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Civil Engineering
CIVL 430
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CIVL 430: Design of Concrete Structures Module #5 Development of reinforcement - understand two basic bond mechanisms between concrete and steel - understand how the uniform bond stress model is used to determine requirements of reinforcement development lengths - understand the factors influencing required development length - be able to calculate required development length according to A23.3 with and without hooks - understand factors influencing bar cut-off locations and how to determine location of bar cut-offs - understand how bar cut-offs are typically determined in building design Beam bending (c) Bar forces Wight and MacGregor, 2009 1 Cracked Beam At cracks, all tensile stress carried by reinforcement. Between cracks, tensile stress shared between concrete and reinforcement. Wight and MacGregor, 2009 Bond mechanisms Two mechanisms transfer stress from reinforcement to concrete: 1. 2. 2 Mechanical interlock Forces on concrete act at an angle, related to the shape of the deformation. Radial component can cause splitting cracks parallel to the bar.  How to protect against splitting cracks? Wight and MacGregor, 2009 Bond Experiment Test Cylinder Splitting crack “Live end” 3 Uniform bond stress model Wight and MacGregor, 2009 Factors influencing constant C: 1. Quality of the concrete/bond. Top bars in beam tend to be surrounded by lower quality concrete due to bleeding of water from the concrete below during vibration. 2. 3. 4. 4 Tension development length: Simplified model (A23.3 Cl. 12.2.3) Restrictions:  Clear cover ≥ d abd bar spacing ≥ 1.4 d b Case 1  Members containing minimum stirrups or ties within l , ord slabs, walls, shells, or folded plates with clear spacing of bars being developed > 2d : b Case 2  All others not satisfying Case 1: Development length factors Factor Type Values  1.3 for top horizontal bars placed so that more k than 300 mm of fresh concrete is below the 1 development length or splice.  1.0 for all other cases  1.0 for uncoated reinforcement  1.5 for epoxy-coated bars with clear cover less k2 than 3b or with clear spacing between bars being developed less thanb6d  1.2 for all other epoxy-coated bars  1.0 for normal-density concrete k3  1.2 for semi-low-density concrete  1.3 for low-density concrete  1.0 for 25M and larger bars k4  0.8 for 20M and smaller bars and deformed wire 5 Compression development length Shorter than tension development length. Why? Development length for bars in compression:  k5= 0.75 when reinforcement enclosed with spiral reinforcement or with 10M ties in conformance with A23.3 Cl. 7.6.5 and spaced not more than 100 mm on centre. Reduction for excess reinforcement If more than the required amount of reinforcement is provided, reinforcement will likely not yield when subjected to ultimate demands.  Hence, can reduce development length by f /f . s y  This is equivalent to reducing by: Is this safe in the case of an unexpected overload?? 6 Headed and Hooked reinforcement Options for developing reinforcement within a length shorter than ld:  Headed reinforcement  Hooked reinforcement Wight and MacGregor, 2009 Standard hooks Standard hooks used to reduce information needed on design drawings Wight and MacGregor, 2009 7 Bend diameter Bend diameter, D, must be large enough to avoid inducing excessive strains in the bar at the bend. D depends on the size of the bar:  10M  15M  20M  25M  30M  35M Hooked development length l depends on: dh  bond along “lead-in-length”    Hooks do not reduce compression development length!
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