lecture7 shoring scaffolding

7/29/2019 Lecture7 Shoring scaffolding

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Shoring and Scaf fo ld ing

Professor Kamran M. NematiWinter Quarter 2007

Temporary StructuresCM 420CM 420CM 420CM 420

Temporary Structures

CM 420CM 420CM 420CM 420

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In multistory work, The shoring whichsupports freshly placed concrete isnecessarily supported by lower floors which

may not yet have attained their their fullstrength,

and which may not have been designedto carry loads as great as those imposedduring construction

Construction Loads may exceed design

loads by an appreciable amount.(Ref: Grundy & Kabaila, ACI J., Dec. 63, pp. 1729-1738)

Shoring

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Shoring

Therefore shoring must be provided forenough floors to develop the neededcapacity to support the imposed loads

without excessive stress or deflection.Whether permanent shores or reshoresare used at the several required lowerfloor levels depends on job plans forreused of materials as well as the rateof strength gain in the structure.


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Professor Kamran M. NematiWinter Quarter 2007 2


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Shoring

There are several types of adjustableindividual shores.

The simplest ofthese, is based onclamping devicewhich permits theoverlapping of two2x4 members.

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Shoring

A portable jacking tool is used tomake vertical adjustments.

Metal shore jack fittings areavailable to fit over the end of4x4 or 6x6 wood shore, thustransforming the piece of lumberinto an adjustable shore.

These devices are capable ofvarying the shore height as much

as 12 in.

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Shoring

A number of patented shoringsystems have been developed withadjustable legs which eliminatecutting, close fitting, and wedging.


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Scaffold-Type Shoring

When tubular steel frame scaffoldingwas first introduced, it was designed to

support the relatively light loads involvedin getting workers to the work area.

Later contractors began to try out thescaffolding as a support for formworkbecause of the apparent advantages ofits modular assembly and system ofjacks for leveling and adjustingelevations.

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Scaffold-Type Shoring End frames assembled with diagonal

braces to form typical shoring tower.

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Scaffolding

Walk-through-typeframes used by masons

Scaffolding has beenused for 5000 years toprovide access areas forbuilding and decorating

structures taller thanpeople who work onthem.


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Scaffolding

The word scaffolding refers to anyraised platform or ramp used for

ingress and egress for pedestrianmovement and/or the passage ofbuilding materials.

Since the mid-1920s the concept ofusing steel pipes fastened togetherwith metal-form or cast clamps(couplers) instead of poles and ropeswas introduced.

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ScaffoldingAluminum alloy pipes and couplerswere developed for their lighter weightand speedier construction.

Aluminum alloy is only two-thirds asstrong as steel, but it is only one-thirdto one-half its weight.

Because of the higher initial cost,aluminum is restricted mostly to

building maintenance scaffolds andsuspended platforms.

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ScaffoldingGeneral Design Considerations

Commonly, all types of scaffold haveincorporated in their design a minimum safetyfactor of 4.

This means that scaffolds and their componentsshall be capable of supporting without failure atleast 4 times the maximum intended load.

To comply with this requirement, multiply thedesign load by 4 and derive the limiting strengthof the component from the yield stress of themetal in accordance with acceptable engineeringcriteria and practices.


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Scaffolding - Design Loads In accordance with OSHA and ANSI criteria and

common practice for many years, design loadratings for scaffold platforms are as follows:

Light-Duty Loading. 25 lb/ft2 maximum workingload for support of people and tools (no equipmentor material storage on the platform).

Medium-Duty Loading. 50 lb/ft2 maximumworking load for people and material restricted not toexceed this rating, often described as applying tobricklayers and plasters work.

Heavy-Duty Loading. 75 lb/ft2 maximum workingload for people and stored material often describedas applying to stone masonry work.

OSHA: Occupational Safety & Health Administration

ANSI: American National Standards Institute

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These ratings assume uniform loaddistribution.

With the exception of the weight of storedmaterials, scaffold loads most often consist ofpersonnel, both stationary and transitory.

