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Building Load

One of the less-thought-about facts of house construction, though certainly one of the most important once you do think about it, is that houses stay up once they are put up. They stay up despite the handicap that many houses are built with less than adequate workmanship or with poor materials, and that the houses are subjected to a great many different and often quite severe stresses during their lifetimes. Some of these stresses are inherent in the structure, others are imposed from outside. The inherent stresses are dead loads. The stresses imposed from outside include live loads, wind loads, snow loads, and earthquake loads.

DEAD LOADS

The dead loads include the weights of all the structural parts of the building—the studs, joists, rafters, roofing, flooring, insulation, plastering, and so on. The dead loads also include the weights of all the mechanical equipment permanently installed in the house—the plumbing and heating equipment, duct work, central air-conditioning equipment, kitchen appliances, and the like. In sum, the dead loads include everything in, on, or attached to the house that can't possibly be omitted or eliminated if the house is to be considered complete and habitable. Since the weights and locations of all these materials are known, the dead loads can easily be calculated and the structure designed to support them.

LIVE LOADS

Live loads include everything that the inhabitants carry into a completed building to make it habitable, including them selves. Live loads, therefore, are additions to the floor dead loads. Anything that's movable is usually considered as part of the live load, though sometimes live loads shade off into dead loads. The permanent installation of large bookcases with their weight of books, a grand piano, or machine-shop equipment, can be considered either as live loads or as dead loads. In the former case, it's assumed that the joists will be able to support this weight without any additional reinforcement. In the latter case, the builder may find it necessary to reinforce the joists to help them support the additional loads.

In American building codes, the first-floor joists must be strong enough to support a live load of 40 lb per sq ft; the second-floor joists must be strong enough to support a live load of 30 lb per sq ft; and attic joists, if the attic is unoccupied, must be strong enough to support a live load of 20 lb per sq ft. If the attic space is to be converted at a later date into a living space, a bedroom or study, then the attic floor joists must be capable of supporting a live load of 30 lb per sq ft. Staircases are required to support a live load of 100 lb per sq ft.

These requirements are very conservative. The weight of all the furniture in a typical living room, apportioned over the entire floor area, will seldom exceed 5 lb per sq ft. If one were to crowd 20 people into a 12 X 15 ft living room for a party, the live floor loading would not exceed 20 lb per sq ft. Nevertheless, one never knows what future loads a floor may be required to support, and it's undoubtedly wiser to expect the worse possible loading than to have the floor start sagging after a period of use.

Wood joists must be strong enough to support both their own weight and whatever live loads are imposed on them. But joists that are designed only for strength will tend, over a period of time, to sag of their own weight as well as the weight of the live loads they are supporting. This sagging will cause the plaster to crack, not to mention what it will do to the peace of mind of the occupants. Joists, therefore, must be large enough to be stiff as well as strong. Building codes require that joists deflect a maximum of 1/360th their unsupported lengths when they are supporting their maximum design loads. For a 20-ft-long joist, this means, for example, that the joist should sag no more than 2/3 in. at the center of its span. Joists will rarely, if ever, be subjected to their maximum design loadings so the actual amount of sag in the typical dwelling will in fact be imperceptible to the occupants.

WIND LOADS

The wind is the most important outside force affecting the structural integrity of a house. The map in Fig. 1 shows the maximum wind pressures that can be expected in different parts of the United States. The contour lines represent the force of the wind in lb per sq ft of pressure, not mi per hr of velocity, since it's the pressure pushing against the house that's of interest to us. The figures represent average maximum wind pressures; that's , the force exerted by the wind over a 5-mm period. Gusts may add perhaps another 50 % to these values. In addition, the shape of a house will sometimes increase the force with which the wind strikes it. For a rectangular-shaped house, for example, a shape factor of 33 % must be added to the basic wind pressure.


Fig. 1. Wind map of the United States. The contour lines represent maximum velocity pressures in lb per sq ft. Use Table 1 to convert these pressures to equivalent miles per hour (U.S. Dept. of Commerce).

Thus, including both gust loads and a shape factor, an engineer designing a new house usually assumes a wind loading of something around 15 lb per sq ft, which is equivalent to a gust velocity of about 75 to 80 mi per hr (refer to Table 1).

