2.2 Behaviour of Steel Structures Under Earthquake Loads

The use of structural steel for building construction is a relatively young concept. Numerous factors have contributed to the growth of this market, and in the World the generally favorable performance of steel buildings in earthquakes prior to 1994 no doubt played a significant role. Specifically four earthquakes in California and Japan (San Francisco, Kanto, Santa Barbara and Long Beach) during the first part of this century gave engineers confidence in steel as a reliable material for earthquake resistant design. During these events, there were typically fewer problems observed in steel structures as compared with concrete and masonry buildings of similar size and scale.

2.2.1 Ductile Behaviour of Structural Steel

In seismic design it is very important to assess the ability of a structure to develop and maintain its bearing resistance in the inelastic range. A measure of this ability is ductility, which may be observed in the material itself, in a structural element, or to a whole building structure. These three kinds of ductility are very different in their numerical values, and each one plays a significant role in seismic design.

Material ductility -µ_e, measures the ability of the material to undergo large plastic deformations. A high value of -µ_e characterizes a ductile material, a low value a brittle one.

Structural element or joint ductility -µ_q, characterizes the behaviour of a member or joint, and particularly its ability to transmit stresses in the elastoplastic range without loss of resistance. For instance a frame structure cannot exhibit ductile behaviour if the plastic hinges are not able to redistribute the bending components.

Structural ductility -µ_d, is an index of the global behaviour of the structure, i.e. the ability of a structure to deform in the inelastic range after some of its parts have exceeded their linear elastic range.

The ductilities µ_e, µ_q and µ_d must meet the condition:

µ_e > µ_q > µ_d

2.2.2 1994 Northridge Earthquake and Moment Resisting Frames

Welded steel Moment-Resisting Frames (MRFs) were long considered as one the most earthquake-resistant types of structures. However, in the 1994 Northridge earthquake, more than 200 buildings of this structural type suffered brittle fractures at connections. None of these steel frame buildings collapsed, but the unexpected type and severity of the damage raised serious questions about the current practice in the design and construction of welded MRFs. Most of these observed fractures occurred at the beam-to-column connections and were usually instigated at the level of the full penetration welds.

A particularly disconcerting aspect of this damage was that it often occurred without accompanying distress to architectural finishes and cladding. As a result, reconnaissance reports immediately following the Northridge earthquake often cited the apparent excellent behavior of steel frame buildings. However, severe damage found in buildings under construction at the time of the earthquake, and detailed investigations of MRFs buildings which suffered increasing amounts of damage during aftershocks, quickly identified the true performance.

Current professional judgment is that the historic practices used for the design and construction of MRF connections do not provide adequate reliability and safety, and should not continue to be used in the construction of new buildings intended to resist earthquake ground shaking through inelastic behavior. As a consequence, pre-qualified connection details and design methods contained in the building codes have been rescinded. Emergency code provisions stipulate that new designs be substantiated by testing or use test-verified calculations.

 

Picture 2.3 - Moment Resisting Connection Details (Weak Beam, Strong Column Concept)

 

Several fundamental questions must be answered in order to develop effective and economical design procedures and construction standards, and to restore public and professional confidence in this form of construction. These questions include:

1. What happened to MRF buildings during the Northridge earthquake? What caused the observed damages?
2. How do we identify MRF buildings that may have sustained damage?
3. How safe are damaged MRF buildings and do they need to be repaired? How can damaged buildings be reliably repaired and/or upgraded?
4. How do we design and construct new buildings so they will not sustain similar damage?
5. Can the vulnerability of existing MRF buildings to future earthquakes be reliably determined and mitigated through effective rehabilitation procedures?
6. What are the economic, social and political costs of new design or construction practices?

Answering these questions involves consideration of many complex technical, professional and economic issues including metallurgy, welding, fracture mechanics, connection behavior, system performance, and practices related to design, fabrication, erection and inspection. Unfortunately, current knowledge is inadequate.

2.2.3 Ductility of Beam-to-Column Connections

The fracture of steel building beam-to-column connections in the Northridge and Kobe earthquakes generated concerns about the reliability of current design and construction technology for steel connections.

 

Picture 2.4 - Common Zone of Fracture Initiation in Beam Column Connection

 

The same type of fracturing may occur at similar steel building beam-to-column connections in other seismically active areas. Recent studies of the newly designed connections under cyclic load show that the ultimate strengths are almost unaltered; however, the plastic rotational capacity can be increased significantly.

2.2.4 Column Panel Region Effects

The panel zone is the portion of the column within the depth of the connecting beams in a moment resisting steel connection. The transfer of moments between beams and columns causes a complicated state of stress and strain in the panel zone. Under the action of forces, the panel zone deforms in three modes: Axial, shear, and bending. Only the shear deformation of the panel zone has a significant effect on the behavior of steel frames.

 

Picture 2.5 - Weak Panel Zone Effect

 

Panel zone design provisions have undergone large changes in the past four decades. The panel zones of steel moment frame structures of the 1960's and 1970's were generally strong in shear. There are two primary reasons for this trend. First, the then existing provisions ignored the contribution of column flanges to the shear strength of the panel zone. Second, the moment demand on the connection was, in many cases, overestimated by: (1) Assuming that framing beams could reach their full plastic capacity, and (2) disregarding gravity moments in calculating connection demands. Since steel frame design is frequently governed by drift limitations, beams are deeper and have larger plastic strength than otherwise required by seismic strength provisions, thus increasing the design demand on the panel zones. Furthermore, gravity moments on interior steel connections tend to counteract seismic moments; however, their effect is rather small, particularly in systems with a few perimeter moment resisting frames. When combined together, these two factors (underestimating strength and overestimating demand) frequently resulted in the need for panel zone reinforcement, which was mainly provided through doubler plates.

2.2.5 1995 Kobe Earthquake

The 1995 Great Hanshin Earthquake (M=6.9), commonly referred to as the Kobe earthquake, was one of the most devastating earthquakes ever to hit Japan. More than 5,500 were killed and over 26,000 injured. The economic loss has been estimated at about €158 billion. The proximity of the epicenter and the rupture propagation directly beneath the highly populated region help explain the great loss of life and the high level of destruction.

Relatively few steel buildings were significantly damaged, with the exception of the 51-building Ashiyahama Seaside Town complex. These buildings, of late 1970's vintage, range in height from about 10 to 29 stories, and are of non-conventional construction. That is, the structure consists of large trussed steel column-beam frames, with typically a two-legged bent per building. A number of 500 m² steel box columns (50 m wall thickness) were observed to have totally fractured.

The major reasons of the damaged steel structures can be categorized following:

• When ductile steel elements are connected by some method, for example by welding, the connection may not be as ductile as the steel.
• Brittle fracture of large size steel members is an another major problem in steel construction.. This type of damage was observed in some high-rise apartment buildings in a large residential development. Steel plates of approximately 50 mm thick were welded to form large square column sections. Before the construction, steel members were tested in the laboratory under simulated earthquake loading to confirm the safety, but using small scale specimens. It appears that the behavior of thick steel members is significantly affected by their size.
• The age of the construction was another reason for the failure problem during the Kobe Earthquake.

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