Tension 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 tie resists that force. Below are two very large beams that have been strapped together with steel tension ties.

Continuity Tie Beams

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 installation notes are based on the tables below:

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.

Size Factor with Arrows

Table listing Steel Tension Ties from Simpson StrongTie

MST_1Let’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.