Precast Construction Lecture Notes PDF

Mousl university 2st semester : Building Construction 3 Architecture department Date : xx / xx / xxxx Iraq lecture no. 9 precast introduction Building Construction 2 Assistant Teacher : Adil Khalil 1. INTRODUCTION...

Mousl university 2st semester : Building Construction 3 Architecture department Date : xx / xx / xxxx Iraq lecture no. 9 precast introduction Building Construction 2 Assistant Teacher : Adil Khalil 1. INTRODUCTION The concept of precast (also known as “prefabricated”) construction includes those buildings, where the majority of structural components are standardized and produced in plants in a location away from the building, and then transported to the site for assembly. These components are manufactured by industrial methods based on mass production in order to build a large number of buildings in a short time at low cost precast construction Systems  closed system Open system In this system will be produce building it is the production of building unites without the knowledge the units non-standardized where project will it used and in any system the parts which is produced where this gives more flexibility from under this system part used the unites that manufactured in closed only in one project which system. leads to wasteful, this work Building on modular coordination can be of these units moldings which used for a wide range of manufacturers process uneconomical but and traditional buildings. the production of 1000 housing units in this system be economic. The main features of this construction process are as follows: The division and specialization of the human workforce The use of tools, machinery, and other equipment, usually automated, in the production of standard, interchangeable parts and products Compared to site-cast concrete, precast concrete erection is faster and less affected by adverse weather conditions Plant casting allows increased efficiency, high quality control and greater control on finishes.. 1.1 TYPES OF PRECAST SYSTEMS Depending on the load-bearing structure, precast systems can be divided into the following categories: Large-panel systems Frame systems Slab-column systems with walls Mixed systems 1. 1. 1 LARGE PANEL SYSTEMS The designation “large-panel system” refers to multistory structures composed of large wall and floor concrete panels connected in the vertical and horizontal directions so that the wall panels enclose appropriate spaces for the rooms within a building. These panels form a box-like structure. Both vertical and horizontal panels resist gravity. load. Wall panels are usually one story high. Horizontal floor and roof panels span either as one-way or two-way slabs. When properly joined together, these horizontal elements act as diaphragms that transfer the lateral loads to the walls. 1. 1. 2 FRAME SYSTEMS Precast frames can be constructed using either linear elements or spatial beam column sub-assemblages. Precast beam-column sub- assemblages have the advantage that the connecting faces between the sub-assemblages can be placed away from the critical frame regions; however, linear elements are generally preferred because of the difficulties associated with forming, handling, and erecting spatial elements. The use of linear elements generally means placing the connecting faces at the beam-column junctions. The beams can. be seated on corbels at the columns, for ease of construction and to aid the shear transfer from the beam to the column. The beam- column joints accomplished in this way are hinged. However, rigid beam-column connections are used in some cases, when the continuity of longitudinal reinforcement through the beam-column joint needs to be ensured. The components of a precast reinforced concrete frame are shown in Figure. 1. 3 SLAB-COLUMN SYSTEMS WITH SHEAR WALLS These systems rely on shear walls to sustain lateral load effects, whereas the slab-column structure resists mainly gravity loads. There are two main systems in this category: precast structural elements are assembled Lift-slab system with walls by means of special joints. Reinforced Prestressed slab-column system concrete slabs are poured on the ground in forms, one on top of the other. Precast In the Lift –slab system, the load-bearing structure consists of precast concrete floor slabs are lifted from the ground up to the final height by lifting reinforced concrete columns and slabs,. cranes. Precast columns are usually two stories high. The slab panels are lifted to the top of the column and then moved downwards to the final position. Temporary supports are used to keep the slabs in the position until the connection with the columns has been achieved. PRECAST CONCRETE FRAMING SYSTEMS Plant-Cast Precast Concrete Concrete elements, cast and cured in a manufacturing plant, then transported to the construction site. Plant casting allows increased efficiency and higher quality control. Durable, permanent steel forms are reused many times, reducing formwork costs compared to sitecast concrete. Use