TYPES OF WOOD BUILDING FRAMING SYSTEMS BASIC AND TUTORIALS

BUILDING FRAME SYSTEM THAT USES WOOD BASIC INFORMATION
What Are The Different Building Framing System Using Wood?


There are various types of framing systems that can be used in wood buildings. The most common type of wood-frame construction uses a system of horizontal diaphragms and vertical shearwalls to resist lateral forces, and specifically with the design of this basic type of building.

At one time building codes classified a shearwall building as a box system, which was a good physical description of the way in which the structure resists lateral forces. However, building codes have dropped this terminology, and most wood-frame shearwall buildings are now classified as bearing wall systems.

It is felt that the designer should first have a firm understanding of the behavior of basic shearwall buildings and the design procedures that are applied to them. With a background of this nature, the designer can acquire from currently available sources the design techniques for other systems.

The basic bearing wall system can be constructed entirely from wood components. See Fig. 1.1. Here the roof, floors, and walls use wood framing.

In addition to buildings that use only wood components, other common types of construction make use of wood components in combination with some other type or types of structural material. Perhaps the most common mix of structural materials is in buildings that use wood roof and floor systems and concrete tiltup or masonry (concrete block or brick) shearwalls.

See Fig. 1.2. This type of construction is common, especially in one-story commercial and industrial buildings. This construction is economical for small buildings, but its economy improves as the size of the building increases.

Trained crews can erect large areas of panelized roof systems in short periods of time. See Fig. 1.3.

Design procedures for the wood components used in buildings with concrete or masonry walls are also illustrated throughout this book. The connections between wood and concrete or masonry elements are particularly important and are treated in considerable detail.

Wind and seismic (earthquake) are the two lateral forces that are normally taken into account in the design of a building. In recent years, design for lateral forces has become a significant portion of the design effort.

The reason for this is an increased awareness of the effects of lateral forces. In addition, the building codes have substantially revised the design requirements for both wind and seismic forces. These changes are the result of extensive research in wind engineering and earthquake-resistant design.

FLOOR FRAMING DESIGN CONSIDERATION BASICS AND TUTORIALS

FLOOR FRAMING DESIGN CONSIDERATION BASIC INFORMATION
What Are The Considerations During Floor Framing Design?


Selection of a suitable and economical floor system for a steel-frame building involves many considerations: load-carrying capacity, durability, fire resistance, dead weight, overall depth, facility for installing power, light, and telephones, facility for installing aid conditioning, sound transmission, appearance, maintenance, and construction time.

Building codes specify minimum design live loads for floor and roof systems. In the absence of a code regulation, one may use ‘‘Minimum Design Loads in Buildings and Other Structures,’’ ASCE 7-93, American Society of Civil Engineers.

Floors should be designed to support the actual loading or these minimum loads, whichever is larger. Most floors can be designed to carry any given load. However, in some instances, a building code may place a maximum load limit on particular floor systems without regard to calculated capacity.

Resistance to lateral forces should not be disregarded, especially in areas of seismic disturbances or for perimeter wind bents. In designs for such conditions, floors may be employed as horizontal diaphragms to distribute lateral forces to walls or vertical framing; those elements then transmit the lateral forces to the
foundations.

When using lightweight floor systems, special reinforcement in the floor slab may be necessary at those points where the floor diaphragm transfers the horizontal forces to the frame elements. Durability becomes a major consideration when a floor is subject to loads other than static or moderately kinetic types of forces.

For example, a light joist system may be just the floor for an apartment or an office building but may be questionable for a manufacturing establishment where a floor must resist heavy impact and severe
vibrations.

Shallow floor systems deflect more than deep floors; the system selected should not permit excessive or objectionable deflections.

Fire resistance and fire rating are very important factors, because building codes in the interest of public safety, specify the degree of resistance that must be provided. Many floor systems are rated by the codes or by fire underwriters for purposes of satisfying code requirements or basing insurance rates.

The dead weight of the floor system, including the framing, is an important factor affecting economy of construction. For one thing, substantial saving in the weight and cost of a steel frame may result with lightweight floor systems.

In addition, low dead weight may also reduce foundation costs. Joist systems, either steel or concrete, require no immediate support, since they are obtainable in lengths to meet normal bay dimensions in tier building construction.

