Lecture 2; Page 2
| Arch 721, Structural Design for Dynamic Loads,
University of Virginia Copyright © 1996-2006 Kirk Martini. 29-Aug-2007 8:39 |
Table
of Contents |
Moment frames in buildings
Recognizing steel moment frames
 |
 |
 |
 |
 |
 |
 |
A large moment frame (frame vs. punctured wall)
 |
 |
Moment frames in timber
 |
 |
 |
 |
 |
 |
 |
 |
(concentric) Braced frames in buildings
 |
 |
Using bracing as a facade element.
 |
 |
 |
Combining braced and moment frames
Along different axes
 |
 |
 |
At different levels
 |
 |
Eccentric braced frames
A variation of the knee brace
 |
 |
 |
 |
Properties of Braced Frames and Moment Frames
| Braced Frames: The Good News | |
| High Strength and Stiffness | |
| Braced frames have much higher initial strength and stiffness. Bracing is a much effective than rigid joints at resisting racking deformation of the frame. | |
| Efficient and Economical | |
| Braced frames use less material and have simpler connections than moment-resisting frames | |
| Compact | |
| Braced frames can lead to lower floor-to-floor heights, which can be an important economic factor in tall buildings, or in a region where there are height limits. | |
| Visual | |
| Braces can be used as a strong visual element (this is not always good news). | |
| Braced Frames: The Bad News | |
| Obstructive | |
| Braces can interfere with architectural requirements for doors, windows, and open floor area. | |
| Low Ductility | |
| Braced frames have bad ductility characteristics under cyclic loading, which is important for seismic design. Braced frames lose strength and stiffness after the braces buckle. (not such bad news for wind design). | |
| This problem can be overcome with structural sleeves to restrain buckling. This approach is gaining application in practice. | |
| Moment Frames: The Good News | |
| Unobstructive | |
| There is a great deal of flexibility in planning: no braces. | |
| Good Ductility | |
| They can have good ductility, if detailed properly (Special Moment Resisting Space Frame = SMRF = "smurf"). The performance is very sensitive to the detailing and workmanship at connections. | |
| Moment Frames: The Bad News | |
| Expensive | |
| Lots of material plus labor-intensive connections. | |
| Low Stiffness | |
| Large deflections can lead to high non-structural damage in earthquakes (i.e. undamaged structure will all glass broken and finishes cracked). | |
| Eccentric Braced Frames | |
| Combine Properties of Moment and Braced Frames | |
| Braces provide stiffness in elastic range, links control strength and provide ductility. | |
| Arch 721, Structural Design for Dynamic Loads,
University of Virginia Copyright © 1996-2006 Kirk Martini. 29-Aug-2007 8:39 |
Table
of Contents |