.1Definitions2.2Advantages of reshoring2.3Types of forming systemsChapter 3Construction loads on formwork,p. 347.2R-33.1Construction loads3.2Load combinations3.
8、3Typical phases of construction3.4Construction load distribution3.5Application of the simplified method3.6Factors affecting the construction load distribution3.7Post-tensioning load redistributionChapter 4Strength adequacy of concrete slabs and formwork, p. 347.2R-84.1Early-age concrete material str
9、ength development4.2Construction load factors4.3Early-age capacity of concrete slabs4.4Serviceability requirements4.5Formwork adequacyChapter 5Construction examples, p. 347.2R-105.1Two-way slab construction5.2Post-tensioned construction exampleChapter 6References, p. 347.2R-176.1Referenced standards
10、 and reports6.2Cited referencesRodney D. Adams Jeffrey C. Erson David W. Johnston Richard C. MooreKenneth L. Berndt N. John Gardner Roger S. Johnston William R. PhillipsRandolph H. Bordner William A. Giorgi Dov Kiminetzky Douglas J. SchoonoverRamon J. Cook Samuel A. Greenberg Harry B. Lancelot W. Th
11、omas ScottJames N. Cornell, II R. Kirk Gregory H. S. Lew Aviad ShapiraJack L. David G. P. Horst Donald M. Marks Rolf A. SpahrWilliam A. Dortch, Jr. Mary K. Hurd Robert G. McCracken*Principal author.Pericles C. Stivaros*ChairKevin L. WheelerSecretary347.2R-2 ACI COMMITTEE REPORTCHAPTER 1INTRODUCTIONI
12、n multistory cast-in-place concrete building construction,freshly cast floors are placed on formwork that is tempo-rarily supported by a system of shores and reshores until theconcrete has the ability to be self-supporting. Constructionloads, imposed by the shoring system on the slabs below,may be s
13、ignificantly larger than the design loads of thosefloors. Furthermore, the concrete of slabs below may nothave attained sufficient strength before the construction loadsare applied. As a result, it is critical to determine the early-ageload strength of the floor slabs, including punching shearstreng
14、th, to avoid the possibility of partial or total failure of thestructural system due to construction overload. To reduce anddistribute the large construction load on the floor immediatelybelow, to several lower floors, it is important to add reshores onlower levels. Therefore, an engineering analysi
15、s that considersboth the construction load distribution and the early-age load-carrying capacity of the concrete slabs should be performedbefore shoring/reshoring operations begin.Formwork failures and failures caused by improperreshoring or premature removal of supports and inadequatelateral bracin
16、g, have periodically occurred throughout thehistory of concrete construction. Premature removal ofshores and reshores can contribute to construction failures ordefects such as permanent excessive deflections (sagging) orcracking in the completed structure. Also, if over-loadedprematurely, time-depen
17、dent deflections under load (creep)will be larger, and sagging is more likely to be both noticeableand objectionable.Decisions regarding the removal of forms and relocationof the shores are too often made without the benefit of aproper analysis of the structural effects, or in many cases,without any
18、 analysis at all. Still, there is no commonlyaccepted method considered as the proper analysis in theconstruction industry.To ensure satisfactory performance and structural safetyduring construction, a thorough understanding of constructionloads applied to the slabs at early ages is necessary. Equal
19、lyimportant is knowledge of the behavior and the strength ofearly-age concrete members that support their own weightand construction loads.The formwork engineer/contractor is usually guided in form-work operations by the following codes, standards, or guides:ACI 347, “Guide to Formwork for Concrete”
20、ACI 318, “Building Code Requirements for StructuralConcrete”ACI 301, “Specifications for Structural Concrete”ANSI A10.9, “American National Standard for Con-struction and Demolition OperationsConcrete andMasonry WorkSafety Requirements”OSHA 29 CFR, “Construction Safety and Health Regu-lations for Co
21、nstruction”SEI/ASCE 37, “Design Loads on Structures DuringConstruction”Other documents that can provide formwork designrequirements or guidelines include state and local buildingcodes, and guidelines prepared by contractors, formworkmanufacturers, and certain construction agencies.The above referenc
22、ed documents provide basic guidelinesfor general formwork operations. At the present time,however, there are no codes or standards that providedetailed design and construction requirements specificallyfor shoring/reshoring operations for multistory reinforcedand post-tensioned concrete construction.
