Structural collapse basic shoring principles

Structural CollapseBasic Shoring Principles

FEMA/Corps of Engineers Overview

FEBRUARY 2006

BUILDING MATERIAL & STRUCTURAL SYSTEMSObjectives:

•Discuss the FEMA Response Team

•Review the basics of how various building materials resist forces, the importance of Ductile vs. Brittle behavior, the concepts of Vertical and Lateral Load Resisting Systems, and Structural Redundancy.

•Discuss the Urban Search & Rescue Structure Marking System.

•Take a look at shoring principles and techniques knowing the how the forces and materials will perform in a collapse situation.

FEMA RESPONSE TEAM

• DISASTER ORGANIZATION• URBAN SEARCH & RESCUE

RESPONSE TEAM• LOCATION OF US&R TEAMS

TYPES OF FORCESIndividual LOADS, usually referred to as FORCES can be

divided into four types: Tension. Compression, Bending, and Shear.

When a FORCE is applied to an individual member, itproduces STRESSES, which are defined as the FORCEdivided by the cross-sectional area on which it acts.

Example: If a 1000lb FORCE acting in Tension on a 2 inch x 2 inch steel bar, will produces a 250 lbs per square inch (psi)

TENSION FORCESTENSION FORCES stretch members of steel or wood. Concreteand masonry have no reliable tension strength.

When a moderate tension force is applied a steel bar willlengthen, and when the force is removed, the bar will return toits original length. This is called ELASTIC Behavior and canbe repeated many times in competent steel or woodmembers.

If a much larger force is applied to the steel bar it will start tolengthen more rapidly. When this rapid lengthening begins,one can observe that the cross-section of the bar will start toget smaller (neck down). When the force is removed, the barwill not return to it’s original length, since it has experiencedpermanent yielding (DUCTILE Behavior)

The DUCTILE behavior of steel in tension provides the specialproperty of forgiveness (warning of failure) and responsewhich makes it especially desirable in resisting dynamicloading.

TENSION FORCESTENSION FORCES (continued)

· Ductile behavior is the ability of a material to stretchand/or bend without suddenly breaking, and after the loadis removed it can remain stretched/bent and then be reloaded.· EXAMPLE: one can bend a hook on a rebar, and evenunbend it without breaking· Brittle behavior means that the material will break without warning (Catastrophic Failure)

COMPRESSION FORCESCOMPRESSION FORCES push on members and can lead to crushing of materials when the members are short and relatively fat. ( small length to width ratios, L/D)

At bearing surfaces between wood or concrete beams and columns, crushing can also occur.

The crushing failures tend to give warning, such as local splitting of concrete and noisy, slow, compression of wood fibers

When long, slender members are loaded in compression, they can fail suddenly by BUCKLING (bowing)

This type of sudden failure wants to be avoided

BENDING FORCESBENDING FORCES occur mostly as a result of Vertical Loads from gravity that are applied to floor slabs and beams. They also occur in sloped roof rafters and sloped slabs in rubble piles.

Bending causes the bottoms of simple beams to become stretched in TENSION and the tops of beams to be pushed together in COMPRESSION.

Continuous beams and cantilever beams have tension forces at the top + compression at the bottom near their supports.

In mid span the forces are in the same locations as for simple beams and slabs.

BENDING FORCES CON’T

•Vertical cracks develop near the mid-span of concrete, sincethe Tension Force causes the concrete to crack in order forthe Reinforcing Steel (Rebar) to resist the Tension Force.

•This cracking can be observed in damaged structures tomonitor and determine the potential for collapse.Stable, hairline cracks are normal, but widening cracksindicate impending failure

•Structural steel and reinforced concrete, moment resistantframes experience tension and compression stresses onopposite faces (similar to continuous beams). These stressescan reverse during earthquakes and high winds.

SHEAR FORCESSHEAR FORCES occur in all beams, and are greatest adjacentto supports. Shear stress can be described as the tendency to tear thebeams surfaces apart.Example: Consider a beam that is made from a group ofindividual books as they sit on a bookcase, with a long threadedrod extending all the way through them, tightened with nuts ateach end. If this beam is placed so that it spans between 2 tablesand one attempts to push one of the books down to the floorbelow, a SHEAR FORCE will be exerted on the surface of thebooks immediately adjacent to the one that is being pushed out

In concrete beams these shear stresses develop diagonaltension cracks, since concrete is very weak in tension.This cracking can also be monitored in a damagedstructure.

Wood beams are strong in tension and compression, but areparticularly weak in shear along the horizontal plane of thesofter spring wood.

