Overloading decks with rebars in a concentrated area or crane hitting side of deck has caused collapses.
Shoring Failures BY C. W. BORDEN* MANAGER OF SAFETY CECO CORPORATION CHICAGO
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he collapse of concrete formwork used to support concrete slabs, decks and roofs is a neve r- e n d i n g problem. Each year hundreds of men are seriously injured or killed and many hundred thousands of dollars are lost in material and equipment. The causes of these failures are many, but there are three general categories into which formwork collapses and shoring failures can be classified: 1. Faulty workmanship in the field; failure to follow construction design and procedures and basic safety rules in shoring and formwork. 2. External forces that overload the formwork before, during, or after the concrete is placed. 3. Inadequate or faulty structural design of the formwork to support the expected design loads. Some of the more common critical areas associated with shoring failures are: 1. Tubular Steel Scaffolding Used for Shoring a. Height of screw jacks at base or top of scaffolding legs extended beyond their load-carrying capacity. b. Unequal spacing of stringers in top U bracket, causing eccentric vertical loading on the hollow leg members. c. Failure to fully block the leg members on ade-
quate sills or failure to place the vertical leg members on level sills in firm, compacted ground. d. Bent, buckled or missing bracing members on the scaffolding. e. Failure to tie high towers of shoring frames together to form a rigid, solid unit to support the deck loads during placing operations. 2. Wood and Steel Shores a. Failure to scab spliced shores on 4 sides with adequate nails, square ends and maintain full contact between the two pieces spliced. b. Height of screw jacks on shores extended too far. c. Size and bearing of sills on grade inadequate to hold shores or sills not properly blocked and leveled. d. Top of shores not adequately tied in to stringers to counteract vibration, uplift or shifting of loads during placing operations. Scabs, wire heads or other fixed assemblies at top of shore not used. e. Inadequate X bracing and horizontal lacing to resist horizontal and vertical loads. f. Failure to brace excavations adjacent to shores on grade and subsequent failure of soil. The functions of X bracing and horizontal lacing on wood and steel shores is not clearly understood by e ve ryone in field supervision on construction jobs. When understood by field supervision, their functions are not always fully appreciated. Cutting corners or taking chances on material by not using enough X bracing and horizontal lacing makes no economic sense. The function of X bracing is to resist any unexpected external lateral forces on the formwork and shoring sys-
Defective mud sills over or near excavations and insufficient scabbing on spliced shores as shown on wood shore in center foreground has caused deck collapse. tem which could cause a collapse. Some of the external lateral forces which have caused collapses in the past a re : 1. Pneumatic pressure loads during placing operations against the bulkhead or deck. 2. Sloped ramps connected to the horizontal deck creating horizontal component or lateral forces on the formwork and shores. 3. Se ve re wind forces against the deck surfaces. 4. Crane booms and other powered or hand equipment hitting side of deck during concrete or loading operations. 5. Any sloped or arched formwork surfaces for sloped floors and ceilings or arched slabwork which create horizontal or lateral forces when concrete is placed. 6. All eccentricities on shores where vertical loading is not fully concentric. 7. Sliding or moving heavy concentrated loads of equipment or material on the deck forms which create lateral forces on the shoring system. Some examples are reinforcing steel, plywood, steel forms, shores and plastic concrete. 8. Use of motor buggies or other high speed equipment on runways over deck forms. High speed acceleration or deceleration in starting and stopping the powered equipment, plus vibration, create heavy lateral forces on the shoring system. Details of the X bracing layout should be clearly indi-
cated on formwork drawings by a qualified engineer. Or, standards should be published on X bracing for va ri e d layouts of common shoring systems which vary in shore height or spacing, only, for horizontal formwork. Ho ri zontal lacing is fastened to shores to reduce the unsupported shore length so that failures will not occur due to vertical loads on the formwork. St ru c t u ra l l y speaking, the safe or allowable vertical load in pounds on any shore depends upon its cross sectional shape and size, its length, and the type of structural material composition. As the length of a shore increases, the safe or allowable vertical load decreases. It is necessary, therefore, to decrease the unsupported shore length with hori zo ntal lacing in order to maintain safe vertical loads on shores as they are increased in length. Here, again, it is most important to remember that X bracing does not replace horizontal lacing, nor does horizontal lacing replace X bracing. They both have separate functions. X bracing resists horizontal or lateral forces on the shores and shoring system only. Horizontal lacing resists vertical loads on each individual shore. To maintain design concrete loadings on 4 x 4 and tubular steel shores over 10 to 12 feet in length, hori zo ntal lacing is required. Additional vertical overloads that have caused past collapses due to insufficient lacing are: 1. Concentrated overloads of reinforcing bars and other material or equipment stacked in small areas on the deck formwork. 2. Powered equipment weight and vibration on the deck. 3. Overloading deck with wet concrete in a concentrated area plus impact loading due to placing concrete from excessive heights above the deck. Some tubular steel scaffolding design in the early 1950’s was inadequate for the vertical loads adve rt i s e d by the manufacturer. Structural design of the steel scaffolding for shoring was not followed up by load tests to failure in order to substantiate the design loading table published by the manufacturer. The Steel Scaffolding and Shoring Institute in Cleveland, Ohio, was later organized to regulate and test steel scaffolding and shoring products on the market. Structural design of formwork systems by the contractor has often been inadequate, and has caused the majority of shoring failures. Formwork and shoring systems are so varied and intricate from project to project that they cannot be classified in specific categories. Shoring failures and concrete formwork collapses are never simple to determine and comprehensive site investigations are always necessary to determine causes. Generally speaking, they are caused by faulty structural design, unplanned and unsafe operational procedures with heavy equipment and material, faulty workmanship by field personnel, severe weather conditions, or any combination of the four factors. PUBLICATION #C670001 Copyright © 1967, The Aberdeen Group, All rights reserved
