Topic 13 - Seismic Design Of Wood Structures

3y ago
29 Views
2 Downloads
1.83 MB
62 Pages
Last View : 2m ago
Last Download : 2m ago
Upload by : Aiyana Dorn
Transcription

WOOD STRUCTURESInstructional Material Complementing FEMA 451, Design ExamplesTimber Structures 13 - 1Interior of the Old Faithful Inn, Yellowstone National Park, taken by author S.Pryor. Note heavy post and beam construction. Will discuss again later.Note that this topic, while complete, does not specifically utilize the examplesin Chapter 10 of FEMA 451, NEHRP Recommended Provisions: DesignExamples. The instructor/student should carefully review Chapter 10 ofFEMA 451 for additional information on the seismic resistant design of woodstructures.FEMA 451B Topic 13 NotesWood Structures 13 - 1

Objectives of TopicUnderstanding of: Basic wood behavior Typical framing methods Main types of lateral force resisting systems Expected response under lateral loads Sources of strength, ductility and energy dissipation Basic shear wall construction methods Shear wall component behavior Analysis methods Code requirementsInstructional Material Complementing FEMA 451, Design ExamplesTimber Structures 13 - 2Self explanatory.FEMA 451B Topic 13 NotesWood Structures 13 - 2

Basic Wood Material PropertiesRadialLongitudinalWood is orthotropicTangential Varies with moisturecontent Main strength axis islongitudinal - parallel tograin Unique, independent,mechanical properties in 3different directions Radial and tangential are"perpendicular" to the grain– substantially weakerInstructional Material Complementing FEMA 451, Design ExamplesTimber Structures 13 - 3Wood is a very complex organic building material. Nevertheless, it has beenused successfully throughout the history of mankind for everything fromstructures to ships to planes to weaponry.Mention that naturally occurring “strength reducing characteristics” such asknots, shakes, and splits will contribute the actual strength of lumber.FEMA 451B Topic 13 NotesWood Structures 13 - 3

Basic Wood Material Properties“Timber is as different from wood asconcrete is from cement.”– Madsen, Structural Behaviour of TimberConcept of “wood” as “clear wood”: design properties used to bederived from clear wood with adjustments for a range of "strengthreducing characteristics." Concept of “timber” as the useful engineering and constructionmaterial: “In-grade” testing (used now) determines engineeringproperties for a specific grade of timber based on full-scale tests oftimber, a mixture of clear wood and strength reducing characteristics.Instructional Material Complementing FEMA 451, Design ExamplesTimber Structures 13 - 4Borg Madsen’s distinction is a good one. The understanding of how timberbehaves must address the natural occurrence of strength reducingcharacteristics.FEMA 451B Topic 13 NotesWood Structures 13 - 4

Basic Wood Material PropertiesLongitudinalSample DFL longitudinal design properties: Modulus of elasticity: 1,800,000 psi Tension (parallel to grain): 1,575 psi Bending: 2,100 psi Compression (parallel to grain): 1875 psiInstructional Material Complementing FEMA 451, Design ExamplesTimber Structures 13 - 5DFL: Douglas Fir-LarchThis slide and the next are intended to provide a feel for general level ofdesign allowable stresses (ASD) unless where noted otherwise. LRFD couldbe used, but ASD is still predominant in the design community.Discuss how bending tension because for tension entire cross section isstressed, which means tension strength reducing characteristics will befound/encountered, whereas for bending, max stresses are at the outeredges of the board and grading rules take into account the size and locationof strength reducing characteristics and how they would affect bending.FEMA 451B Topic 13 NotesWood Structures 13 - 5

Basic Wood Material PropertiesSample DFL perpendicular to grain designproperties: Modulus of elasticity: 45,000 psi (2.5 5 % of Ell!) Tension (perpendicular to grain): 180 to 350 psiFAILURE stresses. Timber is extremely weak forthis stress condition. It should be avoided if at allpossible, and mechanically reinforced if notavoidable. Compression (perpendicular to grain): 625 psi.Note that this is derived from a serviceability limitstate of 0.04” permanent deformation under stressin contact situations. This is the most "ductile"basic wood property.Instructional Material Complementing FEMA 451, Design ExamplesRadialTangentialTimber Structures 13 - 6Intended to provide a feel for general level of design allowable stresses andto emphasize the weakness of wood stressed perpendicular to grain. Incommercial lumber, tension perpendicular is very low and designs must notrely on this type of action.Note how miners have long taken advantage of the ductile nature ofcompression perpendicular to grain in shoring up mine shafts.FEMA 451B Topic 13 NotesWood Structures 13 - 6