On the other hand, concrete arch and cellular-steel floors are usually designed with one or two intermediate beams within the panel. The elimination of secondary beams does not necessarily mean overall economy just because the structural-steel contract is less.

These beams are simple to fabricate and erect and allow much duplication. An analysis of contract price shows that the cost per ton of secondary beams will average 20% under the cost per ton for the whole steel structure; or viewed another way, the omission of secondary beams increases the price per ton on the balance of the steelwork by 31⁄2% on the average. This fact should be taken into account when making a cost analysis of several systems.

Sometimes, the depth of a floor system is important. For example, the height of a building may be limited for a particular type of fire-resistant construction or by zoning laws. The thickness of the floor system may well be the determining factor limiting the number of stories that can be built. Also, the economy of a deep floor
is partly offset by the increase in height of walls, columns, pipes, etc.

Another important consideration, particularly for office buildings and similar type occupancies, is the need for furnishing an economical and flexible electrical wiring system. With the accent on movable partitions and ever-changing office arrangements, the readiness and ease with which telephones, desk lights, computers, and other electric-powered business machines can be relocated are of major importance.

Therefore, the floor system that by its makeup provides large void spaces or cells for concealing wiring possesses a distinct advantage over competitive types of solid construction. Likewise, accommodation of recessed lighting in ceilings may disclose an advantage for one system over another. Furthermore, for economical air conditioning and ventilation, location of ducts and method of support warrant study of several floor systems.

Sound transmission and acoustical treatments are other factors that need to be evaluated. A wealth of data are available in reports of the National Institute of Standards and Technology. In general, floor systems of sandwich type with air spaces between layers afford better resistance to sound transmission than solid systems, which do not interrupt sound waves.

Although the ideal soundproof floor is impractical, because of cost, several reasonably satisfactory systems are available. Much depends on type of occupancy, floor coverings, and ceiling finish—acoustical plaster or tile.

Appearance and maintenance also should be weighed by the designer and the owner. A smooth, neat ceiling is usually a prerequisite for residential occupancy; a less expensive finish may be deemed satisfactory for an institutional building.

Speed of construction is essential. Contractors prefer systems that enable the follow-up trades to work immediately behind the erector and with unimpeded efficiency.

In general, either rolled beams or open-web joists are used to support the floor elements. The most common types of flooring are (a) concrete fill on metal deck, (b) pre-cast concrete plank, and (c) cast-in-place concrete floors with integral joist.

Metal decks may be cellular or plain and are usually stud-welded to the supporting elements to provide composite action. Cast-in-place concrete floors, or concrete pan floors, are becoming less common than in the past. In addition to the systems described, there are several adaptations of these as well as other proprietary systems.

STEEL AND CONCRETE FRAMING IN CONSTRUCTION BASICS AND TUTORIALS

STEEL AND CONCRETE FRAMING TYPES IN CONSTRUCTION BASIC INFO
How Steel and Concrete Framing Works?


In another type of framing system, different from those described, a partial use of structural steel has an important role, namely, composite framing of reinforced concrete and structural steel.framing in carrying loads. The term composite, however, often is used for the specific cases in which concrete slabs act together with flexural members.

Reinforced-concrete columns of conventional materials when employed in tall buildings and for large spans become excessively large. One method of avoiding this objectionable condition is to use high-strength concrete and high-strength reinforcing bars.

Another is to use a structural-steel column core. In principle, the column load is carried by both the steel column and the concrete that surrounds the steel shape. Building codes usually contain an appropriate formula for this condition.

A number of systems employ a combination of concrete and steel in various ways. One method features steel columns supporting a concrete floor system by means of a steel shearhead connected to the columns at each floor level.

The shallow grillage is embedded in the floor slab, thus obtaining a smooth ceiling without drops or capitals.

Another combination system is the lift-slab method. In this system, the floor slabs are cast one on top of another at ground level. Jacks, placed on the permanent steel columns, raise the slabs, one by one, to their final elevation, where they are made secure to the columns.

When fireproofing is required, the columns may be boxed in with any one of many noncombustible materials available for that purpose. The merit of this system is the elimination of form work and shoring that are essential in conventional reinforced-concrete construction.

