In grillage foundations a minimum 15 cm cover is provided on

Solution (By Examveda Team)

Let's understand this question about Grillage Foundations.

First, what is a Grillage Foundation?
Imagine you have a very heavy column, and the soil below isn't super strong. To spread out this heavy load over a larger area of soil, we use a grillage foundation. It's like a criss-cross pattern of strong steel beams (like I-beams) placed in layers (called tiers), one on top of the other, usually at right angles. All these steel beams are then completely covered and encased in concrete.

Now, what does "cover" mean here?
In civil engineering, "cover" is the thickness of concrete between the steel (whether it's reinforcement bars or steel beams) and the outside surface of the concrete structure. Its main job is to protect the steel.

Why is protection important?
Steel can rust (corrode) if it's exposed to moisture, air, or harmful chemicals, especially those found in the soil. Rust weakens the steel, which can make the whole foundation fail. So, the concrete cover acts as a shield!

The question asks about a minimum 15 cm cover. Where is this crucial?
Think about which part of the grillage foundation is most exposed to the harsh conditions of the soil. It's the very bottom part of the foundation, specifically the beams of the lowest tier that are directly resting on or very close to the soil.

If the concrete cover below these lowest beams is not enough, moisture and chemicals from the soil can easily seep through and reach the steel beams, causing them to corrode quickly. A large cover, like 15 cm (which is 150 mm), provides excellent protection against these elements, ensuring the foundation lasts a very long time.

Let's look at the options:
Option A: Upper flange of top tier - While it's covered by concrete, this isn't the most vulnerable spot for corrosion from the soil compared to the bottom.
Option B: Lower beam of lower tier - This is the critical location. The bottom of the lowest tier is in direct contact with or very close to the soil. A large cover here is essential to protect the steel from soil moisture and chemicals.
Option C: Ends of external beams - Protection at the ends is also important, but the continuous bottom surface of the lowest tier is generally considered the most vulnerable and requires this substantial cover for overall durability.
Option D: None of these - This is incorrect as Option B is clearly the most appropriate answer.

Therefore, the correct answer is Option B: Lower beam of lower tier because it is the part most exposed to the corrosive environment of the soil and requires the maximum concrete cover for durability and protection of the steel beams.