Basic Wood Material PropertiesRadialTangentialShrinkage Wood will shrink with changesin moisture content. This is most pronounced in theradial and tangential directions(perpendicular to grain). May need to be addressed inthe LFRS.(Wood Handbook, p. 58)Instructional Material Complementing FEMA 451, Design ExamplesTimber Structures 13 - 7For most designs shrinkage in the longitudinal direction can be ignored.However, that may not be the case for perpendicular to grain shrinkage.Accumulated effects in the boundary chords of shear walls can degrade theperformance of the shear wall system and may need to be addressed withshrinkage compensating devices. While tangential 2x radial, for designpurposes. this is ignored as one won’t know that the orientation will be inservice.Figure is 3-3 from the Wood Handbook.FEMA 451B Topic 13 NotesWood Structures 13 - 7

Wood Structure Construction Methods: GravityPlatformBalloon Walls are interrupted byfloor "platforms."Floors support walls.Most common type oflight-frame constructiontoday. Economical but createsdiscontinuity in the loadpath. Metal connectorsessential for completeload path. Instructional Material Complementing FEMA 451, Design Examples Walls featurefoundation to roofframing members. Floors supported byledgers on walls orlapped with studs. Not very commontoday.Timber Structures 13 - 8Self explanatory. Note the accumulation potential of shrinkageperpendicular to grain in each floor over the height of the structure.FEMA 451B Topic 13 NotesWood Structures 13 - 8

Wood Structure Construction Methods: GravityPost and Beam Space frame for gravityloads.Moment continuity atjoint typically only ifmember is continuousthrough joint.Lateral resistancethrough verticaldiaphragms or bracedframes.Knee braces as seen herefor lateral have no codedesign procedure forseismic.Six story main lobby Old Faithful Inn, Yellowstone, undergoing renovation work in2005. Built in winter of 1903-1904, it withstood a major 7.5 earthquake in 1959.Instructional Material Complementing FEMA 451, Design ExamplesTimber Structures 13 - 9The Old Faithful Inn wasn’t “designed” for seismic but the designers andbuilders provided a structure that suffered only minor damage in the 1959earthquake. Lateral resistance of this structure is a combination of woodmoment frame action due to the knee braces at the post/beam connection(note eccentricity in the braces under axial forces due to architecturalcurvature of the braces, in every brace) and diaphragm action in theroof/walls. Some beam/column connections in the very top of the lobby,which supported a “crows nest” platform where a small orchestra would playand entertain guests, were damaged and so that practice was stopped.Here it is being repaired and strengthened (summer 2005).FEMA 451B Topic 13 NotesWood Structures 13 - 9

Wood Structure Construction Methods: GravityPost and BeamConstructionGravity frameRoof purlinsRoof sheathingFloor sheathingLateral systemFloor joistsInstructional Material Complementing FEMA 451, Design ExamplesTimber Structures 13 - 10For the most part. this slide is self explanatory. Emphasize that the lateralsystem typically will not support gravity load, and while braced frame actionis shown here, it could also be vertical diaphragms (stud walls with nailedwood structural panel sheathing). Note that 1997 UBC had seismic designprovisions for heavy timber braced frames but none are included in NEHRPor IBC provisions. Because the LFRS doesn’t support gravity loads, it is in adifferent category when it comes to the R factor used to determine lateraldemand. Also, spread footings are more likely to support the concentratedloads from the columns as compared to platform style construction.FEMA 451B Topic 13 NotesWood Structures 13 - 10

Typical Light-Frame Foundation: Slab-On-GradeSill bolts atpressure treatedsill to foundationBearing wall supporting gravity loadsSlab-on-grade"Shovel" footing withminimal reinforcingInstructional Material Complementing FEMA 451, Design ExamplesTimber Structures 13 - 11Self explanatory. Note that relatively little engineering goes into the footingsfor the most part.FEMA 451B Topic 13 NotesWood Structures 13 - 11

