CONCRETE: Concrete is a compression material only. Never place concrete in
WIND AND SEISMIC (LATERAL) LOAD DESIGN
7. Introduction. A very high percentage of structural failures occurs due to the natural
7.4. Structural Systems. There are various structural systems that are incorporated into
buildings to provide resistance to wind and earthquake loads. Over the years, the UBC has become progressively stricter in the
requirements of construction of these various systems when they are intended to resist earthquake forces. The UBC recognizes that different types of systems behave differently in earthquakes, and the potential for failure is different as well. As a result, engineers must use different earthquake design forces depending on the type of system used. For example, a heavy timber braced frame which holds up gravity load and is also intended to
resist earthquake loads must be designed with approximately double the seismic force as a common plywood shear wall. There are about 25 possible basic types of structural systems (not including the 15 or so possible
combinations of systems) that the UBC recognizes. The bottom line is that certain structural systems are penalized by the earthquake code and certain systems are encouraged. Table 16-N in the UBC lists the various systems and their corresponding ‘R’
values (the lower the ‘R’ value, the higher the applied load, i.e. the more this type of
construction is penalized) . Following is a listing of the more common types of structural systems.
7.4.1. Shear Walls. A shear wall is a wall that is constructed to resist the racking from the sideways (lateral) loading associated with wind or earthquakes. In actuality, all walls provide a certain amount of racking resistance, but in order to be counted in design they must be constructed in a certain manner. Shear walls may be constructed of wood, gypsum board, metal, or concrete.
7.4.1.1. Plywood or Oriented Strand Board (OSB) Shear Walls Over Wood Framing.
These are light framed wood stud walls covered with plywood or OSB. The shear strength (resistance to lateral loads) depends on:
thickness of plywood or OSB (the thicker – the stronger)
the length of nails used (the longer – the stronger)
the spacing of the nails (the more closely spaced – the stronger, however spacing may not be too close as to cause stud splitting)
the spacing of the studs (the closer – the stronger)
There is a whole laundry list of rules for plywood and OSB shear walls that exists as footnotes to Table 23-II-1 in the UBC. These are quite important to ensure proper construction, but are very frequently ignored. They are summarized below:
All edges must be blocked. This means that all plywood or OSB edges must be nailed to a stud, plate, or 2x blocking. [There is an exemption to this, however, for prescriptive shear walls that qualify as conventional light framed construction. See following section on conventional light framed construction.]
Sheets may be installed either vertically or horizontally.
Field nailing (i.e. non-edge nailing) must be no greater than 6” O.C. for 3/8” or 7/16” thick sheathing with studs at 24” O.C.; and 12”
O.C. for other conditions.
If other than Douglas Fir-Larch or Southern Pine studs are used, shear wall strength values must be reduced.
In two sided shear walls where edge spacing is less than 6” O.C., edges must be
offset (staggered) from side to side, or 3”
wide studs must be used.
In seismic zones 3 and 4 (this includes western Washington) where shear loads exceed 350 pounds per lineal foot (this includes 1/2” plywood nailed with 8d’s at 3”
O.C.; 1/2” plywood nailed with 10d’s at 4”
O.C.; and all stronger configurations) all edge nailed members and mud sills must be at least 3x material. The only exception to this is the allowance of 2x sill plate material if double the number of anchor bolts are used, and 2”x2” washers are installed on the anchor bolts.
7.4.1.2. Aspect Ratio of Wood Diaphragms.
The UBC does not allow tall skinny shear walls or long narrow roof diaphragms. There is a Table, 23-II-G which shows the maximum diaphragm dimension ratios allowed. Following are the most common aspect ratio restrictions that would apply to seismic zone 3 (Western Washington):
For plywood or OSB shear walls, all edges nailed, the maximum height to width ratio is 3-1/2:1
For plywood and OSB floor and roof diaphragms, the maximum span to width ratio is 4:1
As an example in determining if a shear wall panel may be counted or not, if you have a garage with a 7’ high door, in order for shear wall panels at the sides of the door to be counted, they must be at least 2’ wide. If they are narrower than 2’, they may not be counted.
There are other ways of providing lateral resistance in the absence of compliant shear walls, but they are all expensive, and should be
engineered. In general, the code wants you to use wide shear walls; the more - the better.
Question arises as how to measure the height and width of a legal shear wall panel. The two sketches from the UBC, Figure 23-II-1, shows how to do this.
The conservative approach is shown in sketch (a) above where the height is from plate to plate.
But you may also use the approach (b) shown below – the difference being that the height is from top of opening to bottom of opening. Note, however, in this case you must provide design and detailing for force transfer around the openings.
7.4.1.3. Prescriptive (Braced) Shear Walls.
The UBC allows certain types of shear walls to be constructed without engineered design. The UBC calls these ‘braced walls’, and they may be constructed of wood boards placed diagonally, plywood and OSB sheathing, fiberboard sheathing, gyp board, particleboard, or plaster.
Of course there are rules for construction of these in the code (see below section for rules pertaining to plywood and OSB braced walls).
Prescriptive braced walls are allowed for
‘Conventional Light-Framed Construction’ only as defined in Division IV of Chapter 23.
Question naturally arises as to exactly what
qualifies as Conventional Light-Framed