[Illustration: FIG. 16]
=34.= Within the last few years, the cost of the best Portland cement
has been so materially reduced that concrete has become an available
material for the construction of factories. Unless used in great
masses, however, it has not the strength to support the necessary
floor loads without the use of steel reinforcement. As explained in
_Design of Beams_, the fibers on the bottom of all beams subjected to
transverse stress are in tension, and while concrete has considerable
resistance to compression, it offers comparatively little to tensile
stress. It is therefore necessary to reinforce the lower portion of all
beams and floor slabs as indicated at _a_, Fig. 16.
=35. Advantages of Reinforced Concrete.=—In Fig. 16, the details of
a typical reinforced-concrete factory building are illustrated, and
a building of this character may be constructed for a cost of from
10 to 15 per cent. greater than the ordinary slow-burning type of
building. Besides, this construction possesses the advantage of being
practicable for long spans and heavy loads, whereas in buildings of
the slow-burning type, owing to the fact that the size of the wooden
beams is limited to the available commercial timber, it is frequently
impossible to design floors with girders of large spans for floor
loads of over 250 pounds per square foot. While this is a heavy load,
it is too light for some classes of work, such as occur in printing
houses and lithographing establishments where heavy stones are used and
stored. The floor loads in such buildings sometimes amount to as much
as 300 or 400 pounds per square foot, while it is not unusual to find
the load on floors in warehouses amounting to as much as 500 pounds per
square foot.
[Illustration: FIG. 17]
=36. Strength of Concrete Columns With Steel Cores.=—In the building
shown in Fig. 17, it will be noticed that the columns are reduced
in size in the lower floors, increased in the middle portion of the
building, and reduced toward the roof. The reduction in the columns
_a_ and _b_ is due to the fact that these columns are reinforced with
a steel core composed of structural shapes riveted together, angles
usually being employed for this purpose. In proportioning such columns,
it is good practice to figure on the ultimate safe unit compressive
stress of the steel without considering the reduction made by the usual
column formula, but to neglect, in the consideration of the strength
of the column, the resistance of the concrete surrounding the steel
core. To illustrate, if the sectional area of the steel reinforcements
in these columns equals 20 square inches, and a safe unit fiber stress
of 16,000 pounds is assumed, the safe strength of the column will be
320,000 pounds.
Above the second floor, the columns are made much larger, for here
there is less steel reinforcement, and it is necessary to figure on the
safe bearing strength of the concrete.
=37. Strength of Reinforced-Concrete Columns.=—In proportioning
reinforced-concrete columns, it is customary among conservative
engineers to figure the safe strength of the concrete-column section
at 500 pounds per square inch of section; that is, if the column is 20
inches square, its area is 400 square inches, and its safe strength at
500 pounds per square inch will be 200,000 pounds. In the top floor, it
is seldom advisable to use concrete columns less than 10 inches square,
though at this dimension they generally possess several times the
requisite amount of resistance.
All columns in reinforced construction generally have embedded in them
3¾-inch to 1-inch round steel rods, tied together with round iron
binders, or bar iron straps as indicated in Fig. 16 (_b_).
=38. Floor and Roof Construction.=—In considering the floor and roof
construction of buildings built of reinforced concrete, it will be
noted from Fig. 16 that the roof slab is made 3 inches in thickness.
Such a slab made of good concrete, reinforced with ⅜-inch steel rods,
spaced 6 inches from center to center, will carry the usual roof loads
for spans up to 7 feet in the clear.
In forming the gutter for such roofs, as indicated at _b_, the gusset
is made by filling in with cinder concrete. Usually cast-iron eave
boxes are embedded in the concrete, and these in turn connected with
inside rain conductors.
The beams supporting the roof, when the span is from 12 to 14 feet, are
made about 12 inches deep and 8 inches wide, while the girders, also
constructed of reinforced concrete, are usually made about 3 inches
deeper and 11 inches in width.
In order to make the roof impervious to moisture, a covering of felt
and slag is commonly employed. This slag joins the parapet wall with
the usual tin flashing and counter flashing, as at _c_, though copper
is recommended for best work.
In the floor construction of reinforced-concrete factory buildings,
the slabs forming the floor panels are made not less than 4 inches
in thickness, and seldom over 5 inches, with a 1-inch finish coat of
cement besides, if this character of finish is desired. Such a floor
slab is shown in the construction at _d_, Fig. 16, while the wooden
floor construction is shown in Fig. 16 (_c_). Here the structural
feature of the floor is a 4-inch concrete slab upon the top of which
is placed 2" × 3" beveled hemlock sleepers, the space between these
sleepers being filled with cinder concrete, and the floor finish
obtained by laying 1-inch tongued-and-grooved maple floorings.
=39. Reinforced-Concrete Beams and Girders.=—The depth of the beams and
girders in reinforced-concrete construction varies, of course, with
the span and loads to be supported. Their width enters little into
the strength, and they may be made as narrow as possible in order to
cover the reinforcing steel. It is the best practice to make beams and
girders of the same width, for then the process of forming the molds is
greatly simplified and the cost reduced.
In placing the reinforcement in the concrete, it should always be at
least 2 inches from the outside surface, for a distance less than this
is considered inadequate fireproofing. In order that the reinforcing
metal _e_, Fig. 16, may enter over the top of the reinforcing metal at
_f_, it is usual to make the secondary girders, or beams, 3 inches less
in depth than the main girders. To stiffen the building, brackets are
customarily introduced between the column and girders, as illustrated
at _g_. These brackets tend to greatly increase the rigidity of the
connection and shorten the span of the girder somewhat.
[Illustration: FIG. 18]
=40. Construction at Window Heads.=—Where it is necessary to have
the window head near the top of the ceiling, reinforced-concrete
construction lends itself readily to the requirements of this
condition, for even where girders are supported over the window head,
the construction may be followed out, as indicated at _h_, Fig. 16.
Where it is desired to have the window head raised still higher, a
construction similar to that shown in Fig. 18 may be used. In this
case, however, care must be taken to have the girders bear on the piers
between the windows, and to have no intermediate beams.
[Illustration: (_a_)]
[Illustration: (_b_)