Continuity ties connect two structural elements together in order to increase earthquake resistance. These straps connect two very large top plates together which in turn support the floor above. In the photo below, an unseen shear wall is located to the right of the strap. When the floor on the left moves to the left, which in turn tries to move the top top plate to the left, this movement is transferred into the strap, into the top plate on the right, and into the shear wall that the strap is connected to. Remember, the shear wall is nailed to the top plate on the right. These straps thereby transfer the the floor movement into the shear wall which keeps the floor stable.
Two by fours can also be used to bridge breaks in structural members such as top plates and floor joists, just as effectively and more economically than steel straps.
Below is an attempt to explain the engineering.
First you look at table 4B. It tells us the tensile strength per square inch of Douglas Fir. Tensile strength measures the amount of force something can resist when you try and pull it part. A spaghetti noodle has very little, a steel rod has a lot.
Once you figure out how strong you want your steel or wood strap to be (look at the web pages on retrofit designs), use these instructions to figure out how long and what size your steel strap should be.
A 2 by 4 will have a tensile strength of 1.5 (the narrow side of the 2 by 4 measured in inches) x 3.5 (the wide side of the 2 by 4, also measured in inches) x 575# which equals 3,019#. You then multiply this by 1.6 (short term load duration factor used for sudden impacts like earthquakes) = 4830# of tensile strength.
Finally, as shown in Size Factor Adjustment table below, this is multiplied by 1.5.
The complete formula is 1.5 x 3.5 x 575# x 1.6 x 1.5 = 7245#
For a nominal dimensioned 2 by 6: 1.5 (narrow dimension) x 5.5 (wide dimension) x 575# (tensile strength when gravity is pulling on it) x 1.6 (short term load duration factor used for sudden impacts like earthquakes) x 1.3 (Size Factor, note this is different from the 2 by 4 factor of 1.5) = 9,867#.
For a full dimensioned 2 by 6: 2 (narrow dimension) x 6 (wide dimension) x 575# (tensile strength when gravity is pulling on it) x 1.6 (short term load duration factor used for sudden impacts like earthquakes) x 1.3 (Size Factor, note this is different from the 2 by 4 factor of 1.5) = 14,352#.
For a nominal dimensioned 3 by 6, 2.5 (narrow dimension) x 5.5 (wide dimension) x 575# (tensile strength when gravity is pulling on it) x 1.6 (short term load duration factor used for sudden impacts like earthquakes) x 1.3 (Size Factor, note this is different from the 2 by 4 factor of 1.5) = 16,445#.
Comparing Steel StrMore on Steel Straps.
To the left is a table found in the Simpson StrongTie Catalog. This company makes most of the steel straps used in the construction industry. The tensile strength of their strongest strap is 6730#, while a 3 by 6 with adequate fasteners on either side of the break, can resist 16,445# A 3 by 6 is therefore over twice as strong.
A 2×4 will reach its full tension capacity of 7245# if half the nails are on one side of the break and half the nails on the other. The nails on each side of the break must have a capacity of 7245# or greater.
12D nail can resist 200 POUNDS tension with 1” or more penetration.
7245/200=36 nails each side
Simpson SDS lags can resist 550# with 1 1/2″ or more penetration
7245/500 = 15
If we are using a nominal 2 by 6
The Matter of Plywood Nailing
This part of Standard Plan A arose out of a concern the committee had that the blocks would split. However empirical evidence does not support this claim, and has the effect that homeowners and contractors are building shear walls considerably weaker than they could if nails were allowed to be 3″ or even 2″ apart. Both empirical evidence and testing by the American Plywood Association support this. The 14-inch block below was nailed with 8d nails 1 inch apart.
The table below is a summary of test results published by the American Plywood Association, a national laboratory that informs the building code and the public about the best ways to nail plywood in shear walls.
