Unit 4 Structure

The functions of rafters, roof joists, and ceiling joists include the following: 5 things

-Carry the live loads of wind, rain, snow, and people -Carry the dead loads of roof sheathing, roof coverings, and roof-mounted equipment -Support the dead loads of insulation and ceiling finishes -Laterally support the walls of the building, preventing racking and wall or roof spreading -Create an attic space, a ventilation space, and support for soffits and fascia

Piles are used instead of footings, typically where the soil quality is poor (see Figure 4.8). They are, generally speaking, more expensive to install and have to be driven into the ground with specialized equipment. They can work one of the following two ways:

-End bearing. Piles can be driven down to a point where they bear on bedrock or other sound substrate. -Friction. Piles can be driven into soil far enough that the friction of the soil against the sides of the pile is enough to resist any downward movement. Incidentally, if a house is supported on piles, the piles probably won't be visible and you may not know it.

How can you tell whether it's a load-bearing or partition wall? The answer lies in what's above and below them.

-Load-bearing walls have ends of floor or ceiling joists resting on them. Partition walls don't. -If there is another wall directly or almost directly above the wall, it's probably a bearing wall. When removing the wall would remove the support for structural members above, it's a load-bearing wall. -Door openings in partition walls do not normally require headers. Openings in load-bearing walls do need headers.

How can you tell whether it's a load-bearing or partition wall? The answer lies in what's above and below them.

-Partition walls have no structural elements beneath them to transfer the loads to the foundations and footings -Load-bearing walls must have foundations, beams and columns, or another wall system below them to transfer the loads to the soil.

Common joist problems include the following: -Rot and insect damage -Sagging joists (often part of a springy floor) -Poor end bearing -Rotated or twisted joists -Inappropriate notching or holes

-Split or damaged -Weak cantilevers -Weaknesses created by openings around stairs, chimneys, and windows, et cetera. or cutouts for attic or crawlspace access. -Prior repairs -Concentrated loads -Missing joists

From far away, you're looking for things such as? In a roof structure

-a sag in the rafter or ridge system, -dishing in the field of the roof, and -spreading of the roof rafters (often visible at the soffits), or the top of the walls.

In houses that have been around a few years, you will be looking to see whether the structure is responding satisfactorily. In most cases, this means staying put. While all structures move slightly (they must in order to carry their loads), the amount of movement should not: 4 things

-cause failure of interior finishes; -put undue stress on joints or individual components; and -affect the usability and operation of house systems (windows shouldn't jam, pipes shouldn't be broken, ------electrical wires shouldn't be pulled, etc.).

Rafter spread can result in; 2 things

-the rafters, soffits and fascia sliding out away from the walls, and -the walls themselves may be being pushed outward if the rafters are well secured to the walls.

The risk of rot to sills, joists and beams is also increased where earth-floor crawlspace ventilation is less than ______ square foot of venting for every _______ square feet of floor area. Check your local requirements for venting. These vary with climate and type of floor surface.

1 sq ft 150 sq ft

Where beams or joists go into pockets in foundation walls, it's common to keep the sides, top, and ends of the wood _________inch away from the concrete to allow air circulation and keep the wood dry. Where the wood has to be embedded, it should be pressure treated and/or field treated with a wood preservative to prevent rot. It is common in old homes to find joists and beams contacting the foundation.

1/2 inch

Beams should not be notched or contain drilled holes. Where this has been done, check for sag, splitting, or rotation of the beam, especially if the notches are in the middle ________ of the span.

1/3 of the span

Floors also have to be stiff. This means that they have to limit the deflection that takes place when structural members respond to live loads. There will always be some deflection, but if the deflection is too great, damage to the interior finish will result. In some jurisdictions, the maximum allowable deflection is __________th of the length of the joist (see Figure 4.16). This number is not magic, but comes from the amount of deflection that plaster and drywall will tolerate without cracking.

