Ethan Clarke
Compact sections are thick plate thickness cross-sections that can develop a plastic moment without the occurrence of local buckling. They are one of the three commonly known cross-section classifications, the other two being non-compact and slender sections. Compact sections can develop a plastic hinge prior to local buckling of the flange or the web. However, compact sections do not have the reserve ductility to maintain their strength beyond a certain rotation. They are also unable to form a collapse mechanism due to inadequate plastic hinge rotation capacity.
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What You'll Learn
Compact sections have a thick plate thickness
When designing structural steel members, there are several factors to consider, especially when it comes to bending stresses in beams. Structural engineers typically use the plastic moment capacity for structural steel. However, those unfamiliar with structural engineering theory or not using AISC specifications often use the more conservative elastic bending capacity. This can lead to more expensive designs.
The plastic moment capacity is safe and a tried-and-true practice in the structural steel building industry. A beam's plastic moment is the theoretical limit at which the entire section has yielded, and if any more load is applied, a plastic hinge will form in the member. The plastic moment limit prevents the plastic hinge from completely forming, and a part of the section will not yield.
Noncompact sections are sections where the stress in the extreme fibres can reach yield stress, but local buckling is likely to prevent the development of plastic moment resistance. Slender sections are those where local buckling will occur in one or more parts of the cross-section before reaching yield strength.
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They can develop a plastic hinge before local buckling
Compact sections can develop a plastic hinge before local buckling. They can develop a plastic moment of resistance, but they lack the necessary plastic hinge rotation capacity to form a collapse mechanism before buckling. This is due to their insufficient ductility reserve, which causes them to fail as a result of local buckling of the flanges.
A plastic hinge is formed when the entire cross-section has stressed to yield. In the context of structural steel members, the plastic moment capacity is often utilised by structural engineers. However, some engineers who are unfamiliar with structural engineering theory or don't adhere to AISC specifications may opt for the more conservative elastic bending capacity.
The ideal design is to have a compact section or provide adequate bracing to ensure that a plastic hinge can form at the end supports without triggering a collapse mechanism. This is because compact sections, by definition, form a plastic hinge but lack the required rotation capability to collapse.
To prevent local buckling, the flanges and webs of the RHS must be sufficiently stocky, and the unbraced length of the member must be adequate to develop the plastic moment without any buckling limit states failing first. Small deformations under yielding should be acceptable, and a detailed deflection analysis should be performed if deflection is a significant concern.
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They don't have the rotation capacity to form a collapse mechanism
While compact sections can develop a plastic moment of resistance, they lack the necessary ductility for adequate plastic hinge rotation capacity. This means that compact sections cannot form a collapse mechanism before buckling.
In structural engineering, a plastic hinge is formed when the entire cross-section of a member is stressed beyond its yield point. This is distinct from a compact section, which can develop a plastic hinge prior to local buckling of the flange or web. However, compact sections lack the rotation capacity to form a collapse mechanism.
The ability of a structure to form a plastic hinge is crucial in preventing collapse. When a beam is loaded with a uniform torque, a plastic hinge will form in the member if the load exceeds the beam's capacity. This occurs when the entire section has yielded, and further load is applied. However, in the case of compact sections, the lack of ductility prevents them from undergoing additional rotation without a loss of strength.
The reserve ductility of plastic sections allows them to undergo additional rotation without a loss of strength up to a certain point. This additional rotation capacity is what enables the formation of a collapse mechanism. On the other hand, compact sections fail due to the local buckling of their flanges, as they do not possess the same level of ductility.
In summary, while compact sections can develop a plastic moment, they lack the rotation capacity to form a collapse mechanism due to their limited ductility. This is a key distinction between compact and plastic sections in structural engineering.
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Compact sections are susceptible to local buckling
Compact sections are defined as sections that can develop a plastic hinge before the local buckling of the flange or web. They are susceptible to local buckling because they do not have the reserve ductility to maintain their strength beyond a certain point. This is in contrast to plastic sections, which can undergo additional rotation without a loss in strength.
The susceptibility of compact sections to local buckling is due to their large width-thickness ratios. This means that they are more likely to fail due to buckling before they can reach their plastic moment capacity. In other words, they may develop a fully plastic stress distribution across the entire cross-section, but they do not have adequate ductility.
To prevent local buckling in compact sections, it is important to ensure that the unbraced length of the member is sufficient to develop the plastic moment without any buckling limit states failing first. Small deformations under yielding should be acceptable, and a detailed deflection analysis should be performed if deflection is a major concern.
Additionally, the AASHTO LRFD specifications limit the flange slenderness of straight girders to 12 to prevent excessive distortion when welded to the web and to avoid damage to the flange during construction.
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They have limited rotation capacity and are unsuitable for plastic hinge formation
Plastic hinges are a crucial concept in structural engineering, and their formation is integral to the design of structures. While compact sections can develop plastic moments, they have limited rotation capacity, making them unsuitable for plastic hinge formation. This limitation arises from their inadequate ductility, which prevents them from forming a complete collapse mechanism.
In the context of structural engineering, a plastic hinge refers to the deformation of a beam section where plastic bending occurs. This deformation allows for plastic rotation within the structure. When a beam reaches its plastic moment capacity, it can undergo plastic rotation without any further increase in the bending moment. This behaviour is akin to that of a real hinge, hence the term "plastic hinge."
The formation of plastic hinges is essential in the design of structures, particularly in ensuring their stability and safety. By inserting a plastic hinge at any section reaching its plastic moment, engineers can analyse the structure's behaviour and predict its response to increasing loads. This analysis is fundamental to designing structures that can withstand the loads they are subjected to.
However, compact sections have limited rotation capacity due to their lack of reserve ductility. While they can develop plastic moments, they fail to maintain their strength beyond a certain rotation level. This limitation is attributed to the local buckling of their flanges, which occurs when the moment applied exceeds the section's capacity. As a result, compact sections cannot form a complete collapse mechanism, even though they may develop plastic hinges.
To address this challenge, engineers aim for a design that ensures plastic hinge formation at the end supports while avoiding it in the span that could trigger a collapse mechanism. This careful consideration of rotation capacity and ductility is crucial in preventing structural failure and ensuring the safety of buildings and other structures.
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Frequently asked questions
A compact section is a cross-section that can develop a plastic hinge before the local buckling of the flange or web. It has a thick plate thickness and can develop its plastic moment resistance without local buckling.
In structural engineering, the plastic moment (Mp) is a property of a structural section. It is the maximum bending moment that the section can resist before a plastic hinge forms and infinite plastic deformation occurs.
A plastic section has sufficient rotation capacity to form a collapse mechanism, whereas a compact section does not. A compact section will form a plastic hinge but does not have the required rotation capability to collapse.
The ideal design for a steel member is to have a compact section or adequate bracing to ensure a plastic hinge can form at the end supports without triggering a collapse mechanism.
Plastic bending refers to the use of a material's plastic moment capacity, which is the theoretical limit at which the entire section has yielded. Elastic bending uses the more conservative elastic bending capacity, which assumes the material has not yielded.
