Lateral force resisting systems, pros and cons
A large number of design groups want to apply a lateral force-resisting system that works well and produces a result in the long run. Thus, a lateral force-resisting system supposed to be both performance-based and cost-effective. Every lateral resisting system has its unique benefits and difficulties for each design and environmental situations. Here are the different types of lateral force-resisting system.
One major issue to consider when designing a metal structure is how the building’s resistance will respond to lateral movement. You would want to design a structure that can absorb most of the turning natural activities that produce lateral motion in a building, such as an earthquake and wind force. Architects, engineers, and building designers can use any one of three framing systems together or combine them as they deem fit to counter the effect of these occurrences. These three framing systems are Shear wall, Braced Frame, and Moment frame.
The Moment frame has been used for many years and it is the most expensive to execute. Analyzing the costs shows that the difference between Braced and Moment frame structures depends on the range of 200 and 400 percent respectively, based on what is involved in the project. The love for steel Moment framing has been much for 20th and 21st-century structures for many reasons.
First, it works well with multi-story buildings that contain wide, open spaces because building with a moment frame lack the structural walls and/or vertically diagonal braces required in braced frame or shear wall designs.
Second, it is better for accommodating taller, higher quantity and/or expansive window openings.
Third, and most importantly, it was noted by building professionals and building enforcement officials that steel buildings utilizing moment framing in their designs seemed to demonstrate superior earthquake-resisting capability.
Thus, building codes in the latter part of the 20th-century adopted preferential design criteria for steel moment frames, especially in regions of the country that were more prone to seismic activity – although that mode of thinking has evolved a bit as well, which will be covered a bit later.
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Moment resisting frames
Moment-resisting frame is a rectilinear assemblage of beams and columns, with the beams rigidly connected to the columns. Moment-resisting frames allow windows but are not very stiff. Moment resisting frames are made up of beams and columns that resist lateral loads through flexure of members and through stiffness of rigid joints connecting the beams and columns. Moment frames generally cost more than braced frames.
- Provides flexibility for architectural design and layout
- Produces greater deflection and drift compared to that of braced frames or shear walls
- Produces localized stress concentrations at rigid joints
- Requires care in the erection of connections in order to resist lateral loads properly
- Expensive moment connections
Braced frames: stiff and efficient
A braced frame is a structural system, which is designed primarily to resist wind and earthquake forces. Members in a braced frame are designed to work in tension and compression, similar to a truss. Braced frames are almost always composed of steel members. Braced frames resist loads through a series of trusses made of steel members. The diagonal members of the trusses resist lateral loads in the form of axial stresses, by either tension or compression. Braced frames are often the most economical method of resisting wind loads in multi-story buildings.
- Can be located internally or externally for flexibility of architectural design
- Accommodates service penetrations
- Can be located within partition walls
- No need for moment connections
- Produces problems for a layout of windows and doors due to obstruction of bracings
- Requires fireproofing materials for steel members so that takes up space
- Need for large gusset plates for connections between beams and columns.
Shear walls: very stiff, less efficient
In structural engineering, a shear wall is a structural system composed of braced panels (also known as shear panels) to counter the effects of the lateral load acting on a structure. The Wind and Seismic loads are the most common loads that shear walls are designed to carry. Shear walls, no option for windows but very stiff.
Shear walls also provide resistance to lateral forces by cantilever action through shear and bending. The slab connected to the shear wall must function as a horizontal diaphragm. Also, shear walls need to be placed symmetrically in both directions to the plane of loading so that no torsional effect would be produced.
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