Buddhi S. SharmaManager,ACECOMS, AIT

About the Author

Introduction

The government of Thailand has a plan to buildabout 600, 000 units within three years for the lowto medium income level citizens. Most of thesefamily units are detached houses. Commissioneddevelopers and designers have proposed differenthousing systems. Though the structural systemsdiffer in terms of material, production, construction,etc., some common requirements are that the designmust be suitable for large scale construction withina limited time and cost. One of these systems,which has already been approved (July 2004) by theNational Housing Authority (NHA) of Thailand isthe “Precast Large Panel Construction”. Severalthousand PLP houses will be built within this yearand much more in coming years.

What is PLP Construction?

Two very basic types of structural systems incommon use are, firstly the RC framed system and,secondly the load bearing wall system. In the firstcase, the load is taken and transferred by/throughconventional RC slab, beam, column andfoundation. However, in case of load bearing wall,the load from the floor is transferred to foundationthrough walls and they do not have any structuralframework. PLP structural system consists ofnormally reinforced precast concrete panels,precast slabs and foundations. No beam andcolumns are used except at few locations. The sizeof the panels depends primarily upon themaximum size that can be lifted conveniently fromthe casting bed without breaking (excessive cracks/stress and deformation), capacity of the lifting/

PLP structural system consists ofnormally reinforced precast walland slab panels with almost nobeam and column. The panels areconnected by grouted dowels,welded plate and shear keys.

Figure 1: Photographs Showing the CompletedHouses

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Precast Large Panel (PLP) ConstructionSome experience from Thailand

transporting device (crane) and architectural divisionof the wall. The occupants of the house can hardlymake any distinction between conventional and thistype of house when it is completed.

What are the Advantages of Using PLP?

The main advantages of this system are as follows:

• Mass production: This system is highly suitablefor mass production;

• Speed: The total time required is significantlyless than the conventional system. For typical2-3 bed room two-story house, the timerequired for casting, lifting, erection andcompletion of structural system is about 2-4days once the system is setup;

• No Removal of Formwork: As the panels arecasted at the factory and moved to the site whenthe concrete has gained enough strength, noformwork is required at the site;

• Less or Comparable Structural Cost:Generally, by the term ‘structural cost’ wemean the cost of material, formwork and laborfor main structural components (slab, wall,etc.). However, for a developer or investor thereare many other factors that are equallyimportant. Faster construction time allows forgreat saving in financial and administrative costs;

• Quality Control: As minimum wet work iscarried out at the site, it is easy to control thequality of the material, workmanship, etc.Because of the repeated nature of theproduction and construction, the unskilledlabor can be trained and become productive in

a very short time.

• Skilled Labor:Because of thereason that most ofthe production workis carried out at thefactory or the castingyard, skilled laborsare not required fortasks like masonrywork, plasting andfinishing. The onlyarea for use of skilledlabor is the closingof the gaps betweenwall-wall, wall-slabpanels and giving thefinal touch;

• Low InitialInvestment: The casting yard can be“Flat-surface” type or “Tilting Table” type (tobe discussed later). Although the Titling Tabletype may have some advantages in the long-term for large scale project (over 600 units), theFlat-surface option requires nominalinvestment;

• Modularization and Automation: Use ofPLP system gives great flexibility to modulatethe components, adjusting them to theindividual architectural design. Automation inproduction, transportation and erection ispossible to take advantage of the highlyrepeated routine work process;

• Durability and Long-term Performance:Normal reinforced concrete is the majorconstruction material of this century. PLP alsouses the normal RC which has no durabilityand long-term performance problems.Properly sealed joints minimize themaintenance due to water leakage or opening

Figure 2: Checking the Stresses and Crack Widths in a Wall Panelduring Lifting from Casting Yard after 24 Hours

Figure 3: Checking the Stresses and Crack Widths in a Slab Panelduring Transportation

Design of precastc o m p o n e n t sr e q u i r e schecking ofstresses on eachand everyrepresentativepanels forvarious stages,i n c l u d i n gp r o d u c t i o nlifting, stacking,transportationand permanentloading.

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of gap caused by temperature changes,etc.

Why is the Design of PrecastSystem Different fromConventional Systems ?

In design of conventional type ofhouses, the system and componentsare designed under permanent loadcondition. However, the design ofprecast system requires significantly

more steps and checks. The designrecommendations available in various designloads are insufficient for PLP systems.

• System Design Considerations

o Form and appropriateness;

o Material selection;

o Safety and stability;

o Serviceability;

o Constructability and other practicalconsiderations;

o Long term performance;

o Economy, etc.

o Process: Carry out the global analysis andchecks (as 3D system).

• Component Design

Each and every component is checked for thefollowing cases:

Figure 5: Typical Reinforcement Details in a Wall Panel

Figure 4: Connection and Joints

System stability andintegration, effectivec o n n e c t i o n s ,optimum size ofpanels, short-termstress checks are keyconsiderations in thedesign.

Note:CP = Connection Point

DB = Deformed Bar

RB = Round Bar

LT = Lifting Point

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o Perform component - detailed analysis andchecks during:

Casting;

Lifting;

Stacking;

Transporting;

Final position (before living);

Permanent load.

o Design of all structural components:

Code-based design checks;

Stress checks;

Crack width checks;

Both precast and non-precast if any.

o Design of connections:

Type and material considerations;

For safety;

For system integrity;

Constructability;

Long-term performance;

Cost;

Possible problems and solutions.

o Connection Types:

Wall-wall;

Wall-slab;

Column-footing.

o Design of foundation:

From structural point of view;

From geotechnical point of view;

Foundation movement sensitivityanalysis.

o Special requirements:

Fire safety;

Durability;

Resistance to chemical attacks.

Casting Yard

Two commonly used casting yards are describedbelow. To minimize the handling of transportationcost, they should be near the site.

Figure 6: Typical Reinforcement Details in a Slab Panel

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• Flat Beds

This type of casting yard can be as simple as “RCslab on ground” with the shape to match withthe wall or slab panel. Any additional features,like adjustable door/window frames, boundary,lubricant pipes, linked vibrators, etc. can be addedfor better efficiency, quality and convenience.One panel can be used for various shapes orseparate blocks are provided for each panel. Thedecision depends upon the availability of the land,scale and speed of construction, total numberof unique panels, lifting time, etc.

• Tilting Tables

Tilting tables are a kind of formworks or mold,normally made up of steel plates, which can betilted up by using hydraulic or other similarmechanism. The thickness, material strength andamount of reinforcement on the panels isgenerally governed by the stresses on thepremature concrete during lifting from the castingbed, rarely by the permanent load. With concretestrength higher than 30MPa (300 kg/cm2), it ispossible to lift the panel after 24 hours forstacking. The lifting stresses can be minimizedby lifting the panels not from the flat (sleepingposition) but from its inclined position (45 degreeor so). This helps to reduce the reinforcement.Acknowledgements:

- Pictures in this article have been taken from Preuksa Project andBaan Eaer-Arthon.

- Inception and strcutural systems design and development of Preuksa’sfirst phase was done by Dr. Naveed Anwar and ACECOMS.

Construction Sequence

Figure 8: Tilting Table Casting Bed

Figure 7: Flat Bed Casting Yard

Component Layout on Bed(bars, connectors, duct, door/window frame, anchor, etc.)

Casting

Lifting

Stacking

Transporting

Erection

Connecting

Erection

Finishing

More about the Author:The author was involved inthe design, production andconstruction of several PLPprojects. He is one of thekey persons involved in theindependent design reviewof various nonconventionalhousing systems designed byover 20 design teams for theNational Housing Authority,Thailand.

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24 April - August 2004