1/360th

Watch for crawlspaces that are shallow. The earth in subgrade areas should be at least _______ inches below floor joists and ______ inches below beams. Where earth is closer, there is a greater risk of rot and insect damage to sills as well as joists and beams. This goes for deck etc. or use PT lumber

18 inches 12 inches

A wood sill is typically used to connect the top of the foundation to the wood floor system above. Sills see primarily compressive loads, but may experience tension (uplift) or shear (lateral) forces due to wind. Sills might be ____x_____ or _____x______ or even _______x_______ lumber, typically the same species and grade as the rest of the lumber. Sills in some areas have to be pressure-treated lumber, redwood, cedar, or other rot and insect-resistant wood

2x4 or 2x6 or even 2x8

Beams require _______ inches of solid bearing at each support point if resting on concrete or masonry, and ________inches if resting on wood or metal. The bearing should be the full width of the beam to avoid crushing or rotation. The end bearing should be level and continuous.

3 inches ! 1/2 inches

Columns may be concrete, concrete block, brick, steel, or wood, typically. They can be a combination of these. Wood columns should be rot-and-decay resistant, and be at least ______ inches square. Steel columns should be at least ______ inches in diameter.

4 inches 4x4 3 inches

There are three factors that impact structural failures.

=Size of the load—failure due to overloading. =Direction of the load—failure due to component not sized to take differing directional forces. -Time imposed—failure due to a continuous overload for a long time.

What is a pilaster?

A pilaster is a thickening of a foundation wall. It may be thickened to receive the concentrated load of a beam resting on top of the pilaster, or it may be acting as a stiffener to prevent the foundation wall from bowing inward.

pilaster

A thickening of a foundation wall. May be thickened to receive the concentrated load of a beam resting on top of the pilaster, or it may be acting as a stiffener to prevent the foundation wall from bowing inward.

Arches and lintels transfer the loads of walls over openings to the wall systems on either side.

Arches are most often the same masonry as is that used in the wall face (e.g., stone, brick, or concrete), although architectural detailing sometimes dictates the use of different materials. For example, stone arches in brick walls are common. Lintels are typically steel, wood, or a large single piece of masonry.

The load from the column must be carried continuously. Blocking is required under the subfloor immediately below the column. The blocking can carry the load from the column through the subfloor, down to the beam. The beam, in turn, should have a column immediately below the blocking. At the bottom of the column should be a footing. Any discontinuity in this vertical transferring of loads can lead to structural distress. See figure 4.26

As a general rule, when thinking about how structures react, think vertically; try to follow loads down through the building and watch for the weak connections or offsets.

Which is stronger, a poured concrete wall or a masonry foundation wall?

Because of the mortar joints, masonry foundation walls are generally not as strong as poured concrete walls.

Partition walls have considerable dead weight in the lumber and plaster on both sides. A typical wall weighs 15 pounds per square foot. Every foot along an 8-foot-high partition wall weighs 120 pounds. A 10-foot-long partition wall weighs 1,200 pounds!

But more than that, people put heavy things against partition walls. Whether it's a dining room with a buffet, sideboard, or china cabinet; a kitchen with a refrigerator, stove, dishwasher, base cabinets, and upper-wall-hung cabinets; or a bedroom with a triple dresser and/or water bed, there are considerable loads on joists.

Which block is weaker, cinder blocks or concrete blocks? How can you tell the difference between the two?

Cinder blocks installed below grade will deteriorate over time if exposed to chronic moisture. Cinder block is weaker and less moisture resistant than concrete block. If you see cinder-block foundations, (it typically has a rougher, darker texture than concrete block), watch for evidence of deterioration of the blocks. In some cases, this is not visible on the interior face of the foundation, but the cinder block can be deteriorated on the outer side. There is no way to predict this.

piers

Columns that may be completely concealed in the soil, or may project above. Piers usually have footings and either be thought of as posts or columns.

Beams transfer their loads vertically to columns or foundations. Beams may receive their loads on a vertical or a horizontal face. Joists may rest on top of beams or be fastened to the sides of beams (see Figure 4.22). Joists may also be supported on the bottom flange of a steel beam.

