Tension ties, also known as continuity ties, connect two wood framing members found in walls and floors together so they don’t separate when earthquake forces try to pull them apart. This pulling apart action is called tension. A tension/continuity tie resists that force. Below are two very large beams that have been strapped together with steel tension/continuity ties.
If the beam on the left moves laterally to the left because of an earthquake, if will also pull the beam on the right with it because the two beams are tied together with the steel tension/continuity tie. In other words. that movement is moves continuously from the beam on the left to the beam right because the tension/continuity tie connects the two together. The strap is put under tension which is why it is also called a tension tie. Without the strap the movement would not continue from one beam to another, not would the strap be placed in a state of tension.
Sometimes tension/continuity ties are used when joining top plates together, though most of the time nails are used because this manner of establishing continuity is only limited by the number of nails that will fit, they are very fast to install, and with sufficient nails over 3 times as strong as a steel strap.
2 by 4s can also be used. In the figure below the 2 by 4 bridges the break in the cripple wall’s double top plate to keep them from tearing apart in tension. The 2 by 4 below is an example of a tension/continuity tie.
If the earthquake tries to move the upper and lower top plates to the right that movement will put the 2 by 4 in tension and at the same time make sure the earthquake force continues from the top plates on the right to the top plates on the left and vice versa. This is important because the earthquake resisting shear walls might only be on one side of the breaks in the top plates.
Nails, staples, or lag screws are used on each side of the 2 by 4 in order to create this continuity. Be sure and look at the National Design Specification to determine how far apart these fasteners can be from each other and how close they can be to the edge of the 2 by 4. As a rule of thumb the distance between the fasteners is determined by splitting of the wood. If the wood splits, you are too close.
The installation notes for lag screws are based on the tables below. Nails are simpler and as effective as lag screws.
This how to figure out how much tension a certain piece of lumber can resist.
First you look at table 4B. It tells us the tensile strength per square inch of Southern Pine, which has the same strength as Douglas Fir-Larch. This is the wood used in house framing all over the Bay Area.
Tensile strength measures the amount of force measured in pounds something can resist when you try and pull it part. A spaghetti noodle has very little, a steel rod has a lot.
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 = 7,245# Our 2 by 4 can resist 7,245# of tension force.
Table listing Steel Tension Ties from Simpson StrongTie
Let’s say we want to create the equivalent of a MST48 tension tie. Each lag can resist 400″ so we divide 5310#/400″ and we get 13.27 or fourteen 3 1/2″ lags each side of a 2 by 4 tension tie. Considering end and edge distances and using a double row, this can be done with a 60″ long 2 by 4.
If we wanted to match the MST72 strap we would need 6,730#/400# = 16.82 or 17 SDS screws. This can be done with a 72″ long 2 by 4.