Where labor is the principal item of cost in any work, less certainty can be expected in the estimate of the cost, whereas materials that are regularly manufactured should vary but little in cost.
There are some general rules that will be found very useful in making a rough estimate of the cost of structures and checking against large errors in more careful estimates.
The cost per square foot of area covered by a building having practically one floor will be nearly constant for different sizes of buildings of the same class.
Higher buildings will have a cost per cubic foot nearly constant for a given class of build ing.
For ordinary lengths of spans, the cost of re inforced concrete bridges per square foot of floor does not vary much.
The cost per square foot of the area covered by buildings of the World's Columbian Expo sition of 1893 at Chicago, for nine of the princi pal buildings, varied between 75 cents and $2.35, and averaged about $1.50. The Administration Building cost $9.18 per square foot. The cost per square foot under roof of eleven of the principal buildings of the Louisiana Purchase Exposition of 1904, at St. Louis, Mo., varied be tween 61 cents and $1.49, with an average of $1.12. Festival Hall cost $5.23 per square foot; Fine Arts Building (Central Building) cost $9.88 per square foot; U. S. Government Build ing cost $2.31 per square foot. The eleven build ings at St. Louis had timber framework. The Chicago buildings had steel frames. The Fine Arts Building at St. Louis is permanent and fireproof. The U. S. Government Building had steel arches. The cost of the steel work alone was 65 cents per square foot.
The cost in 1903 of a large reinforced con crete factory building was 6.4 cents per cubic foot. This was for the building alone, not in eluding installation of plumbing or furnishings.
Mr. I3. G. Tyrrell ("Railroad Gazette," Vol. 37, No. 18) made camparative estimates of a large factory building designed to carry 100 lbs. per square foot of live load (six stories and base ment), and found that a building of heavy wooden interior construction with brick floors, and cast-iron columns in the lower two tiers, would cost 6.2 cents per cubic foot, or 83 cents per square foot of area of floors; the same build ing with concrete-steel floors on a steel frame work would cost 10.2 cents per cubic foot, or
$1.36 per square foot of area of floors. This did not include furnishings or stairs.
The cost of a reinforced concrete power build ing per cubic foot above ground was 7.7 cents (see description, "Engineering Record," April 15, 1905, p. 438). The total cost of this build ing was $250,000.
The cost of a brick building with slate roof on timber will probably be from 8 to 14 cents per cubic foot of its volume.
The cost of a mill building with sides and roof of corrugated iron will probably be from 75 cents to $1.50 per square foot of plan.
The cost of apartment buildings and depart ment stores as usually constructed will be from 20 to 30 cents per cubic foot of volume.
Office buildings will usually run from 30 to 60 cents per cubic foot.
City dwellings will run from 10 to 30 cents per cubic foot.
Brick veneer dwellings will cost about 8 cents per cubic foot.
Window and door frames as ordinarily made for mill buildings, cost about 25 cents per square foot in place, estimating the dimensions out to out of frames. Galvanized-iron louvers of No. 18 iron cost about the same.
The cost of furnishing clips and rivets and putting up corrugated iron, is about $2.00 per square of 100 square feet.
Erection of plain structural work costs $9.00 to $10.00 per ton; of framework of office build ings, $10.00 to $12.00 per ton; of mill buildings, $11.00 to $15.00 per ton. Complicated work of many small parts and light tonnage, such as angles and tees for roof tile, may run as high as $28.00 to $30.00 per ton to erect. Bridge truss work will cost $15.00 to $20.00 per ton. These figures include furnishing falsework, also the painting.
The painting of structural work costs about $1.00 per ton for each coat.
The driving of field rivets costs from 5 cents to 20 cents each, depending upon the accessibility of the rivets and the number of times that scaf folds must be moved in a day. A riveting gang costs about $8.00 a day. For ordinary work, 12 cents per rivet is a good average.