It is generally impractical to ensure that a steel connection behaves strictly as either pinned or rigid, and structural engineers often adopt one of these classifications out of habit. In many cases, this approach is completely understood and accepted. However, in other situations, it may lead to significant issues.
Let's examine a standardized moment-resistant joint (IM) and explore how choosing exclusively between pinned and rigid connections can lead not only to uneconomical designs but also, more critically, to dangerous ones.
In the RFEM program from Dlubal Software, we use the Steel Joints add-on to first verify whether the results for initial stiffness and moment resistance correspond with those found in the "Standardised Steel Joints in Steel Structures to DIN EN 1993-1-8" publication. The results show considerable similarity.
And now, moving on to the structural design of a continuously loaded beam connected to a column via our connection:
CASE (A) - Uneconomical Design
In a completely rigid connection, the joint transmits a large bending moment. This design approach relieves the bending stress on the beam but may results in an oversized joint.
Conversely, with a pinned connection, the joint itself may be adequate, but the beam is likely to fail due to a significant moment occurring mid-span.
In our example, semi-rigid connection ideally reduces the moment at the joint without excessively increasing the moment in the middle of the span to an unacceptable level. Therefore, both the joint and the beam are satisfactorily designed.
CASE (B) - Dangerous Design (Addition of Tension Normal Force)
Treating a semi-rigid connection as pinned overlooks the bending moment that actually occurs. This oversight can lead to a dangerous design where the capacity of the connection is significantly strained by normal and shear forces alone. Subsequent bending moments increase the utilization of the joint, potentially leading to joint failure.
CASE (C) - Dangerous Design (Longer Beam More Prone to Deflection)
Assuming a semi-rigid connection as rigid implies that the joint is stiffer than it actually is. This assumption falsely improves the conditions for the beam, which then bends less and produce smaller bending moment along its span. However, in reality, the joint is not as rigid as assumed. The beam then experiences greater deflection and mid-span moments, potentially resulting in an unsatisfactory outcome in the serviceability limit state or undesirably affecting structures on which the beam acts.
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