Thechoice of structural system to be adopted mainly depends on the floor layout,functionality of the building, mechanical and electrical services and totalheight of the building. A suitable building system will enhance the robustnessand lifespan of a building.
Therefore, the suitability, as well as pros andcons of the four systems discussed in the previous session shall be consideredbefore design stage. Toensure the standardization and uniformity in designing the structuralcomponents for prefabrication construction, several design codes have beenintroduced. This will then help to facilitate the construction on-site andmanufacturing process in the assembly factory. Till now, three codes that havebeen adopted in Singapore are listed as below:Ø SS EN 1992 Eurocode 2: Design ofConcrete Structures Ø Singapore Standard CP65: Code ofPractice for Structural Ø British Standard BS 8110:Structural Use of Concrete 1997Inthese codes, properties of concrete and steel have been clearly stated andexplained. Besides, the design guidelines on fire resistance and durabilityhave also been included. For instance, the nominal concrete cover needed forreinforced concrete member subjected to different kind of exposure is listed inthe form of table in CP 65.
Figure shows the snapshot of CP65 related tonominal concrete cover needed for various exposure conditions. In addition, therecommended design imposed loads for various kind of building, as well as thedead load for architecture finishing, such as false ceiling, floor tile andscreeding are too listed. Figure: Nominalcover to all reinforcement listed in CP65.Movement Joint forBuilding SystemWhena building is subjected to drastic temperature change due to surroundingweather, it may expand and contract.
Also, differential settlement may occur ifthe building is large and built on several types of foundation systems whichlay on different ground conditions. Such movement will then initiate crackingprocess and deteriorate the serviceability and lifespan of the building. Hence,movement joints are introduced to cater for these problems.Movementjoints are specially formed joints to accommodate relative movement such asthose due to thermal effects between adjoining parts of a structure. Ingeneral, there are three types of movement joints, namelyØ Contraction JointTo allow cracking and contractiondue to concrete shrinkage and thermal effectØ Expansion JointTo provide gap and thus accommodatethe expansion of a building structureØ Construction JointTo be located at point with minimalstress and thus minimise the occurrence of cracksAccordingto Structural Engineering’s Pocket Book, movement can be difficult towaterproof and detail, thus it should be kept to a minimum. Figure: Typical expansion joint (left) andconstruction joint (right)Withoutdetailed calculations, joint should be details to permit 15-25mm movement. Forpreliminary sizing purpose, some approximate guidelines on the spacing ofmovement joint based on the types of building are provided in the figure below:Figure: General guidelines in spacing ofmovement joint extracted from Structural Engineer’s Pocket Book.Robustness DesignIngeneral, a cast in-situ concrete structural frame is very robust due to itsmonolithic nature.
To explain further, it is the normal connection detailing forcast in situ reinforced concrete meet the tying requirement to avoidprogressive failure. In contrast, precast concrete frames require specialconsideration with regards to their robustness. Progressivefailure is defined as the collapse of large part of a building which initiatedby the failure of a relative small of part of it. It is sometimes calleddisproportionate failure. Normally, progressive failure happens due to theaccidental impact load exerts on a small part of building, which causes part ofthe load bearing system to fail and the impact of the accidental load, coupledwith load of fragments of failed rock debris will transfer to the adjacentstructural components. If the adjacent components are inadequate, it will initiatea chain reaction that causes other structural elements to fail in a domino effect,like the progressive failures of house of cards.
Onetypical example of progressive failure is the collapse of 2000 CommonwealthAvenue in United States 1971. The collapse was due to a punching shear failureoccurred at 16th storey of the building due to improper concretedetailing and low concrete strength. Such small failure initiates a chainreaction causing the building to be progressively collapse.
Figure:Progressive failure of 2000 Commonwealth Avenue.Figure: Typicalload transfer diagramAsstated in the CP 65, one of effective means to prevent progressive failure isto provide effective horizontal ties around the periphery, internally and tocolumns and walls. With this, the method of quantifying the required ties forceare shown in the snapshots of EC 1 below:a. Horizontal Tiesb. VerticalTiesWhere H is the clear height of thewall and t is the thickness of the wall in mm.Figure: Examplesof tiesAnotherway to prevent progressive failure is through the design of catenary action.
The underlying ideology of robustness design through catenary action is thatany loss of key load bearing structures, especially column would not result indisproportionate collapse of the building. This can be done by bridging ofdamaged structures (slab) which then leads to a new alternative load path.Consequently, load can be distributed to the remaining functioning columnssecurely. Researchhad been conducted on this field of study by Dr. M J C Wilford and Dr. C. W.
Yu. Several beam and slabs specimens were tested through flexural bending testwith the loss of supporting system. With this, they had proposed a set ofequations which is meant to quantify the total tension force and hence the areaof steel rebar needed for splices between two continuous slabs and beams. Theequations are given as below: ForUDL For singlepoint load