The
choice of structural system to be adopted mainly depends on the floor layout,
functionality of the building, mechanical and electrical services and total
height of the building. A suitable building system will enhance the robustness
and lifespan of a building. Therefore, the suitability, as well as pros and
cons of the four systems discussed in the previous session shall be considered
before design stage.

To
ensure the standardization and uniformity in designing the structural
components for prefabrication construction, several design codes have been
introduced. This will then help to facilitate the construction on-site and
manufacturing process in the assembly factory. Till now, three codes that have
been adopted in Singapore are listed as below:

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Ø  SS EN 1992 Eurocode 2: Design of
Concrete Structures

Ø  Singapore Standard CP65: Code of
Practice for Structural

Ø  British Standard BS 8110:
Structural Use of Concrete 1997

In
these codes, properties of concrete and steel have been clearly stated and
explained. Besides, the design guidelines on fire resistance and durability
have also been included. For instance, the nominal concrete cover needed for
reinforced concrete member subjected to different kind of exposure is listed in
the form of table in CP 65. Figure shows the snapshot of CP65 related to
nominal concrete cover needed for various exposure conditions. In addition, the
recommended design imposed loads for various kind of building, as well as the
dead load for architecture finishing, such as false ceiling, floor tile and
screeding are too listed.

Figure: Nominal
cover to all reinforcement listed in CP65.

Movement Joint for
Building System

When
a building is subjected to drastic temperature change due to surrounding
weather, it may expand and contract. Also, differential settlement may occur if
the building is large and built on several types of foundation systems which
lay on different ground conditions. Such movement will then initiate cracking
process and deteriorate the serviceability and lifespan of the building. Hence,
movement joints are introduced to cater for these problems.

Movement
joints are specially formed joints to accommodate relative movement such as
those due to thermal effects between adjoining parts of a structure. In
general, there are three types of movement joints, namely

Ø  Contraction Joint

To allow cracking and contraction
due to concrete shrinkage and thermal effect

Ø  Expansion Joint

To provide gap and thus accommodate
the expansion of a building structure

Ø  Construction Joint

To be located at point with minimal
stress and thus minimise the occurrence of cracks

According
to Structural Engineering’s Pocket Book, movement can be difficult to
waterproof and detail, thus it should be kept to a minimum.

Figure: Typical expansion joint (left) and
construction joint (right)

Without
detailed calculations, joint should be details to permit 15-25mm movement. For
preliminary sizing purpose, some approximate guidelines on the spacing of
movement joint based on the types of building are provided in the figure below:

Figure: General guidelines in spacing of
movement joint extracted from Structural Engineer’s Pocket Book.

Robustness Design

In
general, a cast in-situ concrete structural frame is very robust due to its
monolithic nature. To explain further, it is the normal connection detailing for
cast in situ reinforced concrete meet the tying requirement to avoid
progressive failure. In contrast, precast concrete frames require special
consideration with regards to their robustness.

Progressive
failure is defined as the collapse of large part of a building which initiated
by the failure of a relative small of part of it. It is sometimes called
disproportionate failure. Normally, progressive failure happens due to the
accidental impact load exerts on a small part of building, which causes part of
the load bearing system to fail and the impact of the accidental load, coupled
with load of fragments of failed rock debris will transfer to the adjacent
structural components. If the adjacent components are inadequate, it will initiate
a chain reaction that causes other structural elements to fail in a domino effect,
like the progressive failures of house of cards.

One
typical example of progressive failure is the collapse of 2000 Commonwealth
Avenue in United States 1971. The collapse was due to a punching shear failure
occurred at 16th storey of the building due to improper concrete
detailing and low concrete strength. Such small failure initiates a chain
reaction causing the building to be progressively collapse.

 

Figure:
Progressive failure of 2000 Commonwealth Avenue.

Figure: Typical
load transfer diagram

As
stated in the CP 65, one of effective means to prevent progressive failure is
to provide effective horizontal ties around the periphery, internally and to
columns and walls. With this, the method of quantifying the required ties force
are shown in the snapshots of EC 1 below:

a.      Horizontal Ties

b.    
Vertical
Ties

Where H is the clear height of the
wall and t is the thickness of the wall in mm.

Figure: Examples
of ties

Another
way to prevent progressive failure is through the design of catenary action.
The underlying ideology of robustness design through catenary action is that
any loss of key load bearing structures, especially column would not result in
disproportionate collapse of the building. This can be done by bridging of
damaged structures (slab) which then leads to a new alternative load path.
Consequently, load can be distributed to the remaining functioning columns
securely.

Research
had 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 test
with the loss of supporting system. With this, they had proposed a set of
equations which is meant to quantify the total tension force and hence the area
of steel rebar needed for splices between two continuous slabs and beams. The
equations are given as below:

                                     For
UDL

                                      For single
point load

 

 

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