FIGURE 4.22: Load Transfer

Brick veneer is a load bearing part of the structure. T or F

False

caissons

Foundation systems created by drilling holes and filling them with concrete.

The rim joist, header joist or band joist as it is sometimes called, runs around the perimeter of the house. This joist, which is usually the same dimension as the floor joists, is toe-nailed down into the sill and end-nailed into the joists. What are the functions of a rim joist? 3 things

Functions of The rim joist -secures the joist ends, -lends support to the walls and floor systems above, and -provides a nailing surface for exterior sheathing and siding. Rim joists can be 1-by-10s, rather than 2-by-10s, for example.

There are three ways of holding the masonry wythes together, two of which are common:

Header bricks Metal ties Diagonal brick bond (rare)

heaving cracks

Heaving refers to the upward movement of the building. Heaving cracks indicate movement of the slab relative to other parts. There are often elevation differences on either side of such cracks. The slab surfaces may be tilted on either side of the crack. Walls may not be plumb or floors may both be level.

lateral thrust

Lateral thrust is also called horizontal force. It causes horizontal foundation cracks that are more serious than vertical or diagonal cracks

Arches and lintel are both common on exterior masonry walls. Lintels are more common with masonry veneer.

Lintels in masonry walls should typically have 6 inches of full and level end bearing. Headers in wood-frame walls typically have 1½ inches of end bearing (on the jack studs below).

Rafters Pulling Away From Ridge

Look at the ridge for evidence of rafters pulling away from a ridge board or slipping off the ridge. On gable ends, you can often see the tops of the rake boards moving apart, indicating movement of the rafters.

Planes of Movement Don't just focus on crack width. You have to think of movement in all three dimensions. Generally speaking, the more planes of movement, the more serious the problem.

One part of the building may be moving horizontally relative to another. A variation on this is a rotational crack. There will be horizontal movement, but the higher up in the building you go, the more movement there will be, usually. The rotation is usually centered around the footing.

What is a pad footing

Pad footings are similar to continuous footings, except they are usually under a single pier or column. Pad footings spread the load out, usually in a square, with the column or pier sitting in the middle of the square. It's common for houses to have strip footings around the perimeter and pad footings on the building interior under columns.

What is a pier?

Piers are columns that may be completely concealed in the soil, or may project above it (see Figure 4.9). You probably are familiar with the piers that are commonly used to build exterior wood decks and porches. These piers may be poured concrete, often with the concrete poured into a cardboard cylinder in a hole dug in the ground. Piers usually, but not always, have footings. Piers can either be thought of as posts or columns, or can be thought of as short piles that bear on their ends.

end bearing pile

Piles that can be driven down to a point where they bear on bedrock or other sound substrate.

Purlins are 2-by-4s or 2-by-6s, typically. The purlin should be the same size as the rafter, at least.

Purlins are supported by 2-by-4 posts or struts. If the purlins are 2-by-4s, the posts are usually every 4 feet. If the purlins are 2-by-6s, the posts are usually every 6 feet. If the posts are longer than 8 feet, they should be braced to prevent them from buckling. Purlin posts can also be up to 45° degrees off vertical.

If a roof system has a slope of 2 in 12 or less the structural member that the roof sheathing is fastened to are called what? What are the members called on a roof > than 2 in 12

Roof Joist Roof Rafters

We'll look at four types of foundation wall cracks:

Shrinkage cracks Settlement cracks Heaving cracks Horizontal force cracks

Sills should also be well above grade level. Sills are often at or below grade on older homes. This is a recipe for problems. Rotted sills are easily crushed by the weight of the house, and will lead to differential settlement of the floors and walls above. Rotted sills also often mean rotted joists and studs. Rotted sills mean the house frame is not well anchored to the foundation. Common sill problems include the following: 6 things

Sills below grade Rot or insect damage Gaps between the sill and foundation Crushed sills Not properly anchored Missing

When looking at cracks in masonry walls, make sure you don't jump to conclusions. Use a step-by-step approach to analyzing cracks and arrive at a carefully considered opinion.

