CE&CR article – Design and detailing process of precast buildings
CE&CR article – Design and detailing process of precast buildings
Design And Detailing Process Of Precast Buildings
Precast building structures are made of precast concrete components of various sizes and shapes which are manufactured using special equipment and machinery. The precast concrete components are later transported to the construction site where they are assembled by cranes which will lift the precast elements to their final position (picture 1). For the purpose of precast production, transportation and construction it is required to prepare structural design calculations and detailed drawings of each precast component. This article will describe the aspects of the structural design process and drawing preparation of precast buildings.
The decision to make a building structure or part of it in precast concrete is initially based on the recommendations given by the architect and the structural engineer who have been appointed by the owner. During the concept design phase the team has to investigate how precast technology for example can reduce cost, shorten the construction time or increase the energy efficiency. There can be many other reasons to opt for precast construction and even at a later stage it can be implemented in the project. The architect and structural engineer shall have to prepare contract documents which will include the specifications and drawings with the sizes of the precast members, the orientation and how they are to be connected to each other. These drawings will enable the contractor and precast fabricator to assess the total size of the work and the difficulties and complexities of execution.
The contract drawings however will not be suitable to start executing the work. At that stage additional design calculations and detail drawings will have to be prepared which can be used for fabrication and execution of the project. This work is generally done by specialized precast engineering firms who employ precast engineers and precast detailers for the preparation of the precast design. These expert design companies have an in-depth understanding of the precast production and assembly process and how to analyze and design the prefabricated building structures.
Precast Engineering Team
Once the owner has awarded the project to the precast manufacturer the role of the precast engineering team (picture 2) begins to take shape. The team generally consists of the following members:
1. Precast engineer
2. Project coordinator
3. Senior drafter
5. Precast detailer
The precast engineer performs the structural analysis and prepares the structural design calculations of each precast concrete component and their connections. The structural engineer shall also review all the drawings prepared by the drafting team and make sure they comply with his design. The engineer will also provide support to the field team in relation to the stability of the structure during construction.
Not all organizations will have a dedicated person working full time in the position of project coordinator and in a lot of teams this role is covered by the senior drafter or engineer. The project coordinator will develop and monitor the overall project planning in assistance with the rest of the team. He will coordinate with the client and attend all project meetings and inform the team. Furthermore he will coordinate the activities with any other consultants and subcontractors involved in the project.
The senior drafter shall perform all drafting functions like preparing the precast shop drawings, erection drawings and material lists. The senior drafter needs to fully understand the contract documents and project information. He will coordinate with the project coordinator and precast engineer with respect to the project schedule. He has to train new staff and improve the skills of the precast drafting team. He has to estimate time required for new projects and monitor time spent on ongoing projects.
The checker reviews and checks the precast detail drawings prepared by the drafters. He has to ensure that contract documents have been are correctly interpreted and included on drawings. He has to check the consistency of drawing presentation with production methods and precast detailing standards. He has to check the accuracy of the precast element drawings, the panel geometry, tolerance conditions and check the bill of materials.
The precast detailer will work under supervision of the senior drafter and has to prepare the precast erection drawings and precast shop drawings. He shall revise the drawings as per the markups provided by the checker.
Following activities have to be executed by the precast engineering team.
1. Design coordination
2. Structural analysis
3. Structural design
4. Precast drawing preparation
Precast Design Coordination
Contract drawings of architect and engineer have to be converted to detail drawings which can be used for the production and execution of the project. In addition many subcontractors are involved in a project such as floor manufacturers, structural steel fabricators, cast in-place contractors, electrical, plumbing, mechanical, window frame manufactures, railing fabricators etc. The precast engineering team shall have to work with a lot of information which has to be integrated in the precast production drawings. It is therefore imperative to control the flow of information and have checking procedures in place. Generally the first set of precast detail drawings will show the exact geometry of the precast elements with connections to other elements and structures. The project coordinator will have to send these drawings to the architect, structural engineer and other consultants for review and checking purpose. It is expected that the drawings are checked and additional information is provided to finalize the drawings. The additional information can be provided as markups on the drawings (redlines), or as digital CAD files or nowadays the use of 3D digital building models is becoming popular. The precast engineering team has to process the information and prepare structural design calculations to finalize the precast production drawings. The major difference with conventional building technology is the integration of all design services in the final production drawings for which all information has to be available and to be coordinated and checked.
The structural engineer shall perform the structural analysis to calculate the strength, rigidity and stability of the building structure and understand how the structure supports and resists the vertical and lateral loads. The engineer requires knowledge of the standard design codes and structural analysis techniques. The most commonly used structural analysis method is the finite element method (FEM).
Specialist FEM computer software can be used to prepare accurate 3D models for structural analysis (picture 3). The structural engineer shall prepare the structural analysis model which includes the geometry of all the structural precast members like columns, beams, walls and slabs. Material properties, supports and connections shall be modelled in such a way that they represent the actual behavior of the precast structure. The engineer shall prepare a very detailed 3D model with exact size and positions of door and window openings, cut-outs and shafts. Precast walls and slabs shall be properly meshed and stiffness modifiers to be applied as per the prescriptions laid down in the standard design codes. The results of the structural analysis are used for the design of each structural member and therefore it is a key part the engineering work.