Typical Light-Frame Foundation: Raised FloorBearing wall supporting gravity loadsRim joistSill bolts atpressure treatedsill to foundationSupplemental blocking under shearwall boundary membersFloor SystemCrawl space under"raised" floor6” to 8” StemwallCMU or Concrete"Shovel" footing withminimal reinforcingInstructional Material Complementing FEMA 451, Design ExamplesTimber Structures 13 - 12As before, not much attention beyond code reinforcing minimums for thefoundation. Shear wall boundary members can create large overturningcompression forces that require supplemental blocking to prevent excessdeformations through elastic compression of the floors (recall that the MOEof wood perpendicular to grain is 2.5% to 5% of the MOE of wood parallel tothe grain. These same issues need to be considered at upper level floors inplatform style construction.Again, note that for uplift forces coming through the walls, careful attentionneeds to be placed on the load path and ensuring that it is continuous. Moreon this later.FEMA 451B Topic 13 NotesWood Structures 13 - 12

Typical Light-Frame Foundation: Post TensioningBearing wall supporting gravity loadsSill bolts atpressure treatedsill to foundationPT SlabVariation in slab thickness,thickened edges, etc.Post- tensioningtendonsInstructional Material Complementing FEMA 451, Design ExamplesTimber Structures 13 - 13This type of footing common in areas with expansive soil. Slab thicknessmay be increased in areas of concentrated load from either gravity oroverturning. Also may be increased as needed for embedment of anchors toresist uplift (from overturning usually).FEMA 451B Topic 13 NotesWood Structures 13 - 13

Wood Structure Construction Methods: LateralResultant inertial forcesHorizontal elementsVertical elementsnourGdntiooM The basic approach to the lateral design of wood structures is the same as for other structures.Instructional Material Complementing FEMA 451, Design ExamplesTimber Structures 13 - 14Slide emphasizes that basic design principles apply to wood structures.Horizontal and vertical elements of resistance need to be identified anddesigned. In the case of prescriptive or nonengineered light-framestructures, this is accomplished through required construction and detailingprovisions of the building code.FEMA 451B Topic 13 NotesWood Structures 13 - 14

Wood Structure Construction Methods: LateralHorizontal elements of LFRSEdge nailing (interior nailingnot shown)Plywood or OSB panelsOffset panel joints (stagger) Most structures rely on some form of nailed wood structural panels to act asdiaphragms for the horizontal elements of the LFRS (plywood or oriented strand board –OSB). Capacity of diaphragm varies with sheathing grade and thickness, nail type and size,framing member size and species, geometric layout of the sheathing (stagger), directionof load relative to the stagger, and whether or not there is blocking behind every joint toensure shear continuity across panel edges.Instructional Material Complementing FEMA 451, Design ExamplesTimber Structures 13 - 15While other types of wood diaphragms are available (single our doublediagonal boards, for instance) nailed wood structural is by far the mostcommon.FEMA 451B Topic 13 NotesWood Structures 13 - 15

Wood Structure Construction Methods: LateralHorizontal elements of LFRSDirectionof loadhapiDragmbnduoNailing at diaphragmboundariesNailing at continuousedges parallel to loadInstructional Material Complementing FEMA 451, Design ExamplesyarInterior or “field"nailingD ia phr agm bou ndar yTimber Structures 13 - 16Additional detail to show difference between nailing at diaphragm boundariesand nailing at continuous panel edges that are parallel to the direction ofload.FEMA 451B Topic 13 NotesWood Structures 13 - 16

Wood Structure Construction Methods: LateralHorizontal element:nailed wood structuralpanel diaphragm The building code has tables of diaphragm design capacity ( ateither ASD or LRFD resistance levels) relative to all of the factorsmentioned above.Instructional Material Complementing FEMA 451, Design ExamplesTimber Structures 13 - 17The previous slide mentions a dizzying array of variables that impact designcapacity. Here we see that it’s not really that bad since someone’s figured itall out and tabularized it in the code. Also mention that while inelastic actionin the diaphragm may be present, it is not expected, and some constructiontechniques will minimize the opportunity for inelastic response anyway (moreon this later).FEMA 451B Topic 13 NotesWood Structures 13 - 17