For high-rise buildings, structural-steel framing often is used around a central, load-bearing, concrete core, which contains elevators, stairways, and services. The thick walls of the core, whose tubular configuration may be circular, square, or rectangular, are designed as shear walls to resist all the wind forces as well as
gravity loads.

Sometimes, the surrounding steel framing is cantilivered from the core, or the perimeter members are hung from trusses or girders atop the core and possibly also, in very tall buildings, at midheight of the core.

GIRDERS USED IN CIVIL CONSTRUCTION LONG SPAN FRAMING BASICS AND TUTORIALS

GIRDERS IN LONG SPAN FRAMES CONSTRUCTION
What Are Girders In Construction?


Girders are the usual choice where depths are limited, as over large unobstructed areas in the lower floors of tall buildings, where column loads from floors above must be carried across the clear area.

Sometimes, when greater strength is required than is available in rolled beams, cover plates are added to the flanges (Fig. 7.10a) to provide the additional strength.

When depths exceed the limit for rolled beams, i.e., for spans exceeding about 67 ft (based on the assumption of a depth-span ratio of 1:22 with 36-in-deep Ws), the girder must be built up from plates and shapes.

Welded girders are used instead of the old-type conventional riveted girds (Fig. 7.10b), composed of web plate, angles, and cover plates.Welded girders generally are composed of three plates (Fig. 7.10c). This
type offers the most opportunity for simple fabrication, efficient use of material, and least weight.

Top and bottom flange plates may be of different size (Fig. 7.10d), an arrangement advantageous in
composite construction, which integrates a concrete floor slab with the girder flange, to function together.

Heavy girders may use cover-plated tee sections (Fig. 7.10e). Where lateral loads are a factor, as in the case of girders supporting cranes, a channel may be fastened to the top flange (Fig. 7.10ƒ).

In exceptionally heavy construction, it is not unusual to use a pair of girders diaphragmed together to share the load (Fig. 7.10g).

The availability of high-strength, weldable steels resulted in development of hybrid girders. For example, a high strength steel, say A572 Grade 50, whose yield stress is 50 ksi, may be used in a girder for the most highly stressed flanges, and the lower-priced A36 steel, whose yield stress is 36 ksi, may be used for lightly stressed flanges and web plate and detail material.

The AISC specification for allowable-stress design requires that the top and bottom flanges at any cross section have the same cross-sectional area, and that the steel in these flanges be of the same grade.

The allowable bending stress may be slightly less than that for conventional homogeneous girders of the high strength steel, to compensate for possible overs tress in the web at the junction with the flanges.

Hybrid girders are efficient and economical for heavy loading and long spans and, consequently, are frequently employed in bridgework.

SKELETON FRAMING METHOD IN STRUCTURES BASICS AND TUTORIALS

SKELETON FRAMING BASIC INFORMATION
What Is Skeleton Framing?


In skeleton framing all the gravity loadings of the structure, including the walls are supported by the steel framework. Such walls are termed non bearing or curtain walls. This system made the skyscraper possible.

Steel, being so much stronger than all forms of masonry, is capable of sustaining far greater load in a given space, thus obstructing less of the floor area in performing its function. With columns properly spaced to provide support for the beams spanning between them, there is no limit to the floor and roof area that can be constructed with this type of framing, merely by duplicating the details for a single bay.

Erected tier upon tier, this type of framing can be built to any desired height. Fabricators refer to this type of construction as ‘‘beam and column.’’ A typical arrangement is illustrated in Fig. 7.8.

The spandrel beams, marked B1 in Fig. 7.8, are located in or under the wall so as to reduce eccentricity caused by wall loads. Figure 7.9 shows two methods for connecting to the spandrel beam the shelf angle that supports the outer course of masonry over window openings 6 ft or more in width.

In order that the masonry contractor may proceed expeditiously with the work, these shelf angles must be in
alignment with the face of the building and at the proper elevation to match a masonry joint. The connection of the angles to the spandrel beams is made by bolting; shims are provided to make the adjustments for line and elevation.

Figure 7.9a illustrates a typical connection arrangement when the outstanding leg of the shelf angle is about 3 in or less below the bottom flange of the spandrel beam; Fig. 7.9b illustrates the corresponding arrangement when the outstanding leg of the shelf angle is more than about 3 in below the bottom flange of the spandrel
beam.