SHEAR FORCESPUNCHING SHEAR occurs where a two-way concrete flat slabis connected to a column and it is the tendency of the slab to dropas a unit around the column. The column appears to punch through the slab. The cracking that indicates the over-stress leading to thistype of collapse is most visible on the top surface of theslab, which is often covered by debris during US&Ractivities.BOLT SHEAR is the tendency of steel pin-like connector (bolt,nail, and screw) to break across its cross section.Example: A roll of coins is Sheared off as each coin slips pastthe other. This type of failure can be sudden.Nail failures in wood structures, which involve some degree of pullout, can occur with enough deformation to give warning.

VERICAL LOADINGVERTICAL LOAD SYSTEMSStructural members in these systems can be divided into twotypes, those that form horizontal (or sloped roof) planes andthose that provide the vertical support for these planes.

Vertical Load Systems• Concept of gravity load path• Loads must be transferred from source to the ground• Top down approach – Plumbing system analogy• Framed and Un-Framed• Connections are particularly vulnerable

TRUSSESTRUSSES are special vertical load resistant members that usegreater depth for structural efficiency, but require more positivelateral bracing of compression members.

Trusses are usually made from wood and/or steel, althoughconcrete has been used for economy in some areas of the world.

Individual members are stressed in either tension orcompression, although stress may reverse in some membersdue to changes in live load (people, vehicles, and rain/snow).

Compression members are normally governed by bucklingand tension members are normally governed by theirconnections.

VERTICAL LOADINGHORIZONTAL MEMBERS support floor and roof planes and arenormally loaded in bending such as:

•Wood - rafters, joists, beams, girders.•Steel - corrugated sheets (filled with concrete), joist, purlins,

beams, girders.•Reinforced concrete floor systems may be of many types. All

have some relationship to the economy of providing adequatestructural depth with available forming materials.

•Pre-cast concrete floors may contain, planks, cored slabs,single or double tees, beams and girders. Most modernsystems in California combine a cast-in-place overlay slab toprovide adequate interconnection of individual members andoverall planar stability.

•These individual members need to be interconnected to theirsupported planes in order to provide the lateral stability toresist the extreme fiber compression forces associated withbending, which occur on the top or bottom of the members.

TYPICAL FLOOR SYSTEMS

VERTICAL LOADINGVERTICAL SUPPORT MEMBERS are normally configured asbearing walls or columns.

In wood and light framed steel systems the bearing walls are made using closely spaced columns (studs at 16-24" o.c.) that must be interconnected by a skin in order to provide the lateral stability that will allow the individual members to be loaded in compression without buckling.

Concrete and masonry bearing walls are proportioned to carry heavy vertical loads depending on their height to thickness ratio.

Individual column (posts) normally carry large compression forces and may be made of wood, steel, or reinforced concrete. In all cases the load capacity is based on the members slenderness ratio (l/r, l/d) as well as the adequacy of the connection between the column and the horizontal system.

All vertical load systems need some system to provide forlateral stability (i.e., the proper alignment of vertical load path).

STRUCTURAL REDUNDANCYREDUNDANCYEspecially in Seismic Zones, it is important for the LateralLoad System to possess some degree of Redundancy.

Redundancy in a structure means that there is more than onepath of resistance for Lateral Forces------Multi Elements

•Box Buildings•Can be achieved by having a Moment Resistant Frame

with many columns and beams, all with ductile connections, •This can be achieved by having a Dual System, like

Shear walls plus a Moment Resistant Frameaka-Collapse Preventor’s

BUILDING MATERIALS• WOOD• STEEL• CONCRETE

– REINFORCED– UNREINFORCED

• MASONARY– UNREINFORCED – URM– REINFORCED

WOOD PROPERTIES•Is tough, light fibrous, fire supporting, •Has defects like knots, splits and non-straight grain that cause

stress concentration.•The growth pattern of fast growing spring wood vs. slower

growing summer wood leads to structural problems. •Connections are best made by bearing one member on it’s

supporting member, however, metal connection devices canbe successfully used.

•Nailed connections perform well as long as splitting isavoided, and bolting may be successful if adequatespacing and edge distances are provided.

Properly proportioned wood structures can exhibit Ductility-When wood posts are kept short and bear on the cross

grain surfaces of beams or sole plates, slow crushing ofthe cross grain can be observed to warn of failure.

•Box Cribbing will exhibit this same failure mode since allthe load is transferred in cross grain bearing.

WOOD PROPERTIES

• Douglas Fir or Southern Pine are the most common types of structural timber used in the U.S.

• Average values for these species are– Compression parallel to grain = 1100 PSI

– Compression perpendicular to grain = 600 PSI

• The capacity of header beam and sole plate is determined by bending an/or horizontal shear strength.