23、 Investigation forusable procedures to establish safe and cost-effectiveshoring/reshoring operations has been ongoing for severaldecades. The effort has focused on two major areas: deter-mining the distribution of loads carried by the concrete struc-ture during construction, and estimating the stren
24、gth of theconcrete members to resist the construction loads.This report outlines the importance of proper formworkdesign for multistory structures and provides basic require-ments for safe construction. ACI SP-4, Formwork forConcrete, serves as an expanded commentary to ACI 347,“Guide to Formwork fo
25、r Concrete,” and provides detailedinformation relative to formwork practices, including adiscussion of and procedures for shoring/reshoring analysis.ACI 318, “Building Code Requirements for StructuralConcrete,” requires contractors to furnish the building offi-cial, upon request, with the structural
26、 calculations andconcrete strength data used in planning and implementingshoring/reshoring operations. Such data and informationshould be furnished to the engineer/architect who shouldevaluate the effects of construction loads to immediate andlong-term deflections. This code requirement obligescontr
27、actors and formwork designers to acquire an under-standing of the construction loads and the structural behaviorof the buildings during construction. This understandingenables them to develop a rational shoring/reshoring systemdesign that is as economical as possible without compro-mising safety, qu
28、ality, and serviceability.The objective of this document is to present practicalguidelines for the design of shoring/reshoring operations.This document provides formwork design tools to evaluatethe safety of construction schedules for multistory reinforcedconcrete and post-tensioned concrete structu
29、res.CHAPTER 2SHORING/RESHORING CONSTRUCTION NEEDS2.1DefinitionsThe following terms will be used in this guide. All theseterms may also be found in ACI 347.backshoresshores placed snugly under a concrete slabor structural member after the original formwork and shoreshave been removed from a small are
30、a at a time, withoutallowing the slab or member to deflect; thus, the slab or othermember does not yet support its own weight or existingconstruction loads from above.centeringspecialized temporary support used in theconstruction of arches, shells, and space structures where theentire temporary supp
31、ort is lowered (struck or decentered) asa unit to avoid introduction of injurious stresses in any partof the structure.engineer/architectthe engineer, architect, engineeringfirm, architectural firm, or other agency issuing project plansSHORING/RESHORING OF CONCRETE MULTISTORY BUILDINGS 347.2R-3and s
32、pecifications for the permanent structure, administeringthe work under contract documents.formworktotal system of support for freshly placedconcrete, including the mold or sheathing that contacts theconcrete as well as all supporting members, hardware, andnecessary bracing.formwork engineer/contract
33、orengineer of the formworksystem, contractor, or competent person in-charge of designatedaspects of formwork design and formwork operations.preshoresadded shores placed snugly under selectedpanels of a deck forming system before any primary (orig-inal) shores are removed. Preshores and the panels th
34、eysupport remain in place until the remainder of the completebay has been stripped and backshored, a small area at a time.reshoresshores placed snugly under a stripped concreteslab or other structural member after the original forms andshores have been removed from a large area, requiring thenew sla
35、b or structural member to deflect and support its ownweight and existing construction loads applied before theinstallation of the reshores. If prefabricated drop-head shoresfor slab formwork systems are used, the shores can becomethe reshores if a large area of shoring is unloaded, permittingthe str
36、uctural members to deflect and support their ownweight. If they are not, then they become backshores.shoresvertical or inclined support members designed tocarry the weight of the formwork, concrete, and constructionloads above.2.2Advantages of reshoringIn multistory cast-in-place construction, rapid
37、 reuse ofform material and shores is desired to allow other trades tofollow concreting operations as closely as possible. Theshores that support the newly placed concrete transmit thatweight to the floor slab below, which can exceed that floorslabs design load capacity. For this reason, shoring orre
38、shoring is provided over a number of floors to distributethe construction load to several floor levels below.Stripping formwork is usually more economical if all theform material is removed at the same time before placingreshores. In this case, the structure system is required tosupport its own weig
39、ht, thus reducing the load in thereshores. A combination of shores and reshores usuallyrequires fewer levels of interconnected slabs, thus freeingmore areas for other trades.Backshoring and preshoring are other methods of supportingnew construction that are less widely used and involve leavingthe or
40、iginal shores in place or replacing them individually so asnot to allow the slab to deflect and carry its own weight. Thesemethods require careful supervision by the formwork engineer/contractor and review by the engineer/architect to ensureexcessive slab and shore loads do not develop.2.3Types of f
41、orming systemsAn important consideration in multistory cast-in-placeconcrete building construction is the type of forming systemto be used. The selection of the forming system forconstructing a cast-in-place concrete structure is a criticaldecision that affects both the construction schedule and cos
42、t.Systems vary from traditional wood post-and-beam formwork/shoring to modern prefabricated systems involvingsophisticated engineering, materials, and equipment.There are several prefabricated forming/shoring systems thatare used to support concrete slabs during construction,including shoring-based
43、systems, flying truss systems, column-mounted systems, and tunnel-forming systems. The followingdescription of these systems is adapted from Jensen (1986).Shoring-based systemsDeck (slab) forms are supportedon shores placed on the slab below. The shores may be singleposts of wood, or metal, or assem
44、bled from frames. Job-builtdeck forms usually consist of wood or aluminum stringersand joists (runners) with the deck surface made of plywood,supported on single-post or frame-type shoring. These formsare sometimes made up in larger panels tied or gangedtogether as tables with attached frame-type sh
45、oring formovement by crane. Deck forms may also be assembled onthe job from proprietary panels framed in wood, steel, oraluminum, sometimes with their own proprietary shoringsystems. Some of these systems allow removal of the slabforms while the shores remain in place until sufficientconcrete streng
46、th is developed to allow the shore removaland reshoring process.Flying truss systemsFlying truss systems are made up ofsteel or aluminum trusses, topped with aluminum or woodjoists and decked with plywood. Adjustable legs or shoressupport the truss on a previously cast slab. The truss-mounted forms
47、are moved as a unit by crane from onecasting position to the next.Column-mounted systemsColumn-mounted systems arelong-span form panels supported by brackets or jacksanchored to concrete columns and shear walls. The deckpanel is generally moved by crane. Similar systems availablefor bearing wall bui
48、ldings support slab forms on bracketsanchored to the walls. These systems make it possible toeliminate most vertical shoring and reshoring.Tunnel-forming systemsTunnel-forming systems arefactory-made, inverted, U-shaped steel form systems thatpermit casting both slab and supporting walls at the same
49、time. When the concrete has gained sufficient strength, thetunnels are collapsed or telescoped and moved to the nextpour. For longer slab spans, the tunnel form may be made intwo inverted L-shapes (termed half-tunnels).CHAPTER 3CONSTRUCTION LOADSON FORMWORK3.1Construction loadsConstruction loads are those loads imposed on a partiallycompleted or temporary structure during the constructionprocess. Construction loads on formwork include verticaldead and live loads of both the formwork and the structure,horizontal loads due to wind, vertical and la
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