MODULE 1c STRUCTURAL ENGINEERING SYSTEMS …

1 FEMA NATIONAL US&R RESPONSE SYSTEMSTRUCTURAL COLLAPSE TECHNICIAN TRAINING MANUAL 02-00 MODULE 1c STRUCTURAL ENGINEERING SYSTEMSPART 1. BUILDING MATERIALS & STRUCTURAL SYSTEMS SM 1c, 1&2 1In this introductory section our Objectives are as listed in theadjacent slides. We will quickly review the basics of how variousbuilding materials resist forces, the importance of Ductile behavior, the concepts of Vertical and Lateral LoadResisting SYSTEMS , and STRUCTURAL TYPESI ndividual LOADS, usually referred to as FORCES can be dividedinto four types: Tension. Compression, Bending, and When a FORCE is applied to an individual member, itproduces STRESSES, which are defined as the FORCE divided by the cross-sectional area on which it Example: If a 1000lb FORCE acting in Tension on a 2 inch x2 inch steel bar, will produces a 250 lbs per square inch (psi)Tension For simplicity we will we will discuss the effects of FORCES,and assume that the student understands the relationshipbetween FORCES and STRESSESTENSION FORCES stretch members of steel or wood.

2 Concreteand masonry have no reliable tension 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 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)n The DUCTILE behavior of steel in tension provides the specialproperty of forgiveness (warning of failure)

3 And responsewhich makes it especially desirable in resisting OBJECTIVESTERMINAL OBJECTIVES The Student shall understand the essentialThe Student shall understand the essentialmaterials and components of structures, andmaterials and components of structures, andhow they behave when subjected to normalhow they behave when subjected to normaland extreme loadingand extreme loadingENABLING OBJECTIVESENABLING OBJECTIVES Understand the Tension, Compression,Understand the Tension, Compression,Bending, and Shear Forces that areBending, and Shear Forces that areexerted on building materials, and howexerted on building materials, and howthey behave. Understand the concepts of Ductile andUnderstand the concepts of Ductile andBrittle behavior. Introduce the concept of Vertical LoadIntroduce the concept of Vertical LoadPath and Vertical Load Resisting SystemsPath and Vertical Load Resisting SystemsENABLING OBJECTIVESENABLING OBJECTIVES Discuss Lateral Load Resisting SYSTEMS ,Discuss Lateral Load Resisting SYSTEMS ,including Box, Moment Frame, andincluding Box, Moment Frame, andDiagonally Braced Frame SystemsDiagonally Braced Frame SYSTEMS Define and discuss STRUCTURAL RedundancyDefine and discuss STRUCTURAL RedundancyFEMA NATIONAL US&R RESPONSE SYSTEMSTRUCTURAL COLLAPSE TECHNICIAN TRAINING MANUAL 02-00 MODULE 1c STRUCTURAL ENGINEERING SYSTEMSPART 1.

4 BUILDING MATERIALS & STRUCTURAL SYSTEMS SM 1c, 1&2 2 TENSION 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 re-loaded. EXAMPLE: one can bend a hook on a rebar, and evenunbend it without breaking Brittle behavior means that the material will break withoutwarning (Catastrophic Failure)Force Type: TensionForce Type: TensionForce Type: TensionCOMPRESSION FORCESn These forces push on members and can lead to crushing ofmaterials when the members are short and relatively fat.( small length to width ratios, L/D)n At bearing surfaces between wood or concrete beams andcolumns, crushing can also occur.

5 The crushing failures tend to give warning, such as localsplitting of concrete and noisy, slow, compression of woodfibersn When long, slender members are loaded in compression, theycan fail suddenly by BUCKLING (bowing) This type of sudden failure wants to be avoidedBENDING FORCESn These occur mostly as a result of Vertical Loads from gravityare applied to floor slabs and beams. They also occur insloped roof rafters and sloped slabs in rubble Bending causes the bottoms of simple beams to becomestretched in TENSION and the tops of beams to be pushedtogether in Continuous beams and cantilever beams have tension forcesat the top + compression at the bottom near their supports. Inmid span the forces are in the same locations as for simplebeams and Type: CompressionForce Type: CompressionForce Type: CompressionForce Type: BendingForce Type: BendingForce Type: BendingFEMA NATIONAL US&R RESPONSE SYSTEMSTRUCTURAL COLLAPSE TECHNICIAN TRAINING MANUAL 02-00 MODULE 1c STRUCTURAL ENGINEERING SYSTEMSPART 1.

6 BUILDING MATERIALS & STRUCTURAL SYSTEMS SM 1c, 1&2 3 BENDING FORCES (continued)n Vertical cracks develop near the midspan 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 failuren 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 Shear Forces are also produce in beams and slabs, and theywill be discussed FORCES occur in all beams, and are greatest adjacentto supportsn Shear stress can be described as the tendency to tear thebeams surfaces : 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.

7 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 outn In concrete beams these shear stresses develop diagonaltension cracks, since concrete is very weak in tension. This cracking can also be monitored in a damagedstructuren Wood beams are strong in tension and compression, but areparticularly weak in shear along the horizontal plane of thesofter spring Type: BendingForce Type: BendingForce Type: BendingForce Type: ShearForce Type: ShearForce Type: ShearFEMA NATIONAL US&R RESPONSE SYSTEMSTRUCTURAL COLLAPSE TECHNICIAN TRAINING MANUAL 02-00 MODULE 1c STRUCTURAL ENGINEERING SYSTEMSPART 1.

8 BUILDING MATERIALS & STRUCTURAL SYSTEMS SM 1c, 1&2 4 PUNCHING 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 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. This can only add to the difficulty of discovering thiscommon hazard under the suspected overload conditionswhere it is most likely to be a SHEAR is the tendency of steel pin-like connector (bolt,nail, and screw) to break across its cross.

9 A roll of coins is Sheared off as each coin slips pastthe othern This type of failure can be Nail failures in wood structures, which involve some degree ofpullout, can occur with enough deformation to give WALL SHEAR AND OVERTURN FORCES n Lateral forces (forces applied horizontally to a structure)derived from winds or earthquakes cause shear and bendingforces in The Shear Forces tend to tear the wall surface, just as if youhad a piece of paper attached to a frame and changed theframe s shape from a Rectangle to a Parallelogram. The changing of shape from a Rectangle to aParallelogram is called RACKING. When Shear Walls are pushed out of plumb in their planethey are said to have been RACKEDn At the ends of Shear Walls there is a tendency for the wall tobe Lifted Up at the end where the Lateral Force is applied,and a tendency for the Wall to be Pushed Down at the endaway from the force.

10 This action is called OVERTURNINGF orce Type: ShearForce Type: ShearForce Type: ShearFEMA NATIONAL US&R RESPONSE SYSTEMSTRUCTURAL COLLAPSE TECHNICIAN TRAINING MANUAL 02-00 MODULE 1c STRUCTURAL ENGINEERING SYSTEMSPART 1. BUILDING MATERIALS & STRUCTURAL SYSTEMS SM 1c, 1&2 5 MATERIAL PROPERTIESWOODn Is tough, light fibrous, fire supporting, cut from living trees andgraded by Has defects like knots, splits and non-straight grain that causestress The growth pattern of fast growing spring wood vs. slowergrowing summer wood leads to STRUCTURAL problems. Theseproblems include: Weakness in cross grain tension and compression. Weakness in parallel to grain, shear strength Shrinkage and Splittingn Live wood may be as much as one half water, while older,seasoned wood (as found in a structure) may contain as littleas 10% water Its volume can change as much as 10% over this Shrinkage (usually in width/depth, not length) causes specialproblems in bolted connections.