Key to Successful Execution of Precast Projects – Meticulous Design and Detailed...

Key to Successful Execution of Precast Projects – Meticulous Design and Detailed Engineering

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Abstract: Precast concrete is widely used all over the world for variety of applications such as buildings, stadiums, infrastructure projects like bridges, airports etc. Although precast technology is widely being used in India in infrastructure projects, the building sector is still exploring this technology very cautiously. Understanding of technology requirements is very important aspect.

In this article, the important aspects of structural design and detailing of building structures using precast technology has been explained which includes parameters affecting the preliminary working of precast system, precast component design, connection design, and types of lateral load resisting system. Another important aspect in precast projects compared to conventional RC structures is Detailed Engineering; which has been briefly explained with type of drawings required to be generated for manufacturing and erection of precast structure.

 

 

Introduction to Design Development phase

This article describes various important aspects involved in designing and detailed engineering of building projects using precast technology.

Structural engineer has a responsibility for calculation of loading on precast elements and deciding loading criteria for the structural design of the entire structure and the effects of the precast erection sequences on individual structural members. Based on these inputs, primary structural system is chalked out.

Once, the supporting system is identified, he or the contracted precaster, as the case may be, works on to achieve practical and economical construction methods.

Following factors shall be considered for preliminary working on precast structural system,

1. Location of site: Proposed construction site should be evaluated on factors such as accessibility, connection with road, distance from precast factory, storage space and sufficient area for working, local regulatory norms, surrounding structures etc.
2. Precaster (producer of precast elements): Capabilities of precaster like production capacity per year, maximum element bed sizes, type of surface finishes he is capable to produce etc should be considered.
3. Erection and transportation equipments: Type of equipments which will be available for erection, maximum load carrying capacity of transportation trailers/trucks, erection sequence and location of storage yard, any restrictions on working timings as per local regulatory bodies etc.
4. Type of connections to be used.

 

 

Once, the preliminary working is done, it is discussed with Architect, owner for their inputs. Precaster should request for clarification of ambiguities, if any, on special conditions not clearly defined in the contract documents.

Structural Configuration

Precast concrete has certain unique features such as greater construction speed, quality control on concrete elements, fire resistance and durability etc., but to fully realize these benefits and thereby gain the most economical and effective use of the material following principles are to be followed:

1. Maximum economy is achieved with maximum repetition. Standard or repetitive sections should be used whenever possible.
2. Architectural precast panels can be used as cladding as well as load bearing member. They can be used to support loads in both the vertical and lateral directions.
3. Prestressing, if possible, improves the economy and performance of precast members, but is usually only feasible with standard shapes which are capable of being cast in long-line beds.

Functional requirements of various structures shall also be considered while finalizing layout of precast elements. Following typical cases are considered:

1. Residential Buildings: Although the span requirement is not demanding, aesthetic, cost effective options needs to be worked out, similarity of plans and elevations shall be maintained to achieve maximum repetition. Hybrid systems can be also be one of the options in Indian scenario such as RC/Steel supporting elements in combination with precast flooring elements etc.
2. Parking Buildings: Precast parking buildings demands long span with unobstructed parking spaces, double tee flooring elements, long span prestressed inverted T-girders are suitable for such requirements.

Other suitable application for precast concrete is distribution centres / ware houses, as architectural cladding panels, large podiums, stadium risers / bleachers etc.

 

 

Design Engineering Process

Design engineering process for precast structures, generally happens on three levels, it starts with overall support system and lateral load resisting system, individual component/element design and then ultimately connection design for the structure.

Design of gravity and lateral load resisting system:

Appropriate building system needs to be selected during the preliminary analysis. The gravity and lateral load resisting system may function separately or they may be combined.

As concrete is a heavy material, this is an advantage for such matters as stability under wind loads, thermal changes, acoustical vibration and fire resistance. Also, the high dead-to-live load ratio will provide a greater safety factor against gravity overloads.

Few examples are shown below of load resisting systems.

(Reference: PCI design handbook){1}

 

 

Moment – resisting frame system

It consist of Moment resisting frames that tie precast beams and columns together with rigid connections . Lateral loads are shared by moment frames proportionate to their stiffness in each direction. Need for Shear walls can be eliminated in case better flexibility in functional use of space is required

Interior Shear wall system

In this system Shear walls are located at the internal core area preferably near ramps. Load bearing spandrels and columns can be provided on external face. Lateral loads are transmitted by floor diaphragms to a structural core of precast shear walls. This can impart better flexibility on elevation aspects of building envelope.

Exterior Shear wall system

In this system Shear walls are located at the external perimeter of building. Load bearing beams and columns can be provided in internal area. This can impart better flexibility in functional usage of internal area of building.