Sketching the cracks may help you to understand the pattern and the mechanism. You're looking at the direction, length, size, direction of movement (the more directions, the worse usually), and age of the cracks.

Cantilevering creates a joist that is unsupported at one end and, consequently, subject to deflection and springiness. The other reality of cantilevers is the teeter-totter effect. If joists are cantilevered over an outside wall, for example, bouncing on the cantilevered end puts an upward force on the other end of the joist inside the house.

The outside wall is the fulcrum. You'll sometimes find humps in floors at the inner end of the cantilevered joists.

People often analyze cracks based on size. This is very dangerous, since there are several things that may confuse you. For example, which of these situations is worse? A ¼-inch-wide crack in a wall Sixteen cracks, each 1/16 inch wide

The several smaller cracks indicate more movement than the one large crack, so this is the more serious of the two situations.

Fire-retardant-treated (FRT) plywood has been used since the late 1970s, often on attached homes (row or town homes) rather than detached homes. With some types of FRT plywood, the chemicals were activated at excessively low temperatures, (around 150°F) commonly reached in attics. As a result, the wood was weakened even though there was no fire situation.

The weakened FRT plywood often loses its ability to hold staples. Roof shingles can be blown off in relatively low winds. The sheathing may sag and/or delaminate and can be weak enough that someone walking on the roof could fall through.

Knee walls are also called dwarf walls, struts, or strongbacks. They are the same as conventional 2-by-4 stud walls elsewhere in the house. There is a top plate, studs, and a bottom plate. Knee walls may be vertical or up to 45° degrees off vertical. If offset, they are often called struts or strongbacks

There is often a single top plate. This is okay if the studs line up with the rafters they are supporting. Think of knee walls as small bearing walls supporting rafters.

Condensation typically occurs in exterior walls. It is the result of warm moist air from the house leaking into the walls during the winter months. As the warm moist air cools in the wall, the moisture falls out of it. When condensation accumulates in walls, the conditions for rot can easily be met.

This process can take place in reverse in hot humid climates. Hot moist outdoor air (over 100°F with 90% relative humidity) leaking into the walls will be cooled if the house has air-conditioning. The inside face of the wall will be much cooler than the outside face. Condensation may form in the walls.

A notch cut in the bottom edge of a beam is more likely to result in failure than one cut in the top edge, although as a general rule, notches and holes should not be cut in beams. T or F

True

Lack of access to a part of a house structure that you ordinarily would see is a red flag. T or F

True

Put simply, the function of a structure is to do nothing. The most successful structures stay still. T or F

True

There are usually no structural implications to shrinkage cracks. The foundation wall may leak through shrinkage cracks, which is a nuisance but not a major structural problem. True or False

True

There should always be access into under floor spaces. While authorities may specify 16 by 24 inches through a wall, 18 by 24 inches through a floor, or something different, use common sense to determine whether the access is large enough to get through. T or F

True

piles

Used instead of footings, typically where the soil quality is poor. They are generally more expensive to install and have to be driven into the ground with specialized equipment.

grade beam

Usually concrete beams that are supported on footings, piles, or piers and are located at grade. They transfer the loads from the building down to the footings or piles.

vertical shear .

When one part of the building drops relative to another. The crack width may be less than 1/16 of an inch. However, there can be significant dislocation. One side of the house could drop 1 inch relative to another, and the crack still won't be very wide but may cause a serious structural problem

Overspanned joists may sag; they can be identified by measurement. There are span tables in code books and wood-design books. Most home inspectors carry some rough rules around in their head. The following are some very broad rules assuming number 1 or 2 grade spruce joists 16 inches on center, and typical 40 pound-force per square foot (psf) live and 20 psf dead floor loads.

assuming number 1 or 2 grade spruce joists 16 inches on center, 2-by-6s can span about 9 feet 2-by-8s can span about 11½ feet 2-by-10s can span about 14 feet 2-by-12s can span about 16 feet Spacing joists 12 inches on center results in longer allowable spans: 2-by-6s can span about 10 feet 2-by-8s can span about 13 feet 2-by-10s can span about 16 feet 2-by-12s can span about 18¾ feet

Beams may sag because they are _______________ for their size. Another way to look at this is to say that they are ______________ for their span.

overspanned undersized

Columns are designed to transfer loads from beams down through footings to the soil. They are located inside the perimeter of the foundation wall, typically, and may or may not be visible during an inspection. Columns are often concealed within interior wall systems. Columns built into perimeter walls are called _____________

pilasters.

