IntroductionHousing is a basic human necessity. In general sense,a house represents something that can be used as ahabitat or living space. The housing demand variesfrom place to place. From socio-economic point ofview, the key determinants of the gross housingdemand are micro- and macro- economics,affordability, government policies, life style, culture,age and population distribution, cost of land etc.Housing demand, supply and technology are relatedin a complex way. Technological developments havegreat influence on various activities related to housingincluding the design, production/construction,marketing, cost and comfort.

From engineering point of view, a house is a type ofstructure that has some special design characteristics(such as the lateral load is not a governing designcriteria). An efficient and economical engineeringdesign of a residential house requires a goodunderstanding of these typical characteristics. Severalmethods ranging from highly simplified manualcalculations to fully automated modeling, analysis anddesign are being practiced. Application of the moderncomputer tools to automate the design, detailing, costestimation and production demands a good workingknowledge, experience and judgment on variousaspects of structural engineering, construction practiceand computer skill. Proper understanding of theunderlying theory and effective application ofcomputer tools empowers engineers with competitiveadvantages.

An attempt has been made in this article to discussthe structural engineering design aspects of aresidential house. Most commonly encountered issuesin design process will be addressed. Use of moderncomputer tools for the design of real world problemswill be illustrated at the end.

Objective and Scope of This ArticleA series of article has been planned on computer aidedanalysis and design of residential and general buildingsystems. This is the first article of this series. Theoverall objective of this first article is to provide anintegrated overview of the housing systems fromstructural engineering point of view. The main focusof this article is the computer application in thestructural modeling, analysis, design and detailing ofhousing systems. The concepts, methods and toolsintroduced in the first part of the article will be appliedto a real project in the second part.

General Classificationof Housing SystemsThe housing systems can beclassified in various ways, whichdepend upon the basis of classification.Some of them are as follows:

Based on primary material of construction

• Reinforced concrete

• Steel

• Timber

• Aluminum

• Composite

• Hybrid

Based on method of construction

• Cast-in-situ

• Precast/Prefabricated

• Combined

Based on connectivity/neighborhood

• Detached

• Twin houses

• Row houses

Structural Classification of HousesFrom structural design point of view, a house can beclassified broadly as follows. Precast and load bearingblock systems which are relatively new will bediscussed here in more details than the conventionaltype of RC framing system.

Load bearing masonry

In this type of houses, the gravityand lateral load are resisted by themasonry walls made up of clay,mortar or concrete bricks/blocks.Basic building blocks of variousforms, sizes, materialcomposition are in current use.Keys and interlockingmechanism (like tong an groove)are generally provided foreffective locking between theblocks, layers and the connectingwalls. Minimum boundedreinforcement may be provided

Computer Aided Analysis and Design ofHOUSE

Interlocking Block House [Nichada,Bangkok System]

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for stability and overall integrity of the system.Designs are based more on laboratory test results ofthe blocks and the unit made up of basic buildingblocks.

Load bearing RC pre-cast large panel houses

This type of houses primarily consist of site-assembledpre-cast concrete panels. The panels act as load bearing

members as well as partition/outer walls. The design of theconnections and joints are themost importantconsiderations in design ofthis type of houses. Variousjoint systems have been triedand are in current use. Suchsystems have beensuccessfully implemented insome large scale housingprojects in Thailand and othercountries. No beams andcolumns are present in thissystem. The main attractive

features of this system is the fast construction, betterquality control and relatively low cost.

Pre-cast RC/PC Members Framed Wet-jointSystems

The final product and the load transfer mechanismsare similar to conventional RC framed systems.However the main difference between theconventional and this system comes from the fact thatin this system frame members at cast at factory. Mostof these members are RC and however pre-stressingcan be used in some situations. The reinforcement barsare exposed only at the end of the pre-cast memberswhich will provide the reinforcement in the joint castat site.

Pre-cast RC/PC Members Framed Weld JointSystems

This system differs from theabove one only on themethod used for beam-column joint. In Metal platesare fixed at the end that needto be connected duringcasting at the factory and themembers are connected bywelding in their finalposition.