• Average values for Douglas Fir and Southern Pine– Fb = extreme fiber bending stress = 1500 PSI– Fh = horizontal shear stress = 90 PSI

WOOD CAPACITY

STEEL PROPERTIES•Is tough, light, strong, ductile, and formable into any shape,

but needs to be fireproofed.•It starts to lose strength above 700° Fahrenheit.•It has magical property of ductility. That is, it can be

stressed beyond it’s Elastic Limit and severely bent, but stillhave enough strength to resist failure.

•Ideal structural material, in that it gives warning of collapse (has forgiveness).•Steel is strong in Tension, Compression, and Shear•Steel beams must be laterally braced so as not to buckle

about their weak axis, especially if the ductile performancerequired for earthquake resistance is expected.

•Steel-framed structures must be properly proportioned inorder to avoid the over loading of columns.

•Steel can be very efficiently connected by bolting or welding(older structures used rivets instead of bolts).

•Welded joints must be properly designed and constructedor they can lead to a brittle failure.

CONCRETE PROPERTIES•Is essentially cast rock, sand & cement •Strong in COMPRESSION, weak in TENSION and SHEAR•Steel bars are cast into concrete to provide for the longitudinal

tension force and enclosing type steel ties and stirrups areadded for confinement and shear resistance.

•Sufficient steel can be added to provide adequatetoughness for seismic resistance, enabling reinforcedconcrete to exhibit ductile properties similar to structuralsteel.

Concrete shrinks, cracks, and creeps under normalcircumstances, and this normal behavior needs to bedifferentiated from the cracking and spalling that indicatesfailure.

•Concrete Construction•Cast in Place•Pre-cast

•Pre-Tensioned•Post Tensioned

CCONCRETE PROPERTIES

MASONARY - UNREIFORCED•Very brittle material•Walls were constructed with a thickness made from three or

more bricks being laid long ways, side by side, for five or sixlayers high (courses) and then a layer was placed with thebricks at 90 degrees (header course), and so on.

•The strength and seismic performance of un-reinforced masonry is highly dependent on the mortar strength.

•The shear strength of mortar can vary from 15 PSI to over 150 PSI, and is determined by the proportion of lime to Portland cement and the workmanship.

•Lime produces a nice buttery mortar, but if too much is used a low strength will result.

•Lime can also be leached out of the mortar by water overtime.

•Used as decorative veneers

MASONARY - REINFORCED

•Is made from clay brick or hollow concrete blocks formed into walls using mortar joints and concrete grout filling of interior cavities.

•Masonry properties are similar to concrete - reinforcing steel bars are normally added to provide tension and shear resistance.

•Highly dependent on the workmanship to provide adequate mortar and grout strength•Can exhibit very good ductility when properly designed

and constructed.

MASONRY - REINFORCEDSolid Brick Unit Masonry

- Two single brick thick outer layers (wythes) are laid up, then rebar and grout are placed between the layers.

- The wythes are connected together with large wire to prevent blow-out when the grout is poured.

- Small heavy wire ladder type reinforcing is used at the joints in some cases.

Concrete Hollow Unit Masonry (CMU)- Each block comes with preformed cavities.- As the units are laid up, horizontal reinforcing (small rebar or

large wire) is placed in the joints.- After the wall reaches a predetermined height, vertical rebar is

placed in specified cells and grout is poured to bond the reinforcing steel to the concrete units.

WEIGHTS OF COMMON BUILDING MATERIALS.•Wood = 35 PCF•Steel = 490 PCF•Concrete = 150 PCF PCF = lbs per cubic ft•Masonry = 125 PCF PSF = lbs per square ft•Concrete/Masonry Rubble=10PSF PER INCH (of thickness)

WEIGHTS OF COMMON BUILDING CONSTRUCTION•Concrete floors weigh from 90 to 150 PSF•Steel beam w/ concrete-filled metal deck = 50-70PSF•Wood floors weigh from 10 to 25 PSF (floors w/ thin concrete fill

are 25 PSF or more)•Add 10 to 15 PSF for wood or metal stud interior walls, each

floor level•Add 10 PSF or more for furniture/contents each floor (more for

storage, etc.)•Add 10 to 20 PSF for Rescuers

· 10 PSF on large slab that spreads out load· 20PSF on wood floors to allow for concentrations

COLLAPSE PATTERNS

• LEAN-TO

• V-SHAPE

• PANCAKE

• CANTILEVER

LEAN-TO COLLAPSE

V-SHAPE COLLAPSE

PANCAKE COLLAPSE

CANTILEVER COLLAPSE

MARKING SYSTEMS

• QUADRANTS WITHIN A BUILDING

• BUILDING MARKING SYSTEM

• ASSEMENT MARKING

SHORING BASICS

SHORING PLACEMENTTwo Main Objectives•Maintain the integrity of all structurally unstable elements•Properly transmit or redirect the collapse loads to stable

ground or other suitable structural elements capable ofhandling the additional loads.