Common truss problems include

sag, buckled webs, weak connectors, rotation or lateral movement, poor end bearing, notches and holes, rot or insect damage, mechanical damage or splitting, and fire damage.

Some of the characteristics of veneer walls include

the presence of weep holes (mostly but not always); the presence of wood studs inside (often visible through electrical switch or outlet boxes from the inside, for example); the absence of header bricks (bricks turned end ways to lock the two wythes of the masonry wall together); the absence of masonry arches (brick veneer construction most often employs steel lintels, although there are some exceptions); and a single wythe of brick supported on a metal angle fastened to the exterior of the foundation. This indicates the brick veneer has been added to a building. The foundation wasn't wide enough to carry the brick, so a shelf was attached to the face of the foundation to support the brick. This situation is relatively rare.

All houses settle when they are built. If the settlement is uniform, there are usually no problems. The house settles slightly as a unit, and because everything moves together, there is no internal stress and no cracking.

true

All modern construction should have sills. Older houses, particularly solid masonry houses, may not have sills. Joists may bear directly on masonry. This is less desirable, but if time has proven the installation acceptable, you should not criticize. Probe the joists carefully where they enter pockets in the foundation walls. Rot is common here, particularly if the joists are at or near grade level.

true

Beams see primarily vertical loads from the weight of floor systems and the live loads above. Lateral loads or tension (uplift) forces may be induced by wind. Beams might fail in bending or shear. Bending is, of course, a combination of compression (felt by the top part of the beam) and tension (felt by the bottom part of the beam).

true

Built-up wood beams are typically made up of three, four, or five 2-by-8s or 2-by-10s. These must be nailed or bolted together so that all the individual members act as a single component. Generally speaking, two sets (one for each side) of two nails are visible every 18 inches along the length of the beam.

true

Concentrated Loads Where a beam is loaded near its midpoint by a column from above, for example, the beam may be overstressed.

true

Floor systems see both vertical and horizontal loads, although most people think of the vertical loads when they think of floors.

true

Floors transfer both live and dead loads to the foundations, footings, and ultimately to the soil below the house.

true

Joists can be thought of as several small beams. Their function is primarily the same as beams. Joists transfer the dead load of the subfloor and floor finishes, as well as the live load of people and furnishings to beams, walls, headers, sills, or foundation walls.

true

Joists see vertical loads from above and lateral soil loads from foundation wall thrust and lateral wind loading from above-grade walls, for example. Joists provide lateral support for walls.

true

Masonry walls may be made of brick, stone, concrete block, cinder block, clay tile, or glass block. Solid masonry walls may be single, double, or triple wythe (thickness). Six-inch single-wythe masonry walls are common on one-story buildings with walls less than 9 feet high

true

Most cracks due to differential settlement are vertical or diagonal, including stepped cracks.

true

Shrinkage cracks do not necessarily have corresponding cracks elsewhere in the building. Cracks due to differential settlement usually do have corresponding cracks in different locations.

true

When one part of the house settles and the rest does not, or two parts settle at different rates, cracking develops. This can be alarming and may or may not be a serious structural problem.

true

Where basement windows or crawlspace vents leave the rim joist and sill plate unsupported (because there's no foundation wall under them), the rim joist should be doubled.

true

Missing columns are unusual on original construction. They are most often caused by homeowners trying to make more clear space available in a room. Depending on the house configuration, the results can be catastrophic failure or sagging.

true Look for signs of HO remodeling