Table 1. Wind Speeds and Velocity Pressures

True wind speed, mi per hr

Velocity pressure, lb per sq ft

5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

95

100

105

110

115

120

0.064

0.256

0.575

1.023

1.600

2.302

3.133

4.092

5.179

6.394

7.73 7

9.208

10.810

12.530

14.390

16.370

18.480

20.720

23.080

25.580

28.200

30.950

33. 830

36.830

In most parts of the United States, a house capable of withstanding a wind load of 15 lb per sq ft will be more than strong enough. However, along the Gulf and Atlantic coasts as far north as Chesapeake Bay, and 50 miles inland from these coastlines, houses must expect to withstand winds of hurricane force sometime during their existence and the anticipated wind loadings must be increased accordingly. It is improbable, however, that wind loads will ever exceed 40 lb per sq ft.

When calculating the wind pressures acting on a house, an engineer assumes that the wind will act in a horizontal direction. It doesn’t always, of course. Most people are also likely to assume that the wind pushes down against a pitched roof. This assumption isn't quite accurate, imagine a strong wind blowing against a house that has a gable roof (see Fig. 2). When the wind strikes the house, it will be deflected to the sides and upward. As the wind passes over the roof, its velocity must increase. As its velocity increases, its pressure must decrease. Depending on the shape of the roof, there may even be a negative pressure acting against the downwind side of the roof.


Fig. 2. Wind blowing against a house with a gable roof.

The roof may, in fact, be pulled up and away from the walls supporting it. Tiles and shingles that are “blown off” a roof aren't pushed off by the wind; they are sucked off by negative air pressures. Everyone has seen photographs of wind-damaged houses that have had their roofs “blown off.” What has actually happened is that a suction pressure has lifted the roof from the wails. I don’t think anyone has seen a photograph of a house that has had its roof blown in during a windstorm.

For the typical gable roof, the negative pressure on the- windward side of the roof increases in a regular way as the pitch of the roof increases. This is shown in Fig. 3. At roof slopes of 30 degrees and more, the wind pressure turns positive and increases as the pitch of the roof increases.


Fig. 3. Wind pressure and suction on a gable roof, as a function of the basic wind pressure. Note that the suction on the downwind side of the roof remains constant at about -0.5 of the basic wind pressure regardless of the roof slope.

As for the walls, wind pressures acting on the downwind side of a house will be less than those acting on the windward side. Much depends on the basic shape of the house. In sum, however, a house is as vulnerable to suction forces as it's to pressure forces. When the windows rattle and the framework shakes during a high wind, it's a combination of positive and negative pressures acting against the house that are responsible for the shakes and rattles. To be more exact, it's the relative changes in air pressure between the inside and outside of the house that are responsible for the shakes and rattles.

The roof is more vulnerable to wind pressures than the walls for two reasons. The first we have mentioned already—the strong negative forces acting against the roof. The second is that in most dwellings the juncture between the walls and roof rafters is structurally one of the weakest parts of the overall house construction. This is especially true for platform-framed houses in which the rafters are nailed to a top plate, which is itself none too securely nailed to the wall studs (see WOOD-FRAME CONSTRUCTION). The great difficulty in attaching the rafters to the walls in a wood-framed house is that it's very difficult to build this joint so that it will resist tensile stresses, that's , stresses that tend to pull the nailed joint apart. The joint must be reinforced if it's to resist high wind loads. For a description of reinforcement methods, see ROOF FRAMING.

In those parts of the country where tornadoes or hurricanes are common, local building codes often require that the roof, walls, and foundation of a platform-framed house be securely joined together with iron strapping. (Incidentally, there is no point in trying to design a house against a direct hit by a tornado—the wind velocities within the funnel of a tornado have been estimated to be as high as 500 mi per hr, which is equivalent to wind pressures of about 75 lb per sq ft.)

SNOW LOADS


Fig. 4. Snow load map of the United States. The contour lines represent the basic snow load in lb per sq ft (U.S. Dept. of Commerce).

A roof is usually designed to support a dead load of 20 lb per sq ft. This includes the weight of the rafters and roofing material, whether it be asbestos shingles, wood shingles, asphalt roll roofing, clay tiles, or whatever, although the roof is usually built a bit stronger than normal if the rafters are required to support the weight of slate or clay-tile roofing. If a flat roof is to be used as a sun deck or roof garden, its design live load must be increased accordingly. The total dead and live loads on a roof garden are assumed to be 100 lb per sq if; on a sun deck, 60 lb per sq ft.

