Earth Quake Resistant Steel Structure

This paper reviews recent innovations that expand the range of applicability of a number of new and emerging structural steel system that can provide effective seismic performance. Focus is the following recent development some of them are as follow and going to be discussed here.

(A) STEEL PLATE SHEAR WALLS

(B) PERFORATED STEEL PLATE SHEAR WALLS

(C) BUCKLING RESTRAINED BRACED WALLS

(D) ECCENTRICALLY BRACE FRAMES

Introduction

In the design of earthquake resistant high rise steel structures, two basic requirements must be met. First, the structure must remain serviceable during the ordinary, frequently occurring load applications. This is usually accomplished by designing the structure so that it remains elastic and provides adequate stiffness to limit deflections. The second requirement is to preclude a disaster during a major earthquake. For such an extreme event, considerable inelastic deformation is usually allowed. Thus, structures must possess sufficient ductility and inelastic stability to withstand these extreme excitations. So researches have been to find a solution that can meet this criteria and how steel can used as high rise structures yet withstand the effect of seismic activity the research focused on Frame Modifications at beams mid span, self centering system, zipper frames, Buckling Restrained Braced Frames. Steel Plate Shear Walls, Plastic & Rotation Limit for building and on shear limits &Truss Pier for BRIDGES. These researches have resulted on the development of various concepts for enhancing the seismic performance of Steel Structures. Here in this PAPER we are going to discuss that how these systems are going to be implemented as being practiced by the Engineering Community over the past few years. Here in this paper were going to discuss about the following:

(A) STEEL PLATE SHEAR WALLS (SPSW)

They are designed to rely on the development of diagonal tension yielding.

(B) PERFORATED STEEL PLATE SHEAR WALLS

They are Steel Plate Shear Walls but with special shapes and designs instead of a conventional steel plate depending on the need.

(C) BUCKLING RESTRAINED BRACED FRAMES (BRBF)

They are special braces that can develop their full axial yield strength for both tension and compression.

(D) ECCENTRICILY BRACED FRAMES

In Eccentrically Braced Frames the bracing are connected to two different points on beams or girder.

(E) CONTROLLED ROCKING OF STEEL BRACED FRAMES

It consists of three components:

(a) Stiff Braced Frames.

(b) Post Tensioning Strands.

(c) Replaceable Energy Dissipating Fuses.

These system are widely being practiced to be used in the construction of large steel bridges as well as high rise steel structures as they are highly DUCTILE system that make it possible to design structures with high lateral stiffness thus indirectly limiting the damage caused by the Non-Structural components of the structure caused by Earth Quake as it makes the building unusable for a long period of time. In the past important seismic evaluation and retrofit of major bridges have occurred in North America since a span of San Francisco-OAKLAND BAY BRIDGE collapsed during 1989 LOMAR PRITA Earth Quake. Similarly large steel trusses bridges and buildings were also evaluated & retrofitted in the states of Oregon. Washington, California & mid west states where important life line exists.

Steel plate shear walls

The selection of (SPSW) as the primary system for resisting lateral forces in the building has increased over the past few years as Design Engineer discovered the benefit of this option. In the past through this system web buckling was prevented and no utilization of post-buckling strength was made possible due toextensive stiffening or by selecting appropriate thickness of web plate and only focused on Elastic & Shear yield plate behavior. So when further research was conducted it became clear that the reduced thickness of web plate played a vital role as it allowed shear buckling and after this buckling the lateral loads are carried to the panel via subsequently developed diagonal tension field action. Smaller panel thickness also reduces forces on adjacent member like beams and columns which make up the boundary elements of the walls as they play a vital role in withstanding the load of the building acting on them so it results in more efficient framing designs. This plate system also dissipates energy under extreme cyclic loading to its infill plates. So understanding the seismic behavior of thin plate shear walls has significantly improved over the past few years. This system can be characterized as a vertical cantilever plate girder in which columns acts as flanges and the steel plate acts as web while the cross beams represent the Transverse stiffness. Although the PLATE GIRDER THEORY that governs the plate design seems appropriate but it should not be used in design of SPSW structure since the relatively high bending strength and stiffness of the beams and columns (BOUNDARY COMPONENTS) have significant effect on the post buckling behaviors. This system is implemented to control failure on a building by Pre-Selecting localized ductile fuses (or weak links) to act as a primary location for energy dissipation when is building is subjected to extreme seismic loading. It is designed in such a way that all the IN-ELASTIC action (or damage) occurs at these critical fuses which are designed to behave in a ductile manner so that they can give appropriate warning before rupture and in this system the infill plates acts a fuses and when damaged they can easily be replaced at a reasonable cost and restore the full integrity of the building. A further benefit of this infill shear plates is that the diagonal tension fields acts as a diagonal braces in a braced frames and thus completes the truss action and is very efficient means to controldiagonal load acting on it. This system of SPSW (Steel Plate Shear Walls) is an efficient alternate to other steel systems and a much beneficial and economical replacement of RCC shear walls as it helps in reducing the area covered by the RCC shear retaining walls as it has much smaller cross section and are much lighter and also minimize the seismic load acting on columns & beams which is proportional to the mass of the structure and the erection procedure of all steel structures is much faster so less manpower thus economical When this system is designed and detailed properly it proves very efficient in dissipating energy at high risk Earth Quake zone. The web plate acts as a diagonal brace and has initial stiffness s it is efficient in limiting wind drift. And all the steel structures are beneficial and much more feasible to the colder regions where concrete construction is not suited. The process of retro fitting of this system is much more feasible and faster to install which is a major issue after the building has been subjected to Earth Quake and needs to be rebuilt. Recently it has been proposed to use light gauge cold rolled and low yield strength (LYS) steel for the infill panels and also the placement of (pattern or PERFORATION) to decrease the strength and stiffness of the panel by a desired amount in addition to use of reduced beam section at the ends of the horizontal boundary members is being investigated as a mean reducing the overall system demand on the vertical boundary members.