of Type III, high early strength cement and steam curing allow concrete members to be cast and cured in as little as 24 hours. Controlled casting conditions and high quality forms allow for greater control of surface finishes.. PRECAST CONCRETE FRAMING SYSTEMS Plant-Cast Precast Concrete Structural elements are commonly reinforced with tightly stretched pretension steel strands, which provide increased structural efficiency. Conventional steel reinforcing is added for resistance to thermal and other secondary stresses. Plant-Cast Precast Concrete On the construction site, precast concrete elements are lifted into place and assembled into structural assemblies in a process similar to that used for structural steel.. Compared to sitecast concrete, precast concrete erection is faster and less affected by adverse weather conditions. A vacuum lifting device is used to lift and place precast concrete pranks. PRECAST CONCRETE FRAMING SYSTEMS PRECAST, PRESTRESSED CONCRETE STRUCTURAL ELEMENTS PRECAST, PRESTRESSED CONCRETE STRUCTURAL ELEMENTS Precast Concrete Slabs Used for floor and roof decks. Deeper elements (toward the right below) span further than those that are shallower (toward the left). Right: Hollow core slabs stacked at the precasting plant. Cop PRECAST, PRESTRESSED CONCRETE STRUCTURAL ELEMENTS Precast Concrete Beams and Girders Provide support for slabs. The projecting reinforcing bars will bond with concrete cast on site. Right: Inverted tee beams supported by precast columns..  PRECAST, PRESTRESSED CONCRETE STRUCTURAL ELEMENTS Precast Concrete Columns and Wall Panels Provide support for beam and slab elements. Since these elements carry mainly axial loads with little bending force, they may be conventionally reinforced without prestressing. Or, long, slender multistory elements may be prestressed to provide resistance to bending forces during handling and erection (columns at right).  PRECAST, PRESTRESSED CONCRETE STRUCTURAL ELEMENTS Precast Concrete Columns and Wall Panels Precast concrete wall panels may be solid (right), hollow, or sandwiched (with an insulating core). Wall panels can be ribbed, to increase their vertical span capacity while minimizing weight, or formed into other special shapes (below).  PRECAST, PRESTRESSED CONCRETE STRUCTURAL ELEMENTS Other Precast Concrete Elements Precast concrete stairs (below) Uniquely shaped structural elements for a sports stadium (right) Etc. PRECAST, PRESTRESSED CONCRETE STRUCTURAL ELEMENTS Assembly Concepts for Precast Concrete Buildings Vertical support can be provided by precast columns and beams wall panels (right), or a combination of all three. The choice of roof and floor slab elements depends mainly on span requirements. Precast slab elements are frequently also used with other vertical loadbearing systems such as sitecast concrete, reinforced masonry, or steel.  PRECAST, PRESTRESSED CONCRETE STRUCTURAL ELEMENTS Assembly Concepts for Precast Concrete Buildings Above: Precast concrete structure consisting of solid wall panels and hollow core slabs. Below: A single story warehouse consisting of double tees supported by insulated sandwich wall panels. PRECAST, PRESTRESSED CONCRETE STRUCTURAL ELEMENTS Assembly Concepts for Precast Concrete Buildings A parking garage structure consisting of precast double tees supported by inverted tee beams on haunched columns. 15 PRECAST CONCRETE FRAMING SYSTEMS MANUFACTURING OF PRECAST CONCRETE STRUCTURAL ELEMENTS. MANUFACTURING OF PRECAST CONCRETE STRUCTURAL ELEMENTS Casting Hollow Core Planks Precast elements are manufactured in casting beds, 800 ft or more in length. High-strength steel strands are strung the. length of the bed and tensioned. Conventional reinforcing, weld plates, blockouts, lifting loops, Untensioned prestressing strands can be seen in the left-most and other embedded casting bed. In the bed second from the right, low-slump items are added as concrete for hollow core slabs is being formed over tensioned strands using an extrusion process. A completed hollow core needed. casting is visible at the far right. Concrete is placed. MANUFACTURING OF PRECAST CONCRETE STRUCTURAL ELEMENTS Prestressing and Reinforcing Steel Many precast elements contain both prestressing strands and conventional reinforcing. Right: The prestressing strands for an AASHTO girder are depressed into a shallow v-shape to most efficiently resist tensile forces in the beam. Shear stirrups are formed from conventional steel reinforcing.. Casting Hollow Core Planks Once the concrete has cured to sufficient strength, the castings are cut into sections of desired length (above). In some cases, transverse bulkheads are inserted to divide the casting bed into sections before concrete is placed. In this case, only the pre-stressing strands need to be cut to separate the sections (below). MANUFACTURING OF PRECAST CONCRETE STRUCTURAL ELEMENTS Casting Hollow Core Planks Individual sections are lifted from the casting bed (right) and stockpiled to await shipping to the construction site.