THE CONCRETE BRIDGE MAGAZINE

Winter 2019

ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.

Issue link: http://www.aspiremagazinebyengineers.com/i/1064399

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C O N C R E T E B R I D G E T E C H N O L O G Y 24 | ASPIRE Winter 2019 by Dr. Krista Brown A c o m m o n mi s c o n c e p t io n i s t h a t increasing the amount of reinforcement in concrete will prevent cracking. Not so! When the stress in concrete reaches its tensile strength, cracking results. Reinforcement will control the width and distribution of cracks, but it does not prevent cracking. Precast concrete deck panels may be more susceptible to cracking during handling and transportation stages, when strengths are typically lower and support points fewer, than in service. Cracking is a risk as panels are stripped from forms, transported to the storage yard, loaded onto a truck, shipped, and installed. Precas ters of building components, especially wall panels, have successfully designed panels with specific pick- point locations to avoid cracking during h a n d ling a n d t ra n s p o r t a t io n . Thi s article presents simple calculations and measures to help designers, owners, precasters, and contractors ensure that precast concrete deck panels do not experience unacceptable cracking during handling and transportation. Cracking that may occur due to drying shrinkage, thermal factors, and other causes will not be addressed. Methodology Section 8.3 of the PCI Design Handbook: Precas t and Pres tressed Concrete 1 provides a procedure and examples for determining stresses in flat panels during handling and transpor tation. With this procedure, which includes the effects of openings and other changes in cross-section geometry, tensile stresses in a precast concrete deck panel are calculated using basic mechanics of materials and compared to the allowable tensile stress. The notation of the PCI Design Handbook is used in this article. B e c a u s e o n ly t e n s il e s t r e s s e s a r e calculated, all stresses will be shown as positive. Stresses are also shown in psi units rather than ksi used in bridge design. The allowable tensile stress, f allowable , is a criterion for no-discernible cracking that is used in the building industry. 1 It is determined by applying a factor of safety of 1.5 to the concrete modulus of rupture, which is a function of compressive strength. f allowable = 7.5 ʹ f c 1.5 = 5 ʹ f c where f' c = concrete compressive strength at the time of handling or transportation, psi Note 5 ʹ f c is equivalent to 0.158 ʹ f c with both f' c and f allowable in ksi. The tensile stress in the concrete section, f t , is calculated by f t = M S where M = moment at the concrete section with a PCI-recommended load multiplier applied for handling or transportation, kip-in. S = section modulus, in. 3 Table 8.3.1 in the PCI Design Handbook lists recommended load multipliers to account for forces caused by form suction and impact during handling and transportation. These multipliers are applied to the weight of the panel and are based on typical experience of precast concrete producers. The multipliers may be modified based on specific conditions and the experience or preferences of precasters or owners. Because the multipliers and concrete s trengths var y with each s tage of handling and transportation, tensile stress and allowable tensile stress must be determined for each stage. The following examples d emons trate calculations for precast concrete deck panels at two stages—stripping from forms and shipping. Example of Calculations for a Rectangular Deck Panel Assume a 10 ft by 32 ft by 8¾ in. precast concrete panel with no openings, internal ducts, or other irregularities in the cross section. Given Normalweight concrete at 150 lb/ft 3 f' ci = 4000 psi at stripping from forms f' c = 5000 psi at shipping The panel will be handled with a four- point pick at locations shown in PCI Design Handbook Fig. 8.3.2, w hich equalizes positive and negative moments. Calculate the allowable tensile stress at stripping: f allowable = 5 4000 = 316 psi Calculate the maximum tensile stress in the panel's longitudinal direction using the equation from Fig. 8.3.2 of the PCI Design Handbook that assumes uniform loading. a = 10 ft b = 32 ft t = 8.75 in. w = 8.75 12 ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ 150 ( ) = 109.4 lb/ft 2 Calculations for Handling and Shipping Precast Concrete Deck Panels

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