Class Day FourteenClass Day Fourteen

Chapter 10Chapter 10

Masonry Load bearing Wall ConstructionMasonry Load bearing Wall Construction

LOAD BEARING MASONRY WALLS are LOAD BEARING MASONRY WALLS are walls designed to be part of the structural system walls designed to be part of the structural system

of a building. of a building.

A load bearing wall will carry a portion of the A load bearing wall will carry a portion of the weight of a roof, or in the case of two or more weight of a roof, or in the case of two or more stories, a portion of the weight of the floorsstories, a portion of the weight of the floors..

A wall is subject to failure for several reasons, A wall is subject to failure for several reasons, which may be due to: which may be due to:

vertical weight imposed; crushing, bucklingvertical weight imposed; crushing, bucklinghorizontal forces, such as wind.horizontal forces, such as wind.horizontal forces due to seismic loads.horizontal forces due to seismic loads.

Structurally, all potential conditions must be Structurally, all potential conditions must be considered – then the wall is assembled according considered – then the wall is assembled according to safe design standards.to safe design standards.

Early buildings made with masonry load bearing Early buildings made with masonry load bearing walls created problems in settlement of the walls created problems in settlement of the foundation, when connected to masonry walls that foundation, when connected to masonry walls that were not load bearing. were not load bearing. Because of uneven Because of uneven

loading, the load loading, the load bearing walls settled bearing walls settled

into the ground, but into the ground, but the non bearing (end) the non bearing (end) walls did not, walls did not,

resulting in resulting in cracking cracking of the masonry of the masonry at at corners. corners. Corners Corners were reinforced were reinforced with with brick or stone called brick or stone called

“ “QuoinsQuoins” ”

non

load

bea

ring

wal

l

load bearing wall

roof structural members

load bearing wall

non

load

bea

ring

wal

l

The photo shows The photo shows quoins installed at quoins installed at the corners of this the corners of this building. building.

Quoins quickly Quoins quickly became a became a decorative feature, decorative feature, but originally were but originally were installed to be a installed to be a structural item, structural item, usually made of usually made of brick or stone.brick or stone.

There are two types of masonry walls: Reinforced There are two types of masonry walls: Reinforced and Non-Reinforced and Non-Reinforced

Because masonry is a brittle material with Because masonry is a brittle material with virtually no allowance for tensile stress, all virtually no allowance for tensile stress, all masonry construction must be reinforced to masonry construction must be reinforced to compensate for tension due to uneven settlement, compensate for tension due to uneven settlement, shrinkage due to loss of moisture, and movement shrinkage due to loss of moisture, and movement due to plastic soils. due to plastic soils. But not necessarily for But not necessarily for structural properties due to imposed loading. structural properties due to imposed loading.

This type of reinforcing is installed within the This type of reinforcing is installed within the mortar bed in horizontal joints. Reinforcing is mortar bed in horizontal joints. Reinforcing is made of steel wire, meshed to a pattern to resist made of steel wire, meshed to a pattern to resist lateral forces, and electrically welded. lateral forces, and electrically welded.

NON REINFORCEDNON REINFORCED

Construction that does not require the integrity Construction that does not require the integrity of structural loading beyond the requirements of of structural loading beyond the requirements of a free standing wall, such as small single story a free standing wall, such as small single story buildings with light loads.buildings with light loads.

These are built by standard procedures, These are built by standard procedures, including mortar fill and wall reinforcing that is including mortar fill and wall reinforcing that is necessary only for resistance to cracking due to necessary only for resistance to cracking due to shrinkage or expansion due to temperature shrinkage or expansion due to temperature difference – not for structural loading.difference – not for structural loading.

– These would also include low rise units which These would also include low rise units which may have cores filled with concrete for use as may have cores filled with concrete for use as stem walls as parts of foundations, generally stem walls as parts of foundations, generally too short to have a tendency to bend.too short to have a tendency to bend.

– The model building codes require analysis on all The model building codes require analysis on all structural membersstructural members, to protect the public , to protect the public against assembly of parts that have not been against assembly of parts that have not been qualified as safe.qualified as safe.

– And, forces can occur that may be somewhat And, forces can occur that may be somewhat indeterminate in structural analysis – due to indeterminate in structural analysis – due to unexpected loads such as wind gusts, soil unexpected loads such as wind gusts, soil movement, and unforseen forces.movement, and unforseen forces.

