Different forces affect buildings in the various parts of the country. Builders have to worry about earthquakes in California, high winds in Florida, and snow loads in Colorado. It’s easier to understand the architect’s or engineer’s plans if you are aware of these factors. The following maps give you an idea of some of the areas of the country that suffer most from the effects of earthquakes, winds, and snow loads.
The most common framing details can be broken down into three categories.
• Shear wall construction
• Diaphragm construction
Each of these categories is covered in this section, including important points for framing.
The factors that affect the strength of any shear wall are:
• The size and type of material used for the plates and studs.
• The size and type of material used for the sheathing.
• Whether one side or both sides have sheathing.
• The nail sizes and patterns.
• Whether or not there is blocking for all the edges of the sheathing.
Engineers and architects are free to use any system they prefer, as long as they can prove that it meets the minimum strength requirements. The easiest and most common method is using the code book tables that provide accepted values for walls with
Seismic Map of Continental U. S.
given resistance capabilities. (Table 2306.3 in the 2009 International Building Code (IBC) shows these values.)
If there are many shear walls in a building, the engineer usually creates a schedule from a code table to show the wall requirements. Unfortunately, there is no standard for labeling shear walls, so the schedules made by the engineers may all be different. They do, however, usually have common
components. You will need to study the shear wall schedule on the plans to understand all the components that apply to framing.
Refer to the Shear Wall Schedule table later in this chapter for an example. It is an easy one to use because the labels also identify the nailing pattern and the type of sheathing. It was developed by the framing council in the state of Washington.
1. Stud sizes—Specified nailing patterns may require changes in the stud sizes. There are three conditions where 3x studs are required for nailing adjoining sheathing edges. A fourth condition is required in seismic design category D, E, or F.
• If the edge nailing is 2%" O. C. or less.
• If there is sheathing on both sides of the wall, the adjoining sheathing edges fall on the same stud on both sides of the wall, and the nailing pattern is less than 6” O. C.
• If 10d (3” x 0.148”) nails are used with more than Ш” penetration, and they are spaced 3” or less O. C.
• (For seismic design categories D, E, or F) where shear design values exceed 350 pounds per linear foot.
2. Penetration—It is very important that the nail does not penetrate the outside veneer of the sheathing (see “Nail Penetration" illustration.) A pressure regulator or nail-depth gage can be used to make sure this doesn’t happen. (See “Nail Regulator and Flush Nailer" illustration.) The top of the nail should be flush with the surface of the sheathing.
3. Nail size—The nail size may change from wall to wall. Check the specified thickness and length of the nails.
4. Nail spacing—The pattern for nailing the sheathing to the intermediate framing members is usually the standard 12” on center. It is the edge nailing that changes to increase the strength. If 3x studs are required, then the pattern must be staggered. Make sure that the nails are at least 3/8” away from the edge of the sheathing.
5. Blocking—The details or shear wall schedule should specify whether blocking is required for panel edges. If the wall is 8′ or less, you can usually satisfy this requirement by running the plywood vertically, so that all the panel edges have backing.
The strength of diaphragms is affected by
• The size and type of material used for the joists or rafters
• The size and type of material used for the sheathing
• The direction of the sheathing in relation to the members it is attached to
• The nail sizes and patterns
• Any blocks, bridging, or stiffeners
Nail depth gage
Nail regulator and flush nailer shown affixed to a pneumatic nailer
Nailing pattern for shear walls utilizing 3x studs
Building codes provide tables for diaphragms similar to those for shear walls. To summarize, the variables used to increase the strength of the diaphragm are the thickness of the sheathing, the size of the nails, the width of the framing member, the nail spacing, and whether or not the diaphragm is blocked.