Steel design – cased beams: For a steel beam encased (cased) in solid concrete, the minimum overall width B of the concrete casing, in terms of the steel beam's flange width b, should be what value to ensure durable side cover and fire protection?

Introduction / Context:
Encased (cased) steel beams are widely used in building construction to improve fire resistance, corrosion protection, and composite action with surrounding concrete or masonry. Choosing the minimum overall casing width B relative to the steel flange width b is critical to guarantee adequate side cover, durability, and space for proper compaction of concrete around the steel.

Given Data / Assumptions:

• Steel I- or H-beam encased fully in concrete (solid casing).
• b denotes the steel beam flange width; B denotes the concrete encasement overall width.
• Normal indoor exposure with a preference for robust fire cover and construction tolerance.

Concept / Approach:

Concrete encasement requires adequate side cover on both faces of the steel flange. Practical detailing rules of thumb provide a minimum of about 50 mm concrete cover on each side to ensure fire resistance, protection against corrosion, and room for vibration/compaction. Therefore, the encasement width B is taken as approximately b + 50 mm + 50 mm = b + 100 mm.

Step-by-Step Solution:

Verification / Alternative check:

Fire design tables and common practice for encased members align with side covers of the order of 40–60 mm per face for standard ratings, making b + 100 mm a serviceable minimum that also accommodates tolerances.

Why Other Options Are Wrong:

• b + 25, b + 50, b + 75 mm: Side cover is insufficient for robust fire and durability performance.
• b + 125 mm: More conservative than typically necessary; adds weight and width without proportional benefit.

Common Pitfalls:

• Ignoring tolerance: too small a margin hampers placement and compaction.
• Not coordinating with stirrup/mesh placement around the encased steel.

Final Answer:

B = b + 100 mm.