Supported Joist Length - SRT Carpentry



Supported Joist Length

To supplement H – 2 – 20 to H – 2 – 24

Supported joist length is simply a calculation that must be done when single span joists are not going to be sufficient to support the floor of a house. Typically, the Supported Joist Length (SJL) is found and then the tables in the building code are used to find the size of the built up beam that is needed for support and the spacing of its supporting posts.

Calculate:

The SJL is defined as the sum of half of the clear span on either side of the beam plus the width of the beam.

SJL is very simple to calculate and is always slightly less that the width of the home in the direction that the floor joists are running. The SJL is found by:

1. Take the width of the building eg. 26’ 6”

2. Convert the width to inches by multiplying by 12. eg. 318”

3. Subtract the width of the sill plates (either 7 or 11 inches) eg. 318 – 11

4. Take the “clear span” (answer from # 3) and subtract 4.5” for the minimum build up of beam. eg. 302.5”

5. Divide 302.5 inches in half and add the beam thickness back in. eg. 155.75”

6. Convert to meters a. 155.75 x 2.54 = 395.60 cm

b. 396 / 100 = 3.96 M

7. Go to the tables in the code for Maximum spans for built up beams for the number of floors required.

8. Since a SJL of 3.8M does not appear, use the longer span data. (if the beam will hold the SJL of 4.2M, then it will be better than code for 3.8M

9. SJL of 4.2 M for SPF # 1 & 2 not more than one floor (p.426) requires a build up beam minimum of 3 pcs. 2 x 8 with posts supporting it spaced 2.45 M apart.

10. Convert the Meters to inches 2.45 x 100 = 245 cm

11. 245cm divide by 2.54 = 96.45” (96 7/16”)

Finding the building width:

From the SJL, you can also find the width of the building by working through the calculations backwards.

a. multiply the SJL x 2

b. subtract the beam thickness (s) you need to know how many built up beams and the number of pieces.

c. add the width of each sill plate

d. this number is the width of the building.

Additional Problem:

Building width of 18’

a. will a 2x 10 16” OC bridged only span?

b. If not, calculate the SJL

216” – 7 – 4.5 = 204.5”

204.5 “ divide by 2 = 102.25

102.25 + 4.5 = 106.75”

SJL 106.75” converted to M = 2.71M (106.75 x 2.54 /100)

SJL of 2.7 M with one story house # 1 & 2 SPF = 3 pcs 38 x 184 with posts spaced at 2.90 M.

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