It is important to remember that the OSHAand ANSI load-rating system is intended forguidance of field personnel in theconstruction and use of nonspecificallyengineered scaffolding applications.

ScaffoldingDesign Loads

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Tube and Coupler Scaff olds Tube and coupler scaffolds are assembled

from three basic structural elements:

the uprights, or posts, which rise from groundor other solid support

the bearer, which supports the workplatforms and / or provide transversehorizontal connections between the posts;

the runners, which attach to the postsdirectly below the bearers and providelongitudinal connections along the length ofthe scaffold.


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Tube and Coupler Scaff olds

These three elements are usually connected withstandard or fixed couplers which provide a 90connection in two places.

The threeelements(uprights,bearer, andrunners) formthe basicstructure

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Tube and Coupler Scaff olds The basic assembly and components of tube and

coupler scaffolds.

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Diagonal bracing is used to stiffen thestructure as necessary most important inthe longitudinal direction.

Bracing is generally connected to the posts

with adjustable or swivel couplers whichhave the facility of adjusting a full 360.

Diagonal bracing should always be attachedto the posts as closely as practical to the

node points formed by the runner-bearerconnections.


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Another important structural element isthe building tie which connects the

scaffold to the wall or structure and isneeded to provide rigidity and anchorageof the scaffold in the transverse direction.

Scaffolds need to be laterally supported;otherwise, they are unstable because oftheir height-to-width ratio and have lowstrength to resist wind and other lateralforces.

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Tube and Coupler Scaff olds Methods of stabilizing against a building

(a) Wall tie and

anchorage

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Methods of stabilizing against a building (cont d):

(b) Window reveal tube


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Methods of stabilizing against a building (cont d):

(c) Reveal between

pilasters

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Tube and Coupler Scaff oldsApplication

Tube and coupler scaffolds can be assembled innumerous ways because of the flexibility of theirassembly dimensions in the horizontal and verticalplanes.

Unlike sectional frame scaffolds they are notrestricted by frame width in the transverse direction,by brace length in the longitudinal direction, or byframe height in the vertical direction.

Consequently, they are preferred for access toworkplaces having irregular dimensions and

contours, e.g., churches, old auditoriums, etc.

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Tube and Coupler Scaff oldsBasic Config urat ion

The basic configurations are as follows:

1. Double Pole. Also called independent wall scaffolds,

these are used for access to vertical surfaces forconstruction, alterations, or surface finishing and repair.

They consist of repetitive pairsof posts along the length,connected by bearers and runners.

2. Single pol e. Also called putlog wall scaffolds, theseare used for the construction of masonry walls. Theyconsist ofsingleposts 3 to 5 feet away from the wallsurface spaced at regular or varying intervals along thewall. The different feature of this type of scaffold is thatthe inside ends of the bearer are supported at joints orcourses in the wall being built instead of the inside posts.


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3. Tower Scaff olds. Theseconsist of one or few bays in either

horizontal plane, constructed torequired height for access toceilings or for specialized loadsupport requirements notconveniently achievable withsectional frames. They may bemounted on casters and becomemobile scaffolds or rolling towers.

An application of tower scaffolds isto provide stair access to to unusualstructures such as cooling towers. Rol l ing scaf fo ld

w i th s tand -o f f

ladder

Tube and Coupler Scaffold s

Basic Config urat ion

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Sect ional Scaffolding The construction principle of sectional

scaffolding is shown below.

Basic assemblyof sectional

scaffolding

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Sect ional Scaffolding The most common material used in the fabrication

of steel frames is 1 5/8-in. OD tubing with a wallthickness between 0.086 and 0.105 in.

The most common grade of steel used for thispurpose is AISI designation A1050, a high-carbon

alloy having a minimum yield stress of 50,000 psiwith a corresponding ultimate stress of over75,000 psi.

The higher carbon steel is generally preferredbecause its lower ductility and greater rigiditymake it more resistant to damaging and bendingof the members and because it has greaterstrength.

lecture7 shoring scaffolding

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