The black and red arrows point at numbers that represent the pounds of earthquake force that each linear foot of plywood can resist if nailed in a manner consistent with the table. For example, if we go to the bottom row of the table that starts with 15/32 and follow it left to right we see that:
(1) The Panel Grade is Structural 1 plywood.
(2) The plywood’s thickness is 15/32″
(3) The penetration into the framing is 1 1/2 inches.
(4) The nail size is 8d (8d or 10d is simply a way of describing nails of a certain length and diameter)
(5) The plywood is nailed on the edges 6″, 4″, 3″ or 2″ apart.
In this case we get different strengths. For example a shear wall will have 430 pounds of resistance per linear foot if the “Nail Spacing at Edges” is 4″ apart with 8d nails, and 870 pounds of resistance when nailed with 10d nails 2″ apart. What kind of nailing would you want for your house?
These higher capacity shear walls are not always feasible, and only the person who determines the shape, size, and condition of your existing house can make that determination.
From a contractor’s point of view, the less linear footage of plywood required translates into less materials and labor required. If the same shear wall strength can be gained by changing the nail spacing, the homeowner and the public stand to gain because the cost of the retrofit will be reduced.
Top Plates and Cripple Wall Retrofits
Cripple wall top plates are actually segments of wall that can move independently of each other–boards that are 8 to 24 feet long butted together. Any point where they butt together is called a top plate break. These breaks create a break in continuity, meaning the pull and push of the earthquake force is not continuous along the cripple wall top plate, and therefore the earthquake force does not have a continuous connection to the shear walls. Steel straps, nails, and 2 by 4s are the three methods by which these breaks are joined together and continuity is restored.
For example, if you have a shear wall at one end of a cripple wall and it is not connected to the floor of the rest of the house, when this floor moves there will be no way for the shear wall to restrain it.
One way to do this is with a Steel Strap
Another way to do this is with nails
This allows a “load path” for the force to bypass the break.
Another way to do this is with a 2 by 4
High values can also be achieved by using straps made of 2 by 4s. A 2 by 4 can resist 7500#, which is more that all but one streel strap. If the two top plates are nailed together to form one single member, and the top plate break (either single or doubled) is bridged with a 2 by 4, the value of an MST48 can be achieved by installing 24 12d nails through the 2 by 4 on each side of the break.
The value of a higher strength MST72 strap can be achieve with 36 nails on each side of the break.
If there is a break created by a vent stack, a 2 by 4 must be used because it will resist both tension and compression forces.
More on Steel Straps
It is fairly common to find breaks in a cripple wall retrofit that need to be addressed so the shear walls all work together in protecting the house. A common way of bridging breaks in a top plate is through the use of MST straps. Below is a table representing their capacity.
MST straps require 16d nails, though substitutions are allowed. For example, as shown in
the adjustment factor table below, if 10d by 1-1/2 nails are used instead of 16d commons, the strength of the strap must be reduced by 16%. This is accomplished by multiplying the value in the table above by 0.84. Thus, an MST48 with a tension value of 5,310 pounds is actually worth 4,460 if 10d ticos are used (0.84 x 5,310).
High values can also be achieved by using straps made of 2 by 4s. If the two top plates are nailed together to form one single member, and the top plate break (either single or doubled) is bridged with a 2 by 4, the value of an MST48 can be achieved by installing 24 12d nails through the 2 by 4 on each side of the break. 5310/188=23.70.
The value of a higher strength MST72 strap can be achieve with 36 nails on each side of the break- 6730/188 = 35.8 nails.
If staples are used, which have a shear value of 80 pounds each and have no potential for splitting, the capacity of this connections is almost limitless.
Narrow straps such as CS16 used in single top plates have a much lower value
There is no reduction from using 10d nails 1-1/2 inch long rather than the 10ds in the catalog.
This can be greatly increased with a 2 by 4 strap. If 20 nails are nailed through the 4 foot section of 2×4 on each side of the break, the tension connection will now be worth 3,760 pounds. If 2 1/2″ long staples are used, the capacity of this connection is almost limitless.