Question

What is the primary advantage of pre-stressing in structural design?

• A. Increased durability
• B. Enhanced aesthetic appeal
• C. Improved fire resistance
• D. Higher load-carrying capacity

Hint:

\textbf{Pre-stressing Concrete:} Inducing internal compressive stresses in concrete to counteract tensile stresses from service loads.
\textbf{Primary Advantages:}
Significantly \textbf{increased load-carrying capacity} and longer span capability.
Better crack control and reduced deflections under service loads.
Allows for more slender and efficient structural members.
Improved durability is a consequence of better crack control.

Answer 1

The Correct Option is D

Pre-stressing is a technique used in concrete construction (and sometimes other materials) where internal stresses are deliberately introduced into a structural member before it is subjected to external service loads. This is usually done by tensioning high-strength steel tendons (wires, strands, or bars) within or adjacent to the concrete. The pre-stressing force induces compressive stresses in the concrete in zones where tensile stresses would normally develop under service loads. Concrete is strong in compression but weak in tension. By pre-compressing these zones, the tensile stresses caused by external loads are counteracted, effectively increasing the load-carrying capacity of the member and controlling cracking. Primary advantages of pre-stressing (especially in concrete):
Higher load-carrying capacity: Pre-stressed concrete members can carry significantly larger loads or span longer distances compared to equivalent non-pre-stressed (reinforced concrete) members. This is because the pre-compression effectively increases the section's capacity to resist tensile stresses from bending. (Matches option d)
Reduced cracking and improved crack control: By keeping the concrete in compression under service loads, tensile cracking is minimized or eliminated, leading to better durability and protection of reinforcement.
Increased stiffness and reduced deflections: Pre-stressed members are generally stiffer and exhibit smaller deflections under service loads.
Lighter and more slender members: Due to increased efficiency, members can often be made more slender, leading to material savings and aesthetic possibilities.
Improved durability (indirectly): Reduced cracking improves durability by protecting steel reinforcement from corrosion. (Option a is a consequence, but higher load capacity is more primary). Let's evaluate the options:
(a) Increased durability: This is an important benefit, largely resulting from better crack control, but the fundamental structural advantage is enhanced load capacity.
(b) Enhanced aesthetic appeal: Pre-stressing allows for more slender sections and longer spans, which can contribute to aesthetic appeal, but this is a secondary benefit.
(c) Improved fire resistance: Pre-stressing itself does not inherently improve the fire resistance of concrete or steel significantly compared to conventional reinforced concrete. Fire resistance depends more on concrete cover, aggregate type, and member size. Special considerations are needed for fire design of pre-stressed concrete.
(d) Higher load-carrying capacity: This is a primary and fundamental advantage. Pre-stressing enables concrete members to resist much higher bending moments and shear forces. The primary structural advantage of pre-stressing is the significant increase in the load-carrying capacity of the member. \[ \boxed{\text{Higher load-carrying capacity}} \]