Perforated steel plate shear wall (P-SPSW)

Over the last two decades, un-stiffened SPSW has become a popular and efficient wind and earthquake load resisting system in North America and Japan. High initial stiffness, excellent ductility, and energy dissipation capacity and tremendous post-buckling strength make this system unique compared to other conventional systems used in the past. As researches have been made for the use of perforated steel plate infill of smaller thickness because this perforated system can accommodate passing of utilities like electric lines water pipes etc through the infill plate and also handling and application of the infill plates becomes more feasible. Thus, CSA/CAN-S16-09 and AISC 2010 have included perforated steel plate shear wall (P-SPSW) system in their current editions. As an improved version of un-stiffened SPSW, this system is equally applicable to new buildings as well as retrofitting of existing buildings. According to Canadian standard, openings should be spread uniformly over the entire plate in a staggered position with a certain distance from the boundary. The openings are oriented in such a way that the plate buckling is independent of loading direction. figure from term paperVian & Bruneau (2004) investigated the seismic performance of SPSW designed and fabrication using low yield strength (LYS) steel panels and reduced beam section (RBS) are added to the ends in order to force all in elastic action in the beams to that position. SPSW with low yield steel webs appear to be the viable option for use in resistance of lateral loads imparted during seismic excitation. The steel with lower yield strength and thickness after testing showed reduced stiffness and earlier onset of energy dissipation by the panel as compared to the conventional hot rolled plate. And the perforated steel panel shows more promise towards alleviating stiffness and more economical. Another paper on this topic (REFERENCE) summarize the test conducted observed the ultimate behavior of this panel and described the adequacy of sample models to replicate the global behavior of SPSW considered therefore these result will not be repeated here.

Buckling restrsined braced frames

BRB frame have received much attention in recent years in U. S authors have extensively covered the latest research and knowledge on this topic(sabelu :eal. 2003, Uang and Nakashima 2003). This system provides lateral resistance to buckling particularly during seismic activity. This system consist of a steel core surrounded by a hollow steel section coated with a low friction material and then grounded with a specialized mortar. The steel core when encased with the (HSS) with the help of mortar prohibits it from buckling when on compression which the coating prevents the axial load to be transformed to the casing. This preventing the strength loss and allowing for better andmore systematic cyclic performances. The concept of BRB was first developed in Japan by NIPPAN STEEL at the end of 1980’s and was known as (UNBOUNDED BRACE). Due to this system seismic load applied to the structure is effectively reduced which result in smaller cross section for the beams and column’s of the braced frame smaller demand on the connecting and most importantly the load’s on the foundation are drastically decreased. Its main purpose is to dissipate the lateral forces from the column’s and beam’s there for the connection of the braces to beam’s and column’s can greatly affect the performance of the brace in the case of the seismic event. Typically the brace is attached to gusset plate which in turn is welded to the beam and column that the brace will be attached to usually three types of connection are used for BRB’s:

• Welded connection:It is fully welded to the gusset plate on the field. This option although reassures additional man hours on site it can increase and improves the performance of brace by improving the force transfer mechanism.
• BOLTED CONNECTION:These connections like welded ones are bolted in the field.
• PINNED CONNECTION: In this connections the brace and gusset are both designed to accept a pin so that can be connected to each other and allow free rotation.

In this system the sizes of column and beam are typically smaller and with simpler connections also BRBS are usually much more easier to erect and can be easily used in seismic retrofitting making it much more economical and easier to work with. Finally in the event of an EARTH QUAKE the damage caused by it is concentrated over a relatively smaller area (brace yielding core) and it also makes post EARTH QUAKE investigation and replacement relatively easy. But it also has its draw backs as BUCKLING RESTRAINED BRACES rely on ductility and must generally be replaced after usage during a major EARTH QUAKE as it rely on ductility of yielding steel core to dissipate energy. As the core yields the material work hardens becoming stiffer. This hardening increases the expected force up to 2 times the initial yield force so it requires stronger connection and foundation strength.

Eccentrically braced frames

Eccentrically braced frames are relatively new lateral force resisting developed to resist seismic events in a predictable manner. Properly designed and (EBF) behave in a ductile manner through shear and flexural yielding of a desired element. This system essentially resembles the features of a moment frame and a concentrically braced frame yet minimizing the drawbacks of each of these systems. The eccentricity in this system provides a ductile fuse that yields in shear and prevents brace buckling. Static and dynamic analyses indicates that this system can perform well during earthquakes, because it combines the stiffness of a concentrically braced frame with the excellent energy dissipation of a moment-resisting frame so utilizing the benefits of these two systems. As they are ductile structure they undergo in-elastic deformation and energy absorption being confined to the region of the joints eccentrically. The brace members are connected to the links or ductile fuse in the girder or beams at separate work points in this system the length of the links are particularly kept short. The beam/girder section or link between them absorbs and dissipates large amounts of energy during seismic activity while maintaining a very stiff structure and it offers many potential applications in the seismic design of structures.