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Tensile Strength

Wood Tensile Strength and Steel Continuity Ties in Residential Seismic Retrofits.

 

Wood and Steel Continuity Ties.

Continuity ties connect two structural elements together in order to increase earthquake resistance.  In the photo to the left a shear wall is unseen to the right of the strap.  It is connected to the wall to the left that is not a shear wall.  When the wall on the left moves to the right that movement pulls through the strap into the shear wall that the strap is connected to.

Two by fours can also be used to bridge breaks in horizontal members such as top plates and floor joists, just as effectively and more economically than steel straps.

 

Continuity Tie Beams

 

When the large beam on the left pulls to the left, it pulls on the beam to the right through the steel into the beam and wall on the left that has a shear wall on it.  In this way the wall to the left with no shear wall on it is protected by the shear wall unseen to the left.   However, wood straps, generally stronger and more economical, will be discussed below.

Tension Tie

Using other information on this website, once you figure out how strong you want your steel or wood strap to be, use these instructions to figure out how long and what size your wooden strap should be.

 

This is how you do it:  First you look at table 4B.  It tells us the tensile strength per square inch of Douglas Fir.

Tension ProblemA 2 by 4 will have a tensile strength of 1.5 (the narrow side of the 2 by 4)  x 3.5  (the wide side of the 2 by 4) x 575# = 3,019#.  Then multiply this by 1.6 (short term load duration factor used for sudden impacts like earthquakes) = 4830#

 

NDS Arrows and Title

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

 

 

 

 

 

 

 

 

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:  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 Straps to Wood 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.

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.

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

 

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