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
(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
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
(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
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
Step 1. Identify the material.
Sheathing: 3/4-inch plywood (strong way)
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
Step 4. Calculate the ULJ.
Step 5. Determine the maximum stringer spacing
by using table 3-2.