What is the purpose of installation of resilient bearings in buildings?

When railway tunnels are built close to buildings, ground-borne vibration is transmitted to the building by means of compression and shear waves.

When structural members (e.g. wall) of a building have natural frequencies similar to the frequency at the source, the response of the structures would be magnified. This effect is even more significant when the building is designed with small number of movement joints. Consequently, the vibration can be felt inside the building and noise associated with such vibration is produced. To avoid this, vibration isolation can be implemented, sometimes by providing resilient bearings at column heads in buildings.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

Where is the desirable location of lifting anchors in precast concrete units?

It is desirable that the position of anchors be located symmetrical to the centre of gravity of the precast concrete units. Otherwise, some anchors would be subject to higher tensile forces when compared the other anchors depending on their distance from the centre of gravity of the precast concrete units. As such, special checks have to be made to verify if the anchor bolts are capable of resisting the increased tensile forces.

concrete anchor

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

Is mild steel or high yield steel suitable as lifting hoops in precast concrete?

The strength of high yield steel is undoubtedly higher than mild steel and hence high yield steel is commonly used as main steel reinforcement in concrete structures. However, mild yield steel is commonly used in links or stirrups because they can be subjected to bending of a lower radius of curvature.

For lifting hoops in precast concrete, it is essential that the hoops can be bent easily and hence mild steel is commonly adopted for lifting hoops because high yield bars may undergo tension cracking when it is bent through a small radius.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

What are the potential problems of excessive concrete covers?

In reinforced concrete structures cover is normally provided to protect steel reinforcement from corrosion and to provide fire resistance. However, the use of cover more than required is undesirable in the following ways:

(i) The size of crack is controlled by the distance of longitudinal bars to the point of section under consideration. The closer a bar is to this point, the smaller is the crack width. Therefore, closely spaced bars with smaller cover will give narrower cracks than widely spaced bars with larger cover. Consequently, with an increase in concrete cover the crack width will increase.

(ii) The weight of the concrete structure is increased by an increase in concrete cover. This effect is a critical factor in the design of floating ships and platforms where self-weight is an important design criterion.

(iii) For the same depth of concrete section, the increase of concrete cover results in the reduction of the lever arm of internal resisting force.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

Do engineers need to cater for corrosion protection of lifting anchors in precast concrete units?

The corrosion of lifting anchors in precast concrete units has to be prevented because the corroded lifting units cause an increase in steel volume leading to the spalling of nearby surface concrete.

Consequently, steel reinforcement of the precast concrete units may be exposed and this in turns results in the corrosion of steel reinforcement and the reduction in the load carrying capacity of the precast units. To combat the potential corrosion problem, the lifting anchors could be covered with a layer of mortar to hide them from the possible external corrosion agents. Alternatively, galvanized or stainless steel lifting anchors can be considered in aggressive environment.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

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