Estimating costs for wood formwork fabricated onsite

Concrete Construction, April, 2003 by Ed Jones

The two tables provided in this article are intended to help the contractor estimate formwork costs on a job. The first table provides the load capacities in pounds per square foot of Plyform plywood forming panels as defined by APA--The Engineered Wood Association. The second table provides the load capacities in pounds per lineal foot for various sizes of Class A dimensional formwork lumber. The values in these tables are based on an acceptable deflection of L/360. More detailed and colorized versions of these tables are available directly from Ed Jones, 859-586-6148.

Plywood load capacity

This table provides values for cost-related conceptual estimating, not for formwork design, because it has not been verified by a professional engineer. The load capacities shown are for partially continuous spans (minimum of three continuous spans over four supports) with the panel's face grain oriented across the supports (the strong way). Supports are assumed to be 1 1/2 inches wide. These values are based on a maximum acceptable deflection of L/360.

Note that support spacings that equate to evenly spaced supports for an 8-foot sheet of plywood are in bold, For example, 19.2 inches equals 5 equally spaced supports; 16 inches is 6 equally spaced supports; 9.6 inches is 10 equally spaced supports.

Support lumber capacity

This chart is not for final design of formwork, which should be done only by a professional engineer; this chart is for conceptual evaluation and cost estimating of formwork. The numbers in red indicate the capacity when the timber beam is configured as a simple span; the values in black are for spans continuous over three spans or more. The Class A lumber that this chart is based on has a modulus of elasticity of 1.6 ksi, bending strength of 1500 psi, and shear strength of 180 psi; capacity values are based on a deflection limit of L/360. The higher limit for load capacity is 3000 pounds per lineal foot.

Load capacities (pounds/square foot) for estimating the thickness
of form panels and center-to-center span across supports

                      Plyform thickness (inch)
Span
(inches)     1/2     5/8     3/4     7/8       1    1 1/8

30.0          --      --      --      --     123      153
29.0          --      --      --      --     136      167
28.0          --      --      --     109     149      184
27.0          --      --      --     121     165      203
26.0          --      --      --     134     183      224
25.0          --      --      --     150     204      249
24.0          --      --     119     168     228      277
23.0          --      --     135     189     255      309
22.0          --     106     153     213     287      346
21.0          --     122     175     242     324      388
20.0          --     141     201     276     368      438
19.2         100     158     225     308     408      484
19.0         103     163     232     317     419      496
18.0         122     191     269     365     480      564
17.0         144     225     314     424     552      645
16.0         173     267     370     495     639      741
15.0         210     320     439     581     744      856
14.0         257     388     526     688     873      995
13.7         274     412     556     722     911     1043
13.0         319     475     625     802    1022     1165
12.0         402     577     733     941    1188     1368
11.0         514     687     873    1120    1411     1570
10.6         553     739     940    1206    1473     1639
10.0         622     831    1056    1355    1577     1755
 9.6         675     901    1146    1470    1655     1841
 9.0         768    1026    1303    1631    1787     1989
 8.7         822    1098    1395    1699    1862     2072
 8.0         972    1258    1581    1882    2062     2295
 7.4        1136    1386    1741    2074    2272     2528
 7.0        1269    1486    1868    2224    2437     2712
 6.8        1345    1543    1939    2308    2529     2814
 6.4        1504    1668    2097    2497    2736     3044
 6.0        1638    1817    2283    2719    2979     3314
 5.6        1797    1994    2506    2984    3269     3638
 5.33       1924    2135    2683    3194    3500     3894
 5.05       2076    2303    2894    3446    3776     4201
 5.0        2105    2336    2936    3495    3830     4261
 4.8        2233    2477    3114    3707    4062     4520
 4.57       2400    2663    3347    3985    4366     4858
 4.36       2577    2859    3593    4278    4687     5215
 4.17       2760    3062    3848    4582    5021     5586
 4.0        2948    3270    4110    4893    5362     5966

Load capacity (pounds per lineal foot) for joists and studs of Class A
formwork lumber

                     Nominal lumber dimensions (inches)
Span
(inches)     2x4     3x4     2x6     4x4     3x6     2x8     4x6

72            --     100     175     175     200     350     400
66            --     100     225     225     300     400     500
60            --     150     275     250     400     500     700
54           100     200     325     350     600     600     800
48           175     300     485     435     750     745    1100
42           240     415     600     575    1035     925    1450
36           330     560     735     800    1325    1200    1850
30           500     850    1000    1125    1725    1650    2400
24           775    1215    1500    1715    2550    2700    3000
18          1180    1950    2800    2700    3000    3000
12          2400    3000    3000    3000

            Nominal lumber dimensions (inches)
Span
(inches)       2x10     3x8    2x12     4x8

72              600     600     800     800
66              700     750     950    1000
60              800     900    1100    1200
54             1000    1200    1400    1550
48             1125    1285    1575    1800
42             1425    1605    2100    2215
36             1900    2075    3000    2815
30             2900    2800            3000
24             3000    3000
18