 

 

In addition to resisting gravity loads, a principal consideration in building design is the lateral force resisting system. Methods used to resist lateral forces, in the approximate order of economy, include:

1. Cantilevered Columns or Wall Panels (Out of Plane): Only feasible in low-rise buildings or warehouses. Base fixity can be attained through a moment couple between the footing and ground floor slab, or by fixing the wall or the column to the footing.
2. Shear Walls: These can be precast concrete, cast-in-place concrete, or masonry. When architectural or structural precast members are used for the exterior cladding, they can often be used as shear walls. Precast concrete box elements have been used effectively in low-rise to high-rise structures. The boxes are created as one complete unit, such as in a precast cell module, or can be created of individual precast walls connected together to create a box unit. Such box units have a much larger moment of inertia than individual walls and therefore can be important members in a lateral force resisting system.
3. Steel or Concrete X-bracing: This system has been used effectively in low and medium rise buildings. A related resistance system usually occurs naturally in parking structures with sloped ramps in the direction of traffic flow.
4. Moment-Resisting Frames: Building function may dictate the use of moment resisting frames. It is sometimes feasible to provide a moment connection at only one end of a member, or a connection that will resist moments with lateral forces in one direction but not in the other, in order to reduce the buildup of volume change restraint forces. To reduce the number of moment frames required, a combined shear wall-moment frame system may be used.

 

 

All of the above systems depend on distribution of lateral loads through diaphragm action of the roof and floor systems. The balance in system design is achieved not only by providing the strength, ductility and toughness to resist lateral forces. It is also important to consider the effects of concrete creep, shrinkage and temperature change. These effects are collectively known as volume changes. Details that result in over-restraint of volume changes can be as damaging as any externally applied force.

In precast concrete structures, individual elements are connected at their joints with a variety of methods. These connections may include embedded steel shapes such as plates and angles, with headed stud or reinforcing bar anchorage; the steel embedments are field bolted or welded. These applications are “dry” connections. A “wet” connection consists of reinforcing bars protruding from the precast members, with the bars mechanically coupled or spliced. Cast-in-place concrete or grout at the joint completes this connection. Either dry or wet connections are used in both Moment-resisting frame and shear wall systems.

 

 

Component Design/ Precast Element Design

Precast components are designed for loads specified by design codes and specifications. While designing, all members are considered as “simple-span” members. Component design is carried by carefully studying various local loads applied on particular element.

In the market, various software are available to design individual precast elements. Precast/prestressed flexural elements are designed using software such as Presto, concise beam etc. Precast/prestressed wall panels are designed using software such as Lecwall etc.

Connection design

Basic purpose of connection is to transfer load and provide stability, which means the design of connection is one of the most important steps in engineering of precast/prestressed concrete structures.

Precast concrete connection must meet a variety of design and performance criteria such as Strength, ductility, volume change accommodation, durability, fire resistance, constructability, aesthetic, seismic requirements, tolerances etc. and not all connections are required to meet these criteria.

Precast concrete is basically a “simple-span” material. However, continuity can be, and often is, effectively achieved with properly conceived connection details.

Precast concrete structures can be built in high seismic areas, provided that the precast members are connected so the unit will perform essentially the same as a cast-in-place concrete unit. These designs and details have become known as “Emulation” of cast-in-place concrete.

Emulation design creates construction that either is monolithic at the critical joint locations or provides connections that act as if they are monolithic at those locations. In general, emulative connections involve connecting reinforcing bars across a joint individually by lap splicing, welding or mechanical couplers. Concrete is made continuous by filling the joint with a high quality non-shrink or fiber-reinforced grout in horizontal joints or a cast-in place closure pour in vertical joints. Engineering follows the rules for reinforced concrete design while taking into account the special circumstances for the connection system chosen, such as maintaining proper cover to mechanical couplers.{2}

 

 

Detailed Engineering / Detailing Process

Detailed engineering process starts almost simultaneously with design process. While designer works on the load resisting system, detailing team starts working on flooring layout of building. Greater amount of efforts are needed when compared to conventional RC structures. Type of drawings required for precast concrete system is described as follows:

1.Erection drawings: {3}

Erection drawings refer to set of drawings from which the precast concrete members are erected. They are prepared using information in contract documents and drawings prepared by sub-contractors such as electrical, mechanical contractor etc..

It should contain all precast concrete members piece marks, completely dimensioned size and shape of each member, the location of each member with respect to building lines and/or column lines and finished floors, and the details and location of all connections from member to member or member to structure. Joints and opening between precast concrete members and any other portion of the structure should be identified.

2.Piece/Shop drawings:{3}

A piece drawing is the drawing from which the plant will produce a precast concrete member. It should contain, or refer to, all the information necessary to accomplish this.

Any piece drawing shall contain information such as piece mark and its located end, dimensions of all cast in items, surface finishes, reinforcement for a member, method of handling, size and location of handling devices. It also should include all related information required for the factory to cast material such as list of materials, concrete quantities, centre of gravity etc.

Conclusions

Precast project needs lot of thought process in planning phase of the project. All related activities such as casting, curing, stacking, transport, erection are required to be planned and finalized on paper at the start as per project timelines.

Structural system can be analyzed and designed using high end computer software and various software packages are also available for precast component design.

Detailed Engineering & drawings are required for casting and erecting precast elements. More number of drawings almost 8 to 10 times are required to be prepared as compared to conventional RC projects.

Large construction volumes are easily manageable with systematic & standardized precast concrete technology.

Stringent control on quality & timelines can be established by using precast technology.

References

1. PCI-MNL-120-04, PCI Design Handbook, Precast and Prestressed Concrete, 7th Edition.
2. ACI Committee 550 report, “Emulating Cast-in-Place Detailing in Precast Concrete Structures (ACI 550.1R-01)”.
3. PCI Drafting handbook, Precast and Prestressed Concrete, 2nd Edition. ?

 

Dhananjay Bhosale
Precision Precast Solutions Pvt. Ltd.

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