Conventional RC Framedsystems

This is the conventional typeof structural system in which the primarily loadtransfer mechanism is the frame action and members/components are cast at site using formworks.

Mixed systems

Mixed system consists of a balanced combination oftwo or more of the above systems.

Characteristics of Common HousingStructural SystemOne of the most popular housing systems is cast-in-situ RC beam and column system with cast-in-placeor pre-cast slab panels. The main characteristic of suchsystems relevant to structural engineers are:

• The primary load transfer mechanism is theframed action. The load transfer path is simpleand well understood.

• Most of the joints and connections are monolithic(rigid) type.

• The components can be clearly classified asstructural and nonstructural.

• Live and other gravity loads are transferred inthe order from slab, to beams, to columns and thento foundation (except flat slab which are not sopopular for houses).

• Isolated footings or piles are used as foundation.

Methods of Design

Two primary methods of design, that using computerand without using computer, will be discussed first asfollows. It is obvious to all that the computers offergreat advantages than the conventional design.

Modern Method of DesignOne of the drastic changes in design practice can beattributed to the use of computers in engineeringworks. Due to increasing popularity of computer, thehouses are, now, designed with the help of computingtools. Refined methods of analysis and design arecommon. Powerful methods and techniques like FiniteElement Analysis allow the designer to easily andrealistically model and analyze the housing systems.It is possible to include inclined roofs, swimmingpools, water tanks, openings, variable section inanalysis models with minimum efforts and time.Spring or other complex elements can be used to

represent the foundation orflexible type of supports. Theselection of proper designsection locations,determination of criticaldesign combinations,alternative section checks canbe automated. The wind andearthquake load can beestimated and appliedconveniently. The deflectionscan be better estimated usingthe effects of cracks on RCsections.

Using Computer for Design of HousesAs introduced in previous sections, computerprograms are highly useful for design of almost alltype of houses. Design software like ETABS use thebuilding specific terminologies to provide intuitiveworking environment for designers. Object-basedmodeling techniques allows us to create the computer

Precast Large Panel Houses[Preuksa System–Designed by

ACECOMS]

Precast RC/PC Members withWelded Joints

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model using basic graphic entities (point, line, areaand solids) instead of core finite element based termslike node, element, boundaries etc. The followingsection discusses further the concepts, methods andtechniques in modeling, analysis and design housesusing computers.

Modeling, Analysis and Design of GeneralComponents

Modeling of most common type components such asslabs, beams, columns, etc. do not require any specialtechniques. However there are certain issues that needto be take care. Important of those will be first asfollows.

Slabs

The important considerations in modeling slabs arewhat type of elements to be used, what should be meshsize, what should be the connectivity conditionbetween slab and beam; and how to transfer the loadfrom slab to other members. Precast panels withtopping are modeled as one way panels (in case ofETABS). In this case, there is no need to divide thepanel into smaller elements. The floor loads can beapplied directly on the panels which are transferredto the supporting beams/columns automatically. Thesepanels are membrane type elements. Two way typeload transfer behavior need to be captured, the slabcan be modeled as membrane or plate type elements.If modeled as membrane, the load is transferred tothe supporting members automatically and there is noneed to divide the slab into smaller elements. In thiscase there will be no interaction between the slab andsupporting elements.

However the slab can also be modeled as Plate/Shellelements. In this case a panel need to be divided intoat least 4x4 elements/panel for the proper transfer ofthe load from slab to the beams. The most importantpoint that the analyst has to understand in this regardis that in FE modeling and analysis, except somespecial techniques are in place, the loads applied toelements (1d, 2d, 3d) are transferred to the nodes andnot to the element edges (even if beams are there inthe model). If a slab panel is divided into 4x4 elements,the trapezoidal or triangular load on beam isrepresented by equivalent 4 point loads which isgenerally considered acceptable for practical design.The design values may not match with the onecomputed using manual transfer of loads (as triangularor trapezoidal). The FE way of transfer of load matchesclosely with manual distribution when the slab meshbecomes finer and finer (more than 6x6 or so). It ishighly recommended that the analyst verify the BMand Shear diagram on the supporting beams beforedesigning these members based the program computedFE results. Another equally important point is thatprogram assumes a full interaction or continuitybetween slab, beams and columns when the slab ismodeled as plate/shell elements (without releases). Itis very easy to verify by viewing the slab moment plots.

