for

Earthquake ResistancePhilosophy of Structures

Date April 5, 2002.

Sanaullah Khan Kakar

Contents

► Shear Walls

► Moment Resisting Frames

► Braced Structures

► Framed Tube Structures

► Hybrid Structures

► Conclusion

Shear Walls

First used in 1940, may be described as vertical, cantilevered beams, which resist lateral wind and seismic loads acting on a building transmitted to them by the floor diaphragms.

Effects in Earthquake

Building frame system with Shear Walls

Shear Walls

Typical arrangement of shear walls

Shear Walls

A simple building with shear walls at its ends. Ground motion enters the building and creates inertial forces which move the floor diaphragms. This movement is resisted by the shear walls, and the forces are transmitted back down to the foundation.

Shear wall: vertical analogy as cantilever beams

Moment-Resisting Frames

Moment-resisting frames are structures having the traditional beam-column framing. They carry the gravity loads that are imposed on the floor system. The floors also function as horizontal diaphragm elements that transfer lateral forces to the girders and columns. In addition, the girders resist high moments and shears at the ends of their lengths, which are, in turn, transferred to the column system. As a result, columns and beams can become quite large. Moment Resisting

Frame System

Effects of Earthquake

Moment-Resisting Frames

Moment-Resisting Frames

■ Moment-resisting frames can be constructed of steel, concrete, or masonry. ■ Consist of beams and columns in which bending of these members provides the resistance to lateral forces.■ There are two primary types of moment frames, ordinary and special.■ Special moment-resisting frames are detailed to ensure ductile behavior of the beam-to-column joints and are normally used in zones of higher seismicity. ■ Steel moment-resisting frames have been under intensive study and testing.

Braced Structures

Braced frames develop their resistance to lateral forces by the bracing action of diagonal members. The braces induce forces in the associated beams and columns so that all work together like a truss with all members subjected to stresses that are primarily axial.

Braced Structure

Braced Structures

■ Braced frames act in the same manner as shear walls, though they may be of lower resistance depending on their detailed design.■ Bracing generally takes the form of steel rolled sections, circular bar sections, or tubes; vibrating forces may cause it to elongate or compress, in this case it loses its effectiveness and permits large deformations or collapse of the vertical structure.

Braced Structure

Braced Structures

Comparison of Moment-Resisting Frames and Braced Structures

Bracing can be used to stop buildings swaying over. It helps buildings stand up to the sideways forces that can occur during earthquakes or high winds. Bracing members can work in tension or in compression.

Effects of Earthquake

Braced Structures

Framed Tube Structures

A framed tube structure is a new addition to the structural systems. Khan is generally credited with its invention in the 1960s.

"a three dimensional space structure composed of three, four, or possibly more frames, braced frames, or shear walls, joined at or near their edges to form a vertical tube-like structural system capable of resisting lateral forces in any direction by cantilevering from the foundation."

Dr.Fazlur Rehman Khan

Framed Tube Structures

The framed-tube structure has its columns closely spaced around the perimeter of the building, rather than scattered throughout the footprint, while stiff spandrel beams connect these columns at every floor level. This structural system was first implemented in 1964 in the construction of the DeWitt-Chestnut Apartments in Chicago, a 43-story reinforced concrete tower designed by Dr.Fazlur Rehman Khan and his colleagues at Skidmore, Owings & Merrill (SOM). Because of its great relative strength and stiffness, the tubular form immediately became a standard in high-rise design.

The revolutionary framed-tube structural system was first seen in the construction of the DeWitt-Chestnut Apartments in Chicago.

Looking up at the Sears Tower, the worlds tallest building for over twenty years. The design for this 1454-foot tall tower introduced the bundled tube structural system, as well as a new vocabulary in architectural form.

Framed Tube Structures

In 1998 the city of Chicago named the intersection of Jackson and Franklin Streets (located at the foot of the Sears Tower) “Fazlur R. Khan Way.”

Framed Tube Structures

Hybrid Structures

Structures are often built in which the lateral resistance is provided by a mixture of structures. The most common are moment resisting frames combined either with structural walls or diagonal bracing.

Both externally and internally the new Toyota building is visually exciting

Hybrid Structures

■ £25 million Toyota headquarters on the North Downs near Epsom racecourse in Surrey.■ The hybrid structural frame which successfully combines precast and insitu concrete with structural steelwork.■ The resulting hybrid structure was erected at a speed to rival that of steel construction and has the bonus of a high quality exposed finish that absorbs temperature fluctuations. To paraphrase the Toyota advertisement: “the structure in front is a hybrid”.

There is a refined juxtaposition of the structural elements

Conclusion

Structural systems that are suitable for different ranges of number of stories