For those reasons, the Building Department of the For those reasons, the Building Department of the City of Lubbock, as do other responsible entities, City of Lubbock, as do other responsible entities, limit the construction height of non reinforced limit the construction height of non reinforced masonry walls to no higher than 18 times their masonry walls to no higher than 18 times their thickness. So, non- reinforced wall limits include:thickness. So, non- reinforced wall limits include:

A wall made of 8” CMU has a maximum height A wall made of 8” CMU has a maximum height of 12’-0”.of 12’-0”.

Likewise, a 12” wall cannot be higher than Likewise, a 12” wall cannot be higher than 18’-0”.18’-0”.

REINFORCEDREINFORCED MASONRY WALLS are units that MASONRY WALLS are units that

– Are designed with the necessary structural Are designed with the necessary structural components to resist loading that induce components to resist loading that induce tension, compressive, and shearing forces. tension, compressive, and shearing forces. Such reinforcement generally is done with steel Such reinforcement generally is done with steel rods imbedded in concrete within the masonry.rods imbedded in concrete within the masonry.

– Since a long, high wall is a relative thin Since a long, high wall is a relative thin membrane of a building envelope, the tendency membrane of a building envelope, the tendency to fail under loading is by buckling, which may to fail under loading is by buckling, which may be a combination of twisting and bending.be a combination of twisting and bending.

– Shorter units are more stiff, and tendency to fail Shorter units are more stiff, and tendency to fail under loading is by crushing and shear.under loading is by crushing and shear.

– Structural components must be added to the Structural components must be added to the assembly of such members that will resist all assembly of such members that will resist all tendencies to fail because of loads. tendencies to fail because of loads.

– Generally, the combination of masonry, mortar, Generally, the combination of masonry, mortar, and concrete fill is sufficient to resist crushing, and concrete fill is sufficient to resist crushing, or compressive forces in small buildings.or compressive forces in small buildings.

– Reinforcing steel is installed through masonry Reinforcing steel is installed through masonry cells, which are then filled with concrete to cells, which are then filled with concrete to become a resisting tensile material held in place become a resisting tensile material held in place by the concrete, to tie the individual masonry by the concrete, to tie the individual masonry units together into a structural mass.units together into a structural mass.

The The tendencytendency for a member to bend because for a member to bend because of building loads, induces all three types of stress; of building loads, induces all three types of stress; compressivecompressive, , tensile,tensile, and and shearingshearing forces. forces. Masonry may be able to resist compression and Masonry may be able to resist compression and shear if the stress is within its allowable limits, but shear if the stress is within its allowable limits, but reinforcing steel is required to resists tension reinforcing steel is required to resists tension forcesforces. .

Since brittle materials rupture at an angle Since brittle materials rupture at an angle relative to their face direction, reinforcing steel, if relative to their face direction, reinforcing steel, if required, resists shear through a concept called required, resists shear through a concept called “diagonal tension.”“diagonal tension.”

Reinforced Masonry WallsReinforced Masonry Walls

Cell Reinforcing – vertical steelBars installed @ 2’-0” oc, thenCells filled with concrete.

A SINGLE LAYER OF MASONRY WALL IS CALLED A ‘WYTHE’

STRUCTURE IN BUILDING MATERIALSSTRUCTURE IN BUILDING MATERIALS

All materials used in construction have All materials used in construction have weight, which may have to be supported by some weight, which may have to be supported by some other material.other material.

All construction materials have a quality All construction materials have a quality about them that can resist the forces imposed by about them that can resist the forces imposed by weights of other materials.weights of other materials.

A A FORCEFORCE is an amount of load caused by is an amount of load caused by weights of building materialsweights of building materials, and anticipated , and anticipated loads caused by snow, water, building contents, loads caused by snow, water, building contents, people, etc. people, etc. Forces are loads due to these weights Forces are loads due to these weights caused by GRAVITYcaused by GRAVITY..

All forces of gravity are vertically downward, All forces of gravity are vertically downward, UNLESS redirected by the resistance of some UNLESS redirected by the resistance of some member, in which case it may become horizontal, member, in which case it may become horizontal, vertical, or a combination.vertical, or a combination.