Based on the output results from the structural analysis model the structural engineer can prepare the reinforcement and connection design for each precast concrete member. Precast design calculations shall be based on the standard design codes and generally the precast members shall have to get connected so that essentially the structure will behave the same as a cast in-situ concrete structure. Other precast systems with different behavior can be used but might not be supported by the design codes and therefore research and actual testing has to show their adequacy. Finally the structural engineer has to prepare the design report which describes the structural analysis approach and results together with the detailed design calculations.
Precast Drawing Preparation
The precast detail drawings are a translation of the contract documents and have to be prepared in such a way that they are having usable information for precast production, handling and erection. Furthermore the precast detail drawings have to provide all information to the architect and engineer to enable them to check the geometry and the relation to the rest of the building. The precast engineering team shall assign a unique identifying tag (called element number or piece ticket) to each precast component. A set of erection drawings consisting of building plans, elevations and sections shall be prepared indicating the exact location and geometry of each precast component and its connections. The precast detail drawings are prepared by expert draftsmen in 2D CAD or with the help of 3D Modelling software to produces the final 2D drawings. This entire process is called precast detailing and it is an essential and important part as errors in these drawings can cause difficulties at site which can increase the cost when precast pieces have to be refabricated.
Generally the following precast shop drawings shall be prepared:
1. Precast erection plans and elevations
2. Precast erection details
3. Embed plans and anchor layouts
4. Precast element form details
5. Precast element reinforcement details
6. Bar bending schedules
7. Hardware details
Precast Erection Drawings And Details
These drawings show how the precast concrete members are erected and they are prepared using the contract documents. The erection drawings contain all precast concrete member piece marks, fully dimensioned size and shape of each member, location of each member with respect to the building lines or column lines and finished floors, and details and locations of all connections. Joints and openings between precast members and any other portions should also be identified.
Erection layout plans show the precast members at each floor looking down and they are useful for indicating precast concrete beams, slab, columns and walls. Sections and details must be marked on the plans. Erection elevations are views that look directly at the structure and are usually taken from the outside looking in (picture 4). Erection details are blow up sections which show all the information to make a connection between precast concrete elements or to other structures. Embed plans and anchor bolt plans are used to show the location of embeds and anchors placed in the foundation or structure for the purpose of connecting the precast elements. The hardware drawings will show the details of inserts and embeds required to be placed inside the precast member when it is manufactured.
Precast Element And Reinforcement Details (Production Drawings)
The precast production drawings are used in the precast plant to manufacture the precast concrete members. These drawings must show each precast member including all the specifications, details, dimensions, inserts, reinforcement details, finishing etc.
The precast element form detail drawings (piece drawings) shows the precast piece in the position from which the precast plant personnel will view it or cast it (casting view). Various views like top, bottom and side views have to be added on the drawing (picture 5). Separate sections and blow up details should be used to clarify any complicated areas. Inserts that are required for connections and lifting have to be shown and provisions for MEP services have to be detailed and specified. Concrete surface finishing has to be specified and marked with symbols on the drawing.
The reinforcement drawings will show the reinforcement required inside the precast element (picture 6). These drawings should show all the necessary information so that they can prepare a reinforcement cage which will fit into the form. Size and location of reinforcing bars (rebars) and welded wire mesh has to be shown in plans, elevations and sections. Blow up details shall be prepared for clarification of areas with reinforcement congestion. Bar bending schedules can be added on the reinforcement drawings which will show the bar mark, bar shape code, size and type of rebar, total number of bars, bar length, bar bending dimensions and total weight.
3D Modeling Of Precast Structures And BIM
Instead of using 2D CAD software to prepare the technical drawings it might be beneficial to utilize more advanced software to prepare accurate 3D digital models of the project from which drawings and other information can be generated and documented. A process called building information modelling (BIM) can be followed where all information is stored in 3D computer models. A BIM model can consist of all the virtual equivalents of the actual building components used to construct a building. These components have all the characteristics of their real counterparts. These intelligent components are the digital prototype of the physical building elements that allow us to simulate the building and understand its behavior in a digital environment way before the actual construction starts. Each consultant and subcontractor can model their building components in separate 3D models which can be integrated into a single model which includes all information (Pictures 7 and 8). From the digital 3D model the precast drawings like plans, sections and elevations can be generated as well as all the production drawings of each precast component can be prepared.
Precast Design Standards
IS 11447 Construction with large panel prefabricates
IS 15916 Building design and erection using prefabricated concrete
IS 15917 Building design and erection using mixed/composite construction
FIB 27 Seismic design of precast concrete building structures
FIB 43 Structural connections for precast concrete buildings
PCI Design Handbook USA – precast and prestressed concrete
CAE Guidelines for the use of structural precast concrete in buildings (New Zealand)