higher load located in the left column of the table, then read right to the column containing the lumber material used as the joist. This is the member to be supported by the stringer. The value at this intersection is the on center (OC) spacing of the stringer.
Step 6. Calculate the uniform load on the stringer.
Step 7. Determine the maximum shore spacing.
(a) Maximum shore spacing is based on the stringer strength. Use table 3-2 or table 3-3 (depending on type of stringer) and the uniform load on the stringer, rounded to the next higher load shown in the left column of the table. Read right to the stringer material column and this intersection is the OC spacing of the shore to assure the stringer is properly supported.
(b) Maximum shore spacing is also dependent on shore strength and end bearing of the stringer on the shore. Use the allowable load (see tables 3-5 and 3-6), based on the shore strength and the bearing stress strength of the stringer.
NOTE: Unsupported Length (UL) = Height above the sill - sheathing thickness joist thickness - stringer thickness. This length has been rounded up to the next higher table value. For example, UL = 8 feet in height, minus 3/4-inch sheathing, minus 3 1/2-inch joist thickness, minus 3 1/2-inch stringer thickness, equals 7 feet 4 1/4 inches (round up to 8 feet), so the UL = 8 feet.
(c) Select the most critical shore spacing. Compare the spacing of the shore, based on the stringer strength (Step 7 (a)) and shore load (Step 7 (b)) and select the smaller of the two spacings.
Step 8. Shore bracing check.
(a) Verify that the unbraced length (1) of the shore (in inches) divided by the least dimension (d) of the shore does not exceed 50. If l/d exceeds 50, the lateral and cross bracing must be provided. Table 3-1 indicates the l/d is greater than 50 shore lengths and can be used if the shore material is sound and unspliced.
(b) In any case, it is good engineering practice to provide both lateral and diagonal bracing to all shore members if the material is available.
Overhead Slab Design
Form Procedure
EXAMPLE PROBLEM:
Design the form for a roof of a concrete structure which is 6 inches thick by 20 feet wide by 30 feet in length. The roof will be 8 feet high above the floor (to the bottom of the slab). The concrete pump truck will be used to place the concrete.
Step 1. Identify the material.
Sheathing: 3/4-inch plywood (strong way)
Joists: 4" x 4" lumber (S4S)
Stringers: 4" x 4" lumber (S4S)
Bracing: 1" x 6" lumber (S4S)
Mudsills: 2" x 12" lumber (S4S)
Step 2. Determine the TL.
Step 3. Determine the maximum joist spacing. Use table 3-1.
Step 4. Calculate the ULJ.
Step 5. Determine the maximum stringer spacing by using table 3-2.