Wind and seismic loads are horizontal Wind and seismic loads are horizontal forces, but may change direction because of forces, but may change direction because of redirection. redirection.

STRESSSTRESS is a is a forceforce over a over a unit of areaunit of area. Stress . Stress may have units such as pounds per square inch, may have units such as pounds per square inch, or pounds per square foot, or in the case of metric or pounds per square foot, or in the case of metric units, grams per square centimeter.units, grams per square centimeter.

If a person stands on two feet, and weighs If a person stands on two feet, and weighs 180 pounds, and the area of each shoe sole is 40 180 pounds, and the area of each shoe sole is 40 sq.in., then a stress on the floor of 180 / 80 equals sq.in., then a stress on the floor of 180 / 80 equals 2.25 pounds per square inch. But, If the person 2.25 pounds per square inch. But, If the person stands on one foot, the stress on the floor 4.5 stands on one foot, the stress on the floor 4.5 pounds per square inch – twice as much.pounds per square inch – twice as much.

All construction materials that are placed for the All construction materials that are placed for the purpose of resisting forces, have a tested purpose of resisting forces, have a tested ALLOWABLEALLOWABLE STRESS STRESS. Or an amount of . Or an amount of FORCEFORCE they can resist over a they can resist over a unit of its areaunit of its area..

FOR INSTANCE, some common building material:FOR INSTANCE, some common building material:

material allowable stress material allowable stress in pounds per squarein pounds per square

ConcreteConcrete 3,000 3,000 BrickBrick 500 500CMUCMU 800 800WoodWood 900 900SteelSteel 24,000 24,000

The preceding allowable figures do not indicate The preceding allowable figures do not indicate WHAT KIND OF STRESSWHAT KIND OF STRESS they resist, and there are they resist, and there are three types:three types:

TENSILE STRESSTENSILE STRESS: The type that tends to stretch a : The type that tends to stretch a material, as though the forces were attempting to material, as though the forces were attempting to pull the molecules of the material apart.pull the molecules of the material apart.

COMPRESSIVE STRESSCOMPRESSIVE STRESS: The type that tends to : The type that tends to compress, or press together, or crush the compress, or press together, or crush the molecules of the material.molecules of the material.

SHEAR STRESSSHEAR STRESS: The type that tends to slide or : The type that tends to slide or twist the molecules of the material apart.twist the molecules of the material apart.

The numerical The numerical value of stressvalue of stress equals the equals the amount amount of forceof force divided by the cross section divided by the cross section areaarea of the of the member that is subject to force.member that is subject to force.

As a result, a mathematical formula can be As a result, a mathematical formula can be realized; realized;

Stress equals force divided by the area affected.Stress equals force divided by the area affected.

or,or, SS = = FF / / AA

wherewhere S = stress in pounds per square inchS = stress in pounds per square inch

F = total poundsF = total pounds

A = area in square inchesA = area in square inches

Another type of stress is created by bending, Another type of stress is created by bending, which is a which is a combinationcombination of the first three of the first three, and it is , and it is through that investigation that beam requirements through that investigation that beam requirements are analyzed and selected.are analyzed and selected.

Imagine a load bearing wall made of concrete Imagine a load bearing wall made of concrete masonry units. An opening in the wall is masonry units. An opening in the wall is necessary to have access to the inside.necessary to have access to the inside.

Since the wall supports a portion of the roof load Since the wall supports a portion of the roof load at the eave and the weight of the CMU above the at the eave and the weight of the CMU above the opening, a beam must be installed to compensate opening, a beam must be installed to compensate for the void of the opening.for the void of the opening.

Elevation of concrete block wall with opening

The beam can be isolated and examined, in order The beam can be isolated and examined, in order to determine what tendencies exist due to the to determine what tendencies exist due to the weight it must support over the opening.weight it must support over the opening.

All building materials are said to be elastic. In All building materials are said to be elastic. In other words they deform when subjected to other words they deform when subjected to loading. If the deformations are not excessive, the loading. If the deformations are not excessive, the members remain viable as structural components.members remain viable as structural components.

Materials under tension tend to stretchMaterials under tension tend to stretchMaterials under compression tend to shrinkMaterials under compression tend to shrinkMaterials under shear tend to deform or twistMaterials under shear tend to deform or twist

When a horizontal beam supported only at the When a horizontal beam supported only at the ends is loaded with a downward force, it has a ends is loaded with a downward force, it has a TENDENCYTENDENCY to bend downward. to bend downward.

