IMPORTANT GENERAL CIVIL ENGINEERING QUESTION AND ANSWERS
1. What is the difference between working stress approach
and limit state approach
For limit state approach, for each material and load, a
partial safety factor is assigned individually depending on the material
properties and load properties. Therefore, each element of load and material
properties is accurately assessed resulting in a more refined and accurate
analysis of the structure. In this connection, the material strength can be
utilized to its maximum value during its lifespan and loads can be assessed
with reasonable probability of occurrence. Limit state approach is commonly
used for the majority of reinforced concrete design because it ensures the
utilization of material strength with the lowest construction cost input.
2. What are the functions of different components of paint?
Primer: This is the first layer of a typical painting system
and it is used to inhabit corrosion and provide a good bond for subsequent
coats.
Undercoat: This component acts as a barrier to corrosion
agents and even out irregularities of bonding surface. It also serves to hide
the underlying background and prevent the details and colour of the area of
application to affect the designed colour and finishing details of paint.
Finishing coat: This is the final layer of a typical
painting system and it protects the underlying layers from the effect of
adverse weather conditions (e.g. sunlight) and to provide the designed
properties of paint like colour, impermeability, wearing resistance, etc.
3. In checking the quality of weld, what are the pros and
cons of various non-destructive weld inspection methods i.e. ultrasonic test,
radiographic inspection and magnetic particle flaw detection test?
The method of radiographic approach was used commonly in the
past until the arrival of ultrasonic inspection technique. The major difference
between the two is that ultrasonic testing detects very narrow flaws which can
hardly be detected by radiographic method.
Moreover, it is very sensitive to gross discontinuities.
Tiny defects, which characterize welding problems, are normally not revealed by
radiographic inspection. Moreover, ultrasonic inspection possesses the
advantages that it can accurately and precisely locate a defect as well as
figure out its depth, location and angle of inclination.
In the past, it was expensive to adopt ultrasonic means for
inspection. Nowadays, the rates for both inspection methods are comparable.
Most importantly, the x-ray and gamma ray used in radiographs are radioactive
and pose potential safety hazard to testing technicians on site. Reference is
made to Paul G. Jonas and Dennis L. Scharosch.
Magnetic flaw detection test can only be used for checking
flaws in any metallic objects. This method is commonly used for inspecting
surface cracks and slightly sub-surface cracks. However, surface and
sub-surface cracks can be readily detected by radiographs and ultrasonic
inspection.
4. Why should acetylene gas cylinders used for gas welding
be erected in upright position?
Acetylene is highly
unstable and flammable and would explode in elevated pressure when reacting
with oxygen in air. Storing acetylene gas in cylinders under pressure is very
dangerous. Hence, for welding purpose, gas acetylene is stored in cylinders of
liquid acetone contained in porous material (like firebrick) to enhance there
is no free space left for acetylene gas and for cooling purpose in the event of
thermal decomposition.
It also prevents the formation of high pressure air pockets
inside the cylinder. Dissolved acetylene in acetone will no longer in contact
with oxygen and is not subject to decomposition. On the other hand, acetone is
used because it is capable of dissolving large amount of acetylene gas under
pressure without changing the nature of the gas.
The cylinders for gas welding i.e. oxygen cylinders and
acetylene cylinders, when not in use should be stored separately because any
mixture of these gases resulting from accidental leakage can be highly
explosive. When in use, acetylene cylinders should always be kept in upright
position because acetone liquid will be drawn from the cylinders with the gas
if they are kept horizontally. Consequently, significant leakage of acetone
liquid will result.
5. Is stainless steel really stainless in construction
application?
A passive layer of chromium oxide is formed on stainless
steel’s surface which renders it corrosion resistant. This chromium oxide layer
acts as a stiff physical barrier to guard against corrosion and makes it
chemically stable. Moreover, when this layer is damaged, it can perform self
repairing where there is a sufficient supply of oxygen. However, stainless
steel will still corrode by pitting in marine environment where chloride attack
occurs.
Therefore, appropriate grades and types of stainless steel
have to be selected in polluted and marine environment to minimize the problem
of corrosion. Reference is made to Euro Inox and the Steel Construction
Institute (2002).
6. Is the procurement of third party insurance necessary to
be incorporated in contract for construction works?
7. What is the mechanism of protection by hot dip
galvanizing?
(i) It forms a metallic zinc and zinc-iron alloy coating on top of steel surface. This zinc coating reacts with moisture in atmosphere to from zinc salts which act as an insulating layer for steel/iron.
(ii) Zinc is higher than steel/iron in the galvanic series and when these dissimilar metals with different electrical potential are in contact, the zinc anode corrodes and offers sacrificial protection to steel/iron and hence steel/iron is protected from corrosion.
8. Should design life be the same as return period for
design conditions?
In selection of return period of certain design conditions,
winds, waves, etc., one should consider the consequences of exceedance. In
fact, there are normally no extreme maximum values of these design conditions
and its selection is based on the probability of exceedance which is related to
return period.
Therefore, design life may not be equal to return period of
design conditions because their selections are based on different
considerations.
9. What is the difference between sureties and security?
For surety bond, the contractor obtains a guarantee from a
third party i.e. a bank or an insurance company, which in return for a fee,
agrees to undertake the financial responsibility for the performance of
contractor’s obligations. This third party will pay to the employer in case
there is a contractor’s default.
For security, a sum of money is deposited in the employer’s
account and upon satisfactory fulfillment of contractor’s obligations, the sum
will be repaid to the contractor.
10. What is the difference between fasteners, bolts and screws?
11. What is the function of washers when using bolts?
12. What is the difference between normal bolts and high
friction grip bolts?
Ans. High friction grip bolts are commonly used in structural steelwork. They normally consist of high tensile strength bolts and nuts with washers. The bolts are tightened to a shank tension so that the transverse load across the joint is resisted by the friction between the plated rather than the bolt shank’s shear strength.
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