Wood Structure Construction Methods: LateralVertical element:nailed woodstructural paneldiaphragm Shear capacities for vertical plywood/OSB diaphragms are also given in the codes, withsimilar variables impacting their strength. Heavy timber braced frames (1997 UBC) and singly or doubly diagonal sheathed walls arealso allowed, but rare.Instructional Material Complementing FEMA 451, Design ExamplesTimber Structures 13 - 18Most vertical elements in wood structures really are vertical diaphragms.Vertical trusses, in the form of heavy timber braced frames as shown on theslide of post and beam construction, are also allowed by code. However,heavy timber braced frames are as common as heavy timber structures.Note the holdowns in the wall corners providing overturning restraint.Results of the testing in the right hand picture show nail pull through as thefailure mode.Note that prescriptive construction will rely heavily on the strength of gypsumwallboard and exterior finishes, such as stucco, to provide strength to theoverall system for seismic resistance, and this is not explicitly addressed bythe prescriptive provisions. Rather, it is provided by default if following therequirements.FEMA 451B Topic 13 NotesWood Structures 13 - 18

Wood Structure LFRS Design Methods: Engineered If a structures does not meet the code requirements for "prescriptive"or "conventional"construction, it must be "engineered." As in other engineered structures, wood structures are only limited by the application ofgood design practices applied through principles of mechanics (and story heightlimitations in the code). A dedicated system of horizontal and vertical elements, along with complete connectivity,must be designed and detailed.Instructional Material Complementing FEMA 451, Design ExamplesTimber Structures 13 - 19Emphasize the importance of engineering in "engineered" wood structures,developing the "complete load path". The structural load path for lateralforces is complex in wood structures. A system of diaphragms and shearwalls, connected through drag struts and shear transfer details, is designed.However, the "nonstructural" sheathing on the inside and outside of thestructure significantly contributes to the performance during an earthquake.While largely ignored, this extra contribution is thought to be inherent in thecode R factors used for design.FEMA 451B Topic 13 NotesWood Structures 13 - 19

Wood Structure LFRS Design Methods: EngineeredDiaphragm BoundaryDiaphragm Terminology“Edge” nailing“Field” nailingContinuous Panel EdgeSupported Edgeof loadDirectiongmon diaphraContinuous Panel EdgeParallel to LoadUnblocked EdgeDiaphragm SheathingInstructional Material Complementing FEMA 451, Design ExamplesTimber Structures 13 - 20Note that a diaphragm boundary exists because of connectivity to a line ofshear resistance containing vertical elements of the lateral force resistingsystem. Blocking is not shown at panel edges (somewhat self explanatory)so be sure to note that.FEMA 451B Topic 13 NotesWood Structures 13 - 20

Wood Structure LFRS Design Methods: EngineeredDiaphragm DesignTables Tables are for DFL or SYP –need to adjust values if framingwith wood species with lowerspecific gravities. Partial reprint of engineeredwood structural paneldiaphragm info in 2003 IBCTable 2306.3.1. Major divisions: Structural 1vs. Rated Sheathing andBlocked vs. Unblocked paneledges.Instructional Material Complementing FEMA 451, Design ExamplesTimber Structures 13 - 21The numbers are small, but that’s the nature of a table like this.Via an example, show the selection of a proper sheathing-framing-blockingnailing solution given a particular shear demand from the diaphragm. Besure to have two examples, one for blocked and another for unblocked.Emphasize that for most residential it is preferred to keep to an unblockedsolution even if that means adding lines of shear resistance to the structureto reduce demands on the diaphragm. Emphasize the reductions (footnotes)for non DFL or SYP lumber, and be sure to note that when using metal plateconnected wood trusses the species of top chord lumber needs to beconfirmed.FEMA 451B Topic 13 NotesWood Structures 13 - 21

Wood Structure LFRS Design Methods: EngineeredShear Wall Design Tables Partial reprint of engineered woodstructural panel diaphragm info in2003 IBC Table 2306.4.1. Tables are for DFL or SYP – needto adjust values if framing withwood species with lower specificgravities. Major divisions: Structural 1 vs.Rated Sheathing and PanelsApplied Directly to Framing vs.Panels Applied Over GypsumWallboard. NO UNBLOCKED edges allowed.Instructional Material Complementing FEMA 451, Design ExamplesTimber Structures 13 - 22Again, small numbers, but it can’t be helped. As for diaphragms, walk theclass through a couple of specific solutions for specific shear demands.Again, emphasize the reductions (footnotes) for non DFL/SYP framingmembers. Point out that it is not uncommon to have pressure treated sillplate material of a softer species of lumber than the framing members, inwhich case the reductions are needed even if using DFL or SYP studs.FEMA 451B Topic 13 NotesWood Structures 13 - 22