Beams

The common practice is to model the beams using 1Delements assuming that they lie on the same plane of

the slab i. e. the centroid of the beams and slab are insame horizontal plane. In real construction the centroidof the beams are below the slab mid-plane. Thissituation can be modeled realistically by providingvertical offset to beams. Rigid offsets are useful toreduce the design moment and for economical design.The effective moment of inertia can be modified (o.5Ig or so ) to estimate the deflection taking into accountthe cracking effects.

One of the most common questions regarding themodeling of beams, especially the secondary beams,is how to handle the torsion on beams? Can we releasethe beam from torsion and design assuming no torsionin introduced in primary beam due to secondary beam?If the secondary beams are released from torsion, howto details the beam ends/joints so that the assumptionmade during analysis are justified? Two key conceptsthat are useful to answer the above questions are: firstis to understand the difference between the equilibriumand compatibility torsions and secondly, is tounderstand the various ways that a secondary beam,primary beam and slab can interact or transfer loadfrom one to another. It is difficult to provide generalanswers which can be applied to all situations.However some guidelines are that it is possible torelease the secondary beam so that it torsion is inducedin main beam due to secondary beam and provide thereinforcement detailing accordingly. The designershould ensure that the final detailing and analysisassumptions are consistent/compatible.

There are several other issues that the author would like to addressin the future issues of this magazine. Those are how to handle thesituations when the beam does not frame centrally into column, dropson beams, beams of varying cross sections, cantilever beams, toppingbeams, etc..

Columns

Columns are relative easier to model but relativedifficult to design if they are too slender. As long asthe columns are short columns, automated designfeatures are generally reliable. However when thecolumn becomes long column and subjected to morethan one action (PMM), a comprehensive designprocedure need to be used. In this case, the softwareestimated factors to compute the effective lengthfactors, moment magnification parameters, sway/non-sway checks, minimum eccentricity calculations, etc.which have direct impact on direct (capacity,reinforcement) need to be carefully examined andverified before accepting the program recommendedreinforcement. It is quite common that programoverestimates the moment magnification factor (5-10)for commonly used column sections in houses (20-30x 20-30 cm and 3 m height) and computes thelongitudinal bars based on excessively magnifiedmoments. Therefore it is extremely important tounderstand the influence of different parameters inthe design of long columns (especially the connectivitycondition) and also to verify whether the automaticallycalculated values are reasonable or not.

Foundations

Foundations can be modeled as fixed, hinge, rolledor spring type supports. Before applying these externalrestraints it is important to understand the possible

April-August 200336

movement of these (base) locations/nodes. Fixed,hinge and rolled restraints/supports are applied tofinite element nodes, normally at the base of columns.However it is recommended to use spring supports tonodes, line elements or area type elements. Spring typesupport take into account the structure-soil interactionand give better results than infinitely fixed or free typesupport. Some general guidelines for modelingfoundations are:

• The spring values can be computed basedon foundation size and the modulus of sub-grade reaction of soil. This modulus can beestimated from ultimate bearing capacitywhich is approximately 40 times the ultimatebearing capacity of soil in (Metric unit).Modulus of sub-grade reaction can beunderstood as an indicative of soil-stiffness.

• The pile foundation can be converted intovertical and lateral springs. The lateralsprings can be estimated using similarconcept that described above and stiffnessvalues for vertical springs can be assumedapproximated as double the structuralstiffness of pile (2EA/L).

Design of Special ComponentsInclined Roof

The may roof consists of multi-faceted sloped surfacesgiving a special architectural effects. Some of the

difficult parts of the modeling and analysis of suchroofs are:

• The ridge of various parts not at the same elevation/level

• The faces of the roof are inclined at 30 degree

• The cantilever parts of the roof are not identical (notequal).