BASIC DEFINITION AND PRINCIPALSShoring is normally the temporary support of structures duringconstruction, demolition, reconstruction, etc. in order to provide thestability that will protect property as well as workers and the public.

SHORING BASIC CON’TA Shoring system is like double funnel. It needs to collect theload with headers/sheathing, deliver it into the post/struts, and thento distribute it safely into the supporting structure below. A heavilyloaded wood post can punch thru a concrete slab etc.

Shoring should be built as a system that has the following:

•Header beam, wall plate, other element collects load•Post or other load carrying element that has adjust ability and

positive end connections•Sole plate, bearing plate, or other element to spread the load

into the ground or other structure below.•Lateral bracing to prevent system from racking (becoming

parallelogram), and prevent system from buckling (moving sideways).•Built-in forgiveness (will give warning before failure)

SHORING PRINCIPLE

TYPES OF SHORING • T-SPOT

• DOUBLE T-POST

• SLOPED FLOOR

• SLOPED FLOOR CRIBBING

• FLYING RAKERS

• FULL TRIANGLE RAKERS

• FULL HEIGHT RAKERS – SOLID SOLE

• LACED POST

THE SHORING TEAM

• OFFICER

• MEASURE PERSON

• SHORES #1

• SHORES #2

• SAFETY

• RUNNER

THE CUTTING TEAM

• CUTTING TEAM OFFICER

• LAYOUT

• FEEDER

• CUTTER

• TOOLS & EQUIPMENT

SHORING EXAMPLES

• “T” SPOT

• FULL TRIANGLE RAKERS

• SLOPED FLOOR CRIBBING

T-SPOT SHOREThe main purpose of the “T” shore is to initially stabilizedamaged floors, ceilings or roofs, so that the more substantialshoring can be constructed at less risk.

The T Shore is basically unstable.•That is if the supported load is not centered directly over

the Shore, it will tend to tip over• The header beam is deliberately kept short so as to

minimize to effect of tipping.

The size of lumber most commonly used in the T shore is 4 X 4 Douglas fir. The estimated weight of the floor and its contents will help to determine the number of shores that will be required.

RAKER SHORE SYSTEMTHE RAKER SHOREThe main purpose of the raker shore is to support leaning orunstable walls and columns by transferring additional weightdown the raker, to the ground or other structural supportingmembers, and away from the wall or column.

RAKER SHORE BASICS

RAKER SHORE COMPONENTS

RAKER SHORE MEASUREMENT

The length of a 45-degree angle raker shore: •Height of the raker shore support point in feet multiplied by 17 will give the length of the raker, tip to tip, in inches.

(8 ft x 17 = 136” or 11’- 4”).The length of a 60-degree angle raker shore:

•Height of the raker shore support point in feet multiplied by 14 will give the length of the raker, tip to tip, in inches.

(8 ft x 14 = 112” or 9’- 4”).

NOTE: WHEN CUTTING RAKERS YOU NEED TO HAVE THE ANGLE ON BOTH ENDS

RAKER SHORE DIAGRAM

CRIBBING•Multi member lay-up of 4x4 to 8x8 lumber in two or three

member per layer configuration.•Capacity is determined by perpendicular to grain load on sum of

all bearing surfaces.•Stability is dependent on height to width of crib and should not

exceed 3 to 1.•Need to overlap corners a minimum of 4” to guard against splitting off corners of individual pieces that can negatively impact overall stability.•Cribs used by contractors (or in short-term emergencies) often rely only

on the friction between bearings for lateral strength, not sufficient for aftershocks or lateral movement.•Individual pieces may be notched like Lincoln logs, to provide lateral resistance in addition to the friction between pieces.

CRIBBING CAPACITY

CRIBBING – SLOPED SURFACES

•For conditions where the shore height is less than 3 feet.•Slope for crib supported floor should not exceed 15%.

- Cribs can be built into the slope, but care must be taken toproperly shim the layers in order to maintain firm, complete bearings.

- Notched crib members could be used since they cantransfer more lateral load than the usual frictioninterconnection.

SLOPED SURFACESSHORING

REMEMBER – Sloped Floor/Surfaces are complicated, not simple. No one or two solutions/shoring systems will work for all conditions.

•One needs to carefully assess the situation, to determinewhich way the floor will move.

•To shore sloped wood floors the header needs to be placedperpendicular to the joist.

QUESTIONS