As a member bends, certain things must be As a member bends, certain things must be realized; First realized; First there must be no failures due to any there must be no failures due to any kind of stresskind of stress, so the square ends of the beam , so the square ends of the beam remain square. Second, if the beam is loaded remain square. Second, if the beam is loaded consistently, the deformation, or bend will be consistently, the deformation, or bend will be downward and downward and will assume the shape of a portion will assume the shape of a portion of the circumference of a circleof the circumference of a circle..

And as the beam bends, the length of the top of And as the beam bends, the length of the top of the beam will become shorter, and the length of the beam will become shorter, and the length of the bottom of the beam will become longer.the bottom of the beam will become longer.

THE STRESS THAT CAUSES THE BEAM TO BENDI.E. THE TENSION AND THE COMPRESSION IS CALLED

THE DASHED LINE AT 0 STRESS DOES NOT CHANGE LENGTH

BENDING STRESS

d-1

supportTENSILE STRESS

d-2

0 STRESS

COMPRESSIVESTRESS

support

It must follow that, It must follow that, if compressive stress is if compressive stress is maximum at the top of the beam and tensile stress maximum at the top of the beam and tensile stress is maximum at the bottom of the beam, there is a is maximum at the bottom of the beam, there is a plane where no stress occursplane where no stress occurs – since compression – since compression is the opposite of tension. is the opposite of tension.

Realize that the definition of stress is an amount of Realize that the definition of stress is an amount of force divided by the area in which the force actsforce divided by the area in which the force acts. . Likewise, properly located, a single force can Likewise, properly located, a single force can represent the sum of the stress multiplied by area.represent the sum of the stress multiplied by area.

stresscompressive

MOMENTis called BENDING

axis; the resulting quantitydistance to the neutral

multiplied by the the SINGLE FORCE

single

forcecompressive

dis

tan

ce

tensile stress

to the top half of the beam,

be represented as a SINGLE

single

forcetensile

the NEUTRAL AXIS

plane of zero stress is called

centroid of the stress.FORCE located at the

the compressive stress can

the cross sectional areastress is multiplied byif the summation of the

TO SUMMARIZE: WHEN STRUCTURAL TO SUMMARIZE: WHEN STRUCTURAL MEMBERS ARE SUBJECTED TO IMPOSED MEMBERS ARE SUBJECTED TO IMPOSED LOADS, FORCES ARE CREATED, WHICH IN TURN LOADS, FORCES ARE CREATED, WHICH IN TURN CAN CAUSE DIRECTCAN CAUSE DIRECT

Tensile stressTensile stressCompressive stressCompressive stressShear stressShear stress

but not necessarily all at the same time.but not necessarily all at the same time.

AND WHEN BENDING OF A MEMBER OCCURS, AND WHEN BENDING OF A MEMBER OCCURS, –Bending stress happens, which Bending stress happens, which

simultaneously causessimultaneously causesTensionTensionCompressionCompressionShearShear

AND NOW EVERYONE TAKE OUT A SHEET AND NOW EVERYONE TAKE OUT A SHEET OFBLANK PAPER AND DO THIS EXERCISE.OFBLANK PAPER AND DO THIS EXERCISE.

THIS CONSTITUTES QUIZ NUMBER THREETHIS CONSTITUTES QUIZ NUMBER THREE

ONE ONE A solid wood block A solid wood block 2’ x 2’ x 2’2’ x 2’ x 2’ is used to is used to support the weight of a car axle which presses support the weight of a car axle which presses down on the ground a total of down on the ground a total of 15001500 pounds. Find pounds. Find the amount of the amount of stressstress the block makes on the the block makes on the ground – in units of pounds per square foot.ground – in units of pounds per square foot.

TWOTWO A square steel strap A square steel strap ½” x 4” in cross ½” x 4” in cross sectionsection can have a maximum can have a maximum stress of 18,000stress of 18,000 pounds per square inch in tension. Find the pounds per square inch in tension. Find the maximum tensilemaximum tensile load it can safely hold. load it can safely hold.

As soon as you finish, place your name on the As soon as you finish, place your name on the paper, then drop it in the chair at the back and you paper, then drop it in the chair at the back and you are free to go.are free to go.