Design bond stress increase for deformed reinforcing bars (ribbed/HYSD) In reinforced concrete design practice, by what percentage may the permissible design bond stress be increased for deformed bars compared to plain (mild) round bars?

Introduction:
Bond stress is the critical interface action that transfers force between reinforcing steel and surrounding concrete. Deformed (ribbed) bars have mechanical interlock that increases bond capacity compared to plain round bars. This question tests the standard percentage increase adopted in design for deformed bars over plain bars.

Given Data / Assumptions:

• Bars are standard deformed/HYSD rebars embedded in normal strength concrete.
• Design situation follows accepted code practice where plain-bar bond values are enhanced for deformed bars.
• No special coatings (e.g., epoxy) or adverse conditions are assumed.

Concept / Approach:

Due to ribs/lugs, deformed bars develop higher bond through both adhesion and mechanical interlock. Codes permit a percentage increase applied to the basic design bond stress specified for plain bars. This increase accounts for improved pull-out resistance and reduced slip in service.

Step-by-Step Solution:

Verification / Alternative check:

Pull-out tests and long-standing code provisions consistently show deformed bars outperform plain bars by a substantial margin. The +60% factor is conservative and widely recognized for general conditions.

Why Other Options Are Wrong:

20%, 30%, 40%, and 50% understate the established enhancement for deformed bars and would be overly conservative for development length and anchorage checks.

Common Pitfalls:

Confusing development length multipliers with bond stress enhancement; forgetting that adverse conditions (e.g., poor compaction, congestion) may still warrant caution even with deformed bars.

Final Answer:

60%