16. Concrete walls

Poured concrete may be used for walls of in­dustrial plants, especially when the general construction is made of reinforced concrete. These may be load-bearing or curtain walls, and, to a certain extent, they possess characteristics that are similar to brick construction except that the construction does not have mul­titudinous joints, the material can be reinforced to resist tension, and the concrete must be poured against forms. Provisions should be made for insulation and avoidance of condensation.

Provision should be made, too, for shrinkage and thermal defor­mation of concrete without harmful cracking. Brickwork, with its mul­titudinous joints, can take up a certain amount of deformation by means of unnoticed hair-cracks at the joints, whereas concrete must crack unless the deformation can occur (preferably unseen) at the specific points or lines that are provided for that purpose.

Construction joints — both vertical and horizontal—are needed to limit the lengths, heights, and volumes of individual monolithic pours. The vertical ones may be used as part of a system of crack control; the horizontal ones are for construction purposes only since the force of gravity presses the parts together regardless of shrinkage and ther­mal action. Dummy joints and planes of weakness may be installed for the purpose of localizing cracks. The locations of all joints should be planned carefully in advance.

In order to minimize costs, concrete curtain walls may be built of precast units that are poured in forms on the ground or elsewhere and then erected as large blocks or tiles with mortared or sealed joints. It is probable that hollow, cellular panels will be manufactur­ed in order to decrease the weight of the walls and to improve in­sulating qualities. It is obvious, however, that any large precast units are difficult to handle in the field. Much thought will undoubtedly be given to such details as the provision of lateral steadiness, anchorage or keying of units, avoid­ance of leakage, exterior and interior finish, vertical supports, light weight, ease of erection, and sizes and thicknesses consistent with durability.

(2152 signs)

17. Roofs

Desirable Qualities.

What qualities or characteristics should a designer look for when he is planning a roof? Here lightness, strength, waterproofness, insulation, fire resistance, cost, durability, and low maintenance charges are of prime importance. Without a good, tight roof, the usefulness and value of a building may be greatly impaired. There is, however, considerable overlapping in practice so that one cannot draw absolute lines of demarcation between roofs to be used on this or that type of structure.

Lightness and strength.

Lightness. — The desirability of light weight is especially great in the case of long-span roofs. For purlin spans under 15 or 16 ft. and for trusses less than 40 ft. long, the sizes of members may be controlled largely by advisable minimum sections, or, at least, the in­crease in the cost of the framework because of larger dead loads will be slight except in the case of timber framing. The designer must be careful to avoid letting his desire for lightweight roof construction, make him negligent of the other characteristics that may be impor­tant, too. He should remember to fasten down such roofs as well as to hold them up, because suction during gales may lift them.

Strength. — There is generally considerable uncertainty regarding the intensity of live loads for which roofs should be designed. Flat roofs in snowy climates may actually have to carry at least 40 lb. per sq. ft. of horizontal projection; when surrounded by parapets, they may be loaded even more heavily. Although unit weights vary greatly, one may assume that 1 ft. depth of hard, dry, wind-blown snow weighs 10 lb. per sq. ft.; if a cold rain falls upon such snow, the load may be increased from 25 to 50 per cent before there will be enough drain­age action to take off the excess water, even when the roof has a moderate slope. If the inlets or scuppers are filled with ice, the load upon flat roofs with parapets may become relatively heavy. Sloping roofs having a pitch of 3:12 or greater may accumulate extensive snowdrifts on the leeward side, but water falling subsequently will drain off to a large extent.

It seems desirable to design a roof for the following minimum live loads, stated in terms of pounds per square foot of horizontal projec­tion:

a. 40 for roofs having 4:12 slope or less in snowy climates; 50, if flat roofs are surrounded with parapets 3 ft. or more high.

b. 30 for all roofs in regions where the winters are moderate.

c. 20 for all roofs in the far South and Southwest.

Besides uniformly distributed live loads, it is advisable to provide for the effect of a 200-lb. man walking around on a reasonably flat roof during construction and maintenance operations or for the purpose of removing snow. Therefore, this feature may need special considera­tion in case the roof is to be made of weak and brittle materials.

The effect of wind loads on sloping roofs of industrial buildings should generally be computed on the basis of the following pressure, pounds per square foot, of vertical projection:

a. 20 for buildings less than 50 ft. high.

b. 30 for structures 50 ft. or more in height.

c. 40 for structures in regions where tropical hurricanes and torna­does are expected.

(3226signs)