Contemporary Design & Traditional Craftsmanship
Lawrence Turner Construction Services
I prefer small and medium size projects with an emphasis on
projects that I have designed.
SUMMARY OF GENERAL STRUCTURAL RULES OF THUMB
FOUNDATION WALL FOOTINGS
Footing thickness to be equal to foundation wall thickness. Footing width to be twice foundation wall thickness. Column footings to be three foot square minimum.
REINFORCED CONCRETE OR MASONRY BEARING WALLS
Maximum ratio of unbraced length or height, whichever is least, in concrete or masonry construction is generally twenty five feet. The length or height in feet can be divided in half to estimate the wall thickness in inches.
The ratios of unbraced length to least thickness of most column types range from 10 to 30. The ratio 20 is a good average for rough estimating, for nominal load and bay size situations. Example: Assume column height of 12', divide by the ratio 20. 12' divided by 20 equals 7-1/2" Certain concrete and masonry columns have minimum required thicknesses that will be greater, than those suggested by using height to thickness ratios.
BEAMS AND JOISTS
The ratios of span to depth for beams and joists of all kinds range from 10 to 24. Lower ratios are for heavy or concentrated load situations. The ratio of 20 is a good all-purpose average for steel, wood and concrete beams and joists.
In general the optimal length of a cantilever is one third the supported span.
Reinforced concrete slabs of various types have span to depth ratios ranging from 20 to 35. The ratio 24 can be used as an average ratio for rough design estimating purposes. As with the ratio used for reinforced walls the assumed span can be divided in half to derive the approximate slab thickness. An 18' slab for example could be conservatively estimated as 9" thick.
Trusses of various types and materials have span to depth ratios ranging from 4 to 12. The lower ratio is appropriate for trusses carrying heavy floor loads or concentrated loads. Otherwise the ratio 8 is a good estimating average for roof trusses. Example: using the 8 ratio and assuming a 64' roof span, the likely truss depth would be: 64' divided by 8 equals 8'
REINFORCED CONCRETE RETAINING WALLS
Thickness for walls up to 10' high: 8" to 10". Typical wall thickness for heights from 10' to 13': 12''. After 13' add 1" in thickness for each additional foot of wall height. Footing thickness is usually 12" for walls up to 13' high. After 13' add 1" in footing thickness for each additional foot of wall height. Footing width is typically.5 to .7 times the wall height. Vertical construction joints are spaced at 20' to 30' intervals and covered at back of wall with membrane waterproofing. Drainage with all foundation walls may be provided by 4" drain pipe weep holes through the wall spaced up to 10' apart or horizontal drain pipe behind wall may be used.
REINFORCED BRICK RETAINING WALLS
Not recommended for walls over 6' without special engineering design. For low walls up to 6' wall thickness may be 10" with 8" to 10" thick footing. Footing width is about .6 times the wall height.
REINFORCED GROUTED CONCRETE BLOCK RETAINING WALLS
Wall thicknesses is 8" for walls up to 6'; 12" tor walls up to 10' high. Footing thicknesses is 12" for walls up to 6', 14" for walls up to 10' high. Footing width is approximately .8 times the wall height.
WOOD FRAME WALLS AND PARTITIONS
Maximum unbraced heights for studs at 16" spacing 2 x 4's to 14' 2 x 6's to 20'
STEEL STUD PARTITIONS
Steel stud widths range from 1-5/8" to 6". Maximum heights range from 9' to 16'. Unbraced length is unlimited if partition is supported both at floor and at ceiling. Unbraced length is limited to 14' if not supported at ceiling.
REINFORCED CONCRETE COLUMNS
Minimum sizes for columns: Principal round columns: 12". Round posts: 6". Rectangular columns: minimum total cross sectional area of 120 sq. in. with 8" minimum thickness. Usual minimum for a rectangular tied column: 10" x 12". Single story rectangular post: 6" minimum thickness. Square or round spiral columns: 14" minimum thickness. Most columns used are "short". Maximum height for short columns is10 times the least cross sectional dimension.
REINFORCED MASONRY COLUMNS
Usual minimum dimension for any masonry column: 12". If column is designed for 1/2 total allowable stress or less, the minimum dimension may be 8". Maximum height: 20 times the least dimension.
UNREINFORCED MASONRY COLUMNS
Minimum least dimension is 12". Maximum height is 10 times the least dimension.
SOLID WOOD COLUMNS
Assuming nominal loads, the maximum allowable length of a solid wood column can be figured as 30 times its least actual dimension.
SOLID WOOD BEAMS
Thickness range is from 2" to 14". Spacing is 4' to 20' on center. Spans typically range from 8' to 32'. Range of commonly used span to depth ratios is16 to 20; use lower range for heavier load conditions, higher range for roof beams.
SOLID WOOD GIRDERS
Commonly used span to depth ratio for girders with concentrated loads is 12. Width will be three-fourths to one-half of depth. To estimate depth in inches multiply span in feet by 1.
GLUED LAMINATED WOOD BEAMS
Usual span range is l6' to 50'. Spacing is 4' to 20' on center Thicknesses range from 3-1/4" to 9". Approximate span to depth ratio is 24. Ratio of beam depth to width is about 2 to 1 for light beams and 3 to1 for heavy beams.
WOOD FLOOR JOISTS
Usual span range is 8' to 25'. Usual spacing is l6" on center. Usual span to depth ratio for standard construction grade pine and fir is 20.
WOOD ROOF JOISTS, CEILING JOISTS AND RAFTERS
Quick estimates of joist depths in inches can be made by multiplying span in feet by .5 for roof joists, .45 for ceiling joists and .4 for wood rafters with light roofing.
PLYWOOD STRESSED SKIN FLOOR CONSTRUCTION
Thicknesses are 3-1/2" to 12-1/2". Spans are 8' to 30'. Span to depth ratio is 30.
WOOD PLANK FLOOR CONSTRUCTION
Thicknesses are 2" to 4". Spans are 4' to 22'. Span to depth ratio is 48.
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