Wood Structure LFRS Design Methods: EngineeredProprietary Moment Frames Traditional vertical diaphragm shear walls less effective at high aspect ratios.Prefabricated proprietary code-approved solutions available.Instructional Material Complementing FEMA 451, Design ExamplesTimber Structures 13 - 23The code places limits on maximum aspect rations for nailed wood structuralpanel diaphragms, with 3.5:1 being the maximum, along with reductions inthe tabulated capacities when the AR exceeds 2:1. Some areas of typicallight framed structures, such as garage returns, are problematic in usingwood as a solution to the LFRS. Companies such as Simpson Strong-Tiehave developed tested, code-approved solutions for these areas usingadvanced materials and construction techniques. One such system, shownabove, employs a partially restrained moment connection between the walland the header to enhance the performance of high AR panels.FEMA 451B Topic 13 NotesWood Structures 13 - 23

Wood Structure LFRS Design Methods: EngineeredComplete Load Path Earthquakes move the foundations of astructure.If the structure doesn’t keep up with themovements of the foundations, failure will occur.Keeping a structure on its foundations requiresa complet

The basic approach to the lateral design of wood structures is the same as for other structures. Horizontal elements Vertical elements Resultant inertial forces G r o u n d M o t io n Slide emphasizes that basic design principles apply to wood structures. Horizontal and vertical elements of resistance need to be identified and designed.

Related Documents:

EXAMPLE 9 SEISMIC ZONE 1 DESIGN 1 2018 Design Example 9 Example 9: Seismic Zone 1 Design Example Problem Statement Most bridges in Colorado fall into the Seismic Zone 1 category. Per AASHTO, no seismic analysis is required for structures in Zone 1. However, seismic criteria must be addressed in this case.

the seismic design of dams. KEYWORDS: Dam Foundation, Probabilistic Seismic Hazard Maps, Seismic Design 1. INTRODUCTION To perform seismic design or seismic diagnosis, it is very important to evaluate the earthquake hazard predicted for a dam site in order to predict earthquake damage and propose disaster prevention measures. There are two .

seismic hazard maps, the NEHRP Recommended Provisions seismic design maps, site effects, directionality effects, and the NEHRP Recommended Provisions response spectrum. FEMA 451B Topic 5a Notes Seismic Hazard Analysis 2 Instructional Material Complementing FEMA 451, Design Examples Seismic Hazard Analysis 5a - 2

The Seismic Tables defined in Pages 5 & 6 are for a seismic factor of 1.0g and can be used to determine brace location, sizes, and anchorage of pipe/duct/conduit and trapeze supports. The development of a new seismic table is required for seismic factors other than 1.0g and must be reviewed by OSHPD prior to seismic bracing. For OSHPD,

SC2493 Seismic Technical Guide, Light Fixture Hanger Wire Requirements SC2494 Seismic Technical Guide, Specialty and Decorative Ceilings SC2495 Seismic Technical Guide, Suspended Drywall Ceiling Construction SC2496 Seismic Technical Guide, Seismic Expansion joints SC2497 Seismic

Peterson, M.D., and others, 2008, United States National Seismic Hazard Maps ․ Frankel, A. and others, Documentation for the 2002 Update of the National Seismic Hazard Maps ․ Frankel, A. and others, 1996, National Seismic Hazard Maps Evaluation of the Seismic Zoninig Method ․ Cornell, C.A., 1968, Engineering seismic risk analysis

To develop the seismic hazard and seismic risk maps of Taungoo. In developing the seismic hazard maps, probabilistic seismic hazard assessment (PSHA) method is used. We developed the seismic hazard maps for 10% probability of exceedance in 50 years (475 years return period) and 2 % probability in 50 years (2475 years return period). The seisic

This analysis complied with these provisions by using the USGS 2014 National Seismic Hazard Map seismic model as implemented for the EZ-FRISK seismic hazard analysis software from Fugro Consultants, Inc. For this analysis, we used a catalog of seismic sources similar to the one used to produce the 2014 National Seismic Hazard Maps developed by .