• 3D arbitrary orientation of the rafters, purlins etc

• Difficult to visualize and set the convenient workingviews in creating the model and verifying the results

• No straight forward or automated methods to applyand transfer the roof loads on purling, rafter and roofbeams.

• Sections available in market may not be availableon software database

• Manual transfer of load is too complex and timeconsuming

3D Finite Element Model of slopedroof in SAP2000

April-August 2003 37

One of the techniques to overcome most of the abovedifficulties is to model the roof in general finiteelement programs like SAP2000 instead of ETABS.The model may include the purlins, rafters, roof beamsand roof columns. The roofing material (tiles, sheets)can be modeled as plate and shell element with highlyreduced modulus of elasticity so that even if it isincluded in the model, it will not act as structuralmember or provide any stiffness. However the roofdead, live and wind loads can be easily applied assurface pressure on shell elements and automaticallytransferred to the supporting elements (purling, rafteretc.) Concepts similar to slabs for FE meshing can beapplied.

The following figure shows an example of such modeland some important parameters used in modeling. Theplan view of the roof can be generated AutoCAD andchanged to sloped faces by modifying the coordinateof the ridge nodes in analysis software. Alternativelyit might be convenient to draw the 3D frame of theroof in AutoCAD and import in the analysis software.

Staircases

There are various ways to include the Staircase into3D models ranging from treating staircases as lineloads on supporting beams at top, landing and base tophysically/actually modeling as shell elements. Directdrawing or extrusion from line element are some ofthe efficient tools for creating staircases. If staircasesare modeled as inclined shell elements, the loads canbe applied directly on the flights and stiffnesses areincluded into the model automatically. Equivalent slabthickness should be used. Landing slabs can bemodeled n Reference Planes (not full fledge floor).Simply suspported staircases (steel or precast) shouldbe treated as loads and not the structural members.

Split Floors

Number of floors for given height of a house isincreasing. Partial floors covering less than full planarea of the house are called Split Floors. Modeling ofsuch floors requires special techniques both incustomized or general finite element software. Incustomized software such as ETABS, there are noexplicit provisions for partial floors. The verticalelements (columns, walls, braces) are divided (createnodes) by all defined floors irrespective of they arefull or part. To overcome this problem, features likeReference Plans can be used. However both of thesemodeling techniques do not handle the split floorseffectively. The first method, divides the verticalmembers into more than required number of elementsand in the second method modeling, viewing,reporting, etc. are not convenient. The selection is amatter of choice.

Openings

Openings on slab, walls, roofs, etc. can be modeledeasily by diving the mesh so that one of more elementsof required size are created at opening location anddeleting the element in that location or setting theelement at those locations to Null type areas. In manysituations, the inclusion of the opening in model

become important to study the stress concentrationand detailing.

Swimming Pools

Isolated swimming pools which are not integral to thebuilding system can be designed as separate structure.If the pool forms the part of the house, they can bemodeled as they are using shell elements. The waterloads can be applied as triangular load on the sidewalls and uniform pressure on the base slab. Both theempty and water-full situations need to be evaluated.The walls can be designed using the procedure similarto design of vertical or shear walls.

ConclusionsIt is very efficient to model and analyze the residentialhouses with modern computer tools. Almost all typesof houses of different material, geometry, loads andlocations can be handled easily. Working knowledgeof finite element modeling, member design and aproper software tool are essential requirements.Special modeling techniques are required to handlesplit floors, integral swimming pools, stair cases,inclined roofs, joints and foundations.

About the Authors

1.) Buddhi S. SharmaManager, ACECOMS, AITemail: buddhi@ait.ac.th

2.) Suda TaleongpongAssociate (Chief Structural Designer),Palmer&Turner (Thailand) Co., Ltd.email: wnt_eng@yahoo.com

3.) Somyot SinpaiboonResidential Project Chief, Civil Park International(CPI).email: somyot@civilpark.com

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