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. Metal  straps are the most common type of tension tie, though lumber can also be used just as effectively. Below are two very large beams that have been strapped together with steel strap tension ties.

Continuity Tie Beams

 

Lumber such as 2 by 4s can also be used, just as effectively and more economically than steel straps.  In the image below the 2 by 4 joist bridge connects both sides of the joist together to keep them from tearing apart in tension.

 

Tension Problem



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 which is used in 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 = 7245#

Size Factor with Arrows

Our 2 by 4 can thus resist 7,245#

Another Example:

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 that actually measure two inches and six inches, : 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#.

MST_1

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#, which is less than a 2 by 4.

 

Fastener Requirements

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 on each side.

12D nail can resist 188 POUNDS tension with 1” or more penetration.

7245/200=36 nails each side

A lag bolt with 1 ½ inches penetration can resist 300 pounds of earthquake force.

A lag bolt from 1 1/2″-2 1/4 inch penetration can resist 450 pounds of earthquake force.

A lag bolt with over 2 1/4 inch penetration can resist 550 pounds of earthquake force.

7245/450 = 16 lags each side to get a tension tie stronger than the strongest tension tie Simpson StrongTie makes.