Official Website of NIAZIT Industrial Group

**Negative Moment Reinforcement (ODKA Bars) in Open Web Steel Joists:

Why NIAZIT Joists Do Not Require Top Reinforcement**

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Negative Moment Reinforcement in Open Web Steel Joists | ODKA Bars vs NIAZIT System

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Technical comparison of negative moment behavior in chromite open web steel joists and NIAZIT joists based on SJI principles. Why ODKA bars are required in chromite systems but not in NIAZIT.

1. Introduction

In steel joist floor systems, negative bending moments at supports play a critical role in the structural design of joists.
In conventional chromite open web steel joists, these moments are typically resisted using top reinforcement bars (ODKA bars).
However, in NIAZIT joist systems, which are steel joists without concrete in the web, such reinforcement is not required.

This article explains this difference based on structural mechanics, section properties, and SJI design philosophy.

2. Negative Moment Behavior in Steel Joists

According to SJI design principles:

• Positive bending governs mid-span behavior

• Negative bending governs support regions

• In negative moment zones:

  • Top chord → tension

  • Bottom chord → compression

Since concrete has negligible tensile capacity, the steel section alone must resist negative moment tension forces.

3. Chromite Joists and the Necessity of ODKA Bars

3.1 Structural Configuration

Chromite joists typically consist of:

• Top chord: steel angle (L-section)

• Bottom chord: steel bar or plate

• Web: steel lattice, filled with concrete after installation

• ODKA bars: additional top reinforcement at supports

3.2 Structural Limitations of the Top Angle

From a section mechanics perspective:

• Angle sections have:

  • Low radius of gyration

  • Asymmetrical stress distribution

  • Reduced tensile efficiency under negative bending

As a result, the top angle alone cannot reliably resist support tension forces.

3.3 Concrete Dependency and Cracking

Although the web is filled with concrete:

• Concrete cracks under tensile stress at supports

• Once cracked, it cannot participate in negative moment resistance

• The load path shifts entirely to steel elements

This creates a structural gap, which is compensated by ODKA bars.

3.4 Function of ODKA Bars

ODKA bars:

• Are placed near the top chord at supports

• Provide tensile capacity for negative bending

• Control cracking and rotation at supports

In chromite systems, ODKA bars are structurally mandatory, not optional.

4. NIAZIT Joists: Why ODKA Bars Are Not Required

4.1 Correct Definition of NIAZIT System

NIAZIT joists:

• Are not classified as precast joists under Iranian regulations

• Have no concrete in the web

• Rely entirely on steel section behavior and geometry

4.2 Tee Section as Top Chord

The most significant difference is the top chord section.

• NIAZIT uses a steel Tee section

• Compared to angles, Tee sections offer:

  • Higher radius of gyration

  • Symmetrical stress distribution

  • Improved tensile stability

A higher radius of gyration directly improves resistance to tensile deformation under negative moments.

4.3 Negative Moment Distribution

In NIAZIT joists:

• Load transfer is entirely steel-based

• No reliance on cracked concrete

• Negative moment stresses are:

  • Distributed across the entire Tee section

  • Not concentrated at a single reinforcement bar

This eliminates the need for localized top reinforcement.

5. Comparison Summary

6. Conclusion

• Chromite joists require ODKA bars due to:

  • Limited tensile capacity of angle sections

  • Concrete cracking at negative moment zones

  • Low radius of gyration

• NIAZIT joists:

  • Use Tee top chords with superior section properties

  • Rely on steel-only load paths

  • Satisfy negative moment requirements without additional reinforcement

This distinction reflects engineering optimization, not material reduction.