Lateral resisting systems

Many design teams want to implement a lateral force resisting system that is both performance-based and cost effective. Each lateral resisting system has its own advantages and disadvantages for each specific design and environmental conditions. We will explore these different types.

One of the most pressing concerns when designing a metal building is the structure's resistance to lateral movement. Specifically, you want to design a building that can withstand the most dramatic natural events that cause lateral motion in a building, including wind force and seismic activity.

In order to combat these occurrences, architects and building designers can use one of three framing systems or combine them as they see fit: Shear wall, Braced frame and Moment frame.

While the third option, the moment frame, has been used for thousands of year it is the most costly to execute. Costs analysis reveals that the differential between braced and moment frame buildings lies somewhere between 200% and 400%, depending on the scope of the project.

The attraction towards steel moment framing has been multi-fold for 20th and 21st century buildings for several 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.

Thirdly, 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.

Moment resisting system

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.

Advantages

Provides flexibility for architectural design and layout

Disadvantages

Produces greater deflection and drift compared to that of braced frames or shear walls

Produces localized stress concentrations at rigid joints

Requires care in 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.

Advantages

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

 Disadvantages

Produces problems for 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 lateral load acting on a structure. 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.