Working-stress “moment of resistance factor” for M150 (1:2:4) concrete: For concrete of nominal grade M150 (mix 1:2:4), which of the following is the typical moment of resistance factor used in classic working-stress design tables?

Introduction / Context:
Older working-stress design (WSD) methods tabulate a “moment of resistance factor” for common concrete mixes, enabling rapid preliminary sizing of plain and reinforced members. Although limit-state design is prevalent today, many exam questions still reference these WSD values.

Given Data / Assumptions:

• Nominal concrete grade M150 corresponding approximately to mix 1:2:4.
• Use within classic WSD tables for preliminary/educational calculations.
• No limit-state partial safety factors are applied here.

Concept / Approach:
These tabulated factors encapsulate allowable stresses and section properties to simplify hand calculations. For M150 concrete, a commonly cited factor is about 7.5 (in appropriate consistent units as used in those tables), enabling quick estimation of section capacity before detailed checks.

Step-by-Step Solution:
Identify grade: M150 (1:2:4).Consult/WSD convention: standard factor ≈ 7.50 for this grade.Select value: 7.50.

Verification / Alternative check:
Numerous traditional textbooks list the factor 7.5 for M150; it aligns with the allowable stress set used in WSD tables.

Why Other Options Are Wrong:

• 0.87: Not a WSD moment factor; it is typically the steel stress factor used in LSM (0.87fy).
• 5.80 or 8.50: Values associated with different mix strengths/assumptions, not the standard for M150.

Common Pitfalls:
Applying WSD factors to limit-state design without conversion; always be consistent with the design philosophy and units.

Final Answer:
7.50