In the cases represented by Fig. 7.9b, the shelf angles are usually shipped attached to the spandrel beam. If the distance from the bottom flange to the horizontal leg of the shelf angle is greater than 10 in, a hanger may be required.

In some cases, as over door openings, the accurate adjustment features provided by Fig. 7.9a and b may not be needed. It may then be more economical to simplify the detail, as shown in Fig. 7.9c. The elevation and alignment will then conform to the permissible tolerances associated with the steel framework.

WALL BEARING FRAMING BASICS AND TUTORIALS

WALL BEARING FRAMING METHODS BASIC INFORMATION
What Is Wall Bearing Framing Method?


Probably the oldest and commonest type of framing, wall-bearing (not to be confused with bearing-wall construction), occurs whenever a wall of a building, interior or exterior, is used to support ends of main structural elements carrying roof or floor loads.

The walls must be strong enough to carry the reaction from the supported members and thick enough to ensure stability against any horizontal forces that may be imposed. Such construction often is limited to relatively low structures, because load-bearing walls become massive in tall structures. Nevertheless, a wall bearing system may be advantageous for tall buildings when designed with reinforcing steel.

A common application of wall-bearing construction may be found in many single-family homes. A steel beam, usually 8 or 10 in deep, is used to carry the interior walls and floor loads across the basement with no intermediate supports, the ends of the beam being supported on the foundation walls. The relatively shallow
beam depth affords maximum headroom for the span. In some cases, the spans may be so large that an intermediate support becomes necessary to minimize deflection.

Usually a steel pipe column serves this purpose. Another example of wall-bearing framing is the member used to support masonry over windows, doors, and other openings in a wall. Such members, called lintels, may be a steel angle section (commonly used for brick walls in residences) or, on longer spans and for heavier walls, a fabricated assembly.

A variety of frequently used types is shown in Fig. 7.6. In types b, c, and e, a continuous plate is used to close the bottom, or soffit, of the lintel, and to join the load-carrying beams and channels into a single shipping unit.

The gap between the toes of the channel flanges in type d may be covered by a door frame or window trim, to be installed later. Pipe and bolt separators are used to hold the two channels together to form a single member for handling.

Bearing Plates. Because of low allowable pressures on masonry, bearing plates (sometimes called masonry plates) are usually required under the ends of all beams that rest on masonry walls, as illustrated in Fig. 7.7.

Even when the pressure on the wall under a member is such that an area no greater than the contact portion
of the member itself is required, wall plates are sometimes prescribed, if the member is of such weight that it must be set by the steel erector. The plates, shipped loose and in advance of steel erection, are then set by the mason to provide a satisfactory seat at the proper elevation.

Anchors. The beams are usually anchored to the masonry. Government anchors, as illustrated in Fig. 7.7, are generally preferred.

Nonresidential Uses. Another common application for the wall-bearing system is in one-story commercial and light industrial-type construction. The masonry side walls support the roof system, which may be rolled beams, open-web joists, or light trusses.

Clear spans of moderate size are usually economical, but for longer spans (probably over 40 ft), wall thickness and size of buttresses (pilasters) must be built to certain specified minimum proportions commensurate with the span—a requirement of building codes to assure stability. Therefore, the economical aspect should be carefully investigated.

It may cost less to introduce steel columns and keep wall size to the minimum permissable. On the other hand, it may be feasible to reduce the span by introducing intermediate columns and still retain the wall-bearing system for the outer end reactions.

Planning for Erection. One disadvantage of wall-bearing construction needs emphasizing: Before steel can be set by the iron workers, the masonry must be built up to the proper elevation to receive it. When these elevations vary, as is the case at the end of a pitched or arched roof, then it may be necessary to proceed in alternate stages, progress of erection being interrupted by the work that must be performed by the masons, and vice versa.

The necessary timing to avoid delays is seldom obtained. A few columns or an additional rigid frame at the end of a building may cost less than using trades to fit an intermittent and expensive schedule. Remember, too, that labor-union regulations may prevent the trades from handling any material other than that belonging to their own craft.

An economical rule may well be: Lay out the work so that the erector and iron workers can place and connect all the steel work in one continuous operation.