In addition, in northern parts of the United States, roofs are required to support the live load of snow for several months of the year. Snow weighs about 8 lb per cu ft when dry and from 10 to 1 5 lb per cu ft when wet or tightly packed. This snow load is calculated on the basis of the horizontal area covered by the roof, not the actual area of the roof itself, which, of course, will be greater when the surfaces of the roof are slanted. If the total anticipated snowfall for two months (the usual method of calculating the total snow load) exceeds 20 lb per sq ft, then it's the snow load that will determine how strong the roof must be constructed, not the weight of the rafters and roofing material.

Figure 4, a map showing the average snowfall in different parts of the United States, indicates in a rough way how strongly a roof must be built in different parts of the country. The requirements of local building codes can be expected to reflect local experiences. One place in the Northwest, for example, has had snow loads greater than 240 lb per sq if, and it's not uncommon in many parts of the Rockies for the snowfall to reach 25 ft, which is equivalent to a snow load of 300 lb per sq ft.

There is an alternative to strengthening the roof rafters when exceptionally heavy snow loads are anticipated. This is to make the roof steeper. The steeper a roof, the less the maxi mum anticipated snow load for the obvious reason that snow has difficulty sticking to a steep surface. A roof that slopes 60 degrees or more hasn’t any snow-load requirement at all.

The question arises: How does one calculate combined snow and wind loads? The answer is that one never assumes that heavy accumulations of snow and strong winds will occur simultaneously, which seems sensible. If the anticipated snow loads are greater than the anticipated wind loads, the snow loads will determine the roof design; otherwise the wind loads will be the determining factor.

EARTHQUAKE LOADS


Fig. 5. Earthquake zones of the United States, as described in the text. The relative sizes of the dots indicates the relative intensity of the earthquake shocks experienced.

The map in Fig. 5 shows the relative frequency and intensity of earthquakes in the United States. In zone 0, no danger what ever exists from earthquake shocks and no special precautions need be taken against earthquakes in the construction of a house. In zones 1, 2, and 3, the probable amount of damage resulting from an earthquake doubles as the number of the zone increases. Thus, zone 2 is expected to experience twice as much damage from an earthquake, on the average, as zone 1 will; and zone 3 is expected to experience twice as much damage from an earthquake, on the average, as zone 2 will. The positions of the dots indicate roughly the locations of earthquake epicenters; their size indicates the intensity of the earthquake shocks.

An earthquake occurs whenever the earth relieves an internal stress that has built up within it. These stresses occur between two great masses of rock that meet each other along what is called a fault. Whatever the reason for the buildup of the stress, it's relieved when these two great masses adjust their positions relative to each other along the fault. It is this suddent adjustment in positions that's felt as an earthquake shock. The shock is immediately followed by several smaller shocks, which are actually vibratory motions as these masses settle down to their new positions.

The shock and the vibrations that follow it result in horizontal movements on the surface of the earth, movements that tend to whip a house back and forth several times in a series of short, sharp, horizontal jolts. Most building codes ignore these earthquake loads in their required design loads for one- and two-family dwellings when the dwellings have a wood frame work. The reason is that wood is peculiar among construction materials in that it can absorb extremely large—but momentary—stresses without suffering any damage or failure. Since the loads imposed on a house by an earthquake are basically horizontal loads of the same sort that are imposed by wind loads, though the earthquake loads are much shorter and sharper, of course, a wood-frame house that can resist the usual design wind loads of 15 lb per sq ft is considered to be capable of resisting the shock loads produced by most earth quakes. Therefore, for a wood house, there is no need to take earthquake loads into account in the design of the house.

Masonry houses, on the other hand, are far more likely to require special construction techniques to resist earthquake loads. Brick, concrete, concrete blocks, and stonework are much more brittle materials than wood, and they are held together by rigid mortar joints. Masonry hasn’t the resilience of wood to sudden, sharp loadings. An un-reinforced masonry wall will very likely suffer some damage in even a moderate earthquake. If the shock is severe, an entire masonry wall may collapse. In earthquake-prone areas, therefore, masonry buildings must be especially reinforced to resist sudden horizontal loads. For a description of the necessary reinforcements, see CONCRETE-BLOCK CONSTRUCTION.

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