2 CEE 7402 Forensic Engineering OutlineReview of Progress Report #1 PresentationsIBC Concrete Design RequirementsBeam & One Way Slab DesignSlab Thickness ConsiderationsLoad Path and Framing PossibilitiesConnection & Analysis IssuesSeismic Detailing RequirementsWork Tasks
3 Progress Report #1 Comments Overall, a very good jobComments on presentations:Timing goodDon’t worry about the intro stuff next timeKnow where our site is located – you have coordinates that are accurate to within 3 miles!!!
4 Progress Report #1 Comments Range of values:100 to 150 mph design wind speedSeismic Design Category D (unanimous)2000 to 2800 psi concrete strength49000 to psi steel yield strength
5 IBC Concrete Design Requirements IBC Chapter 19Mimics ACI 318 CodeIBC 2000 version based on 1999 ACI 318IBC 2003 will use 2002 version of ACI 318First seven sections (1901 – 1907) correspond to ACI 318 Chapters 1 to 7
6 IBC Concrete Design Requirements Section 1908 gives specific modifications to ACI 318Deals with “meat” of ACI CodeSections 1909 – 1916 deal with specialized areasSec – Seismic Design RequirementsSec – Anchorage to ConcreteGet to know this document!!!
7 Load Path / Framing Issues Building Frame SystemFrame for gravity loadShear walls for lateral loadConsider support of the chapel gravity loads:Where do the columns go?What beams do I need?How do I design my slab?
8 Beam & One Way Slab Design Review We presumably know how to do the following from CEE 3422:Design a rectangular beam of unknown cross-section sizeDesign a rectangular beam of known cross-section sizeDesign a simply supported one way slab
9 Beam & One Way Slab Design Review We presumably know how to do the following from CEE 3422:Design a T-beam for positive momentDesign a T-beam for negative momentDesign a doubly reinforced beam (beam with compression reinforcement)Design a beam for shear
10 Design of Continuous Beams and Slabs GapYou know how to design cross-sections for positive or negative momentReinforcement follows the moment diagramWhy continuous spans?MomentsDeflectionsdMTwo Simple SpansContinuous over Center Support
11 Design Moments (Uniform Dist. Loading) Simple SpanswL2/8Continuous SpansAnalysis far more complicatedWhat type of fixity do we actually have?Must consider effects of patterned loadingFormation of plastic hinges allows for moment redistribution
12 Design Moments – Continuous Spans We have four analysis optionsElastic Analysis (preferably STAAD)Elastic Analysis w/ Moment RedistributionApproximate Frame AnalysisACI Approximate Moment CoefficientsSee McCormac text Chapter 13
13 Slab Thickness Considerations What governs the thickness of a slab?Flexural StrengthShearDeflectionsUsually, deflections will govern the thickness requirements for a one-way slabSize slab based on deflection requirementsCheck shearDesign reinforcement for flexure
14 Slab Thickness Considerations Review McCormac text, Ch. 5 (serviceability) and Ch. 3 (one-way slabs)Review notes from CEE 3422, lectures on one-way slab design and serviceabilityACI Sec
15 For simply-supported beams, total beam depth ‘h’ must be at least L/16 Slab Thickness Considerations (such that we do not need to compute deflections)For simply-supported beams, total beam depth ‘h’ must be at least L/16A 16 ft. long simply supported beam must be at least 12 in. deep.For simply-supported one-way slabs, total slab thickness ‘h’ must be at least L/20A 10 ft. long simply supported one-way slab must be at least 6 in. deep.You will have to look up other values!!!
16 Slab Thickness Considerations Something to keep in mind….Your material properties!These tables are based on normal strength concreteYou may wish to consider creative ways to adjust tables for your low concrete strengthHint: Think about what the key concrete material property related to deflections is…
17 Load Path / Framing Possibilities Now we can begin to develop a framing plan for our structureTypical practice on site is a 5 in. thick slabWe have a methodology to determine how far a slab of a given thickness can spanDo our material properties have any effect?Let’s look at a plan view of the two-story section…
18 CEE 7402 Forensic Engineering Note: columns automatically placed at each wall end or cornerLn = 27.0 ft.Ln = 10.5 ft.Ln = 14.5 ft.Ln = 12.0 ft.Think we’ll need some additional framing members???
19 Framing Concepts Let’s use a simple example for our discussion… Column spacing30 ft. on centerThink about relating it to your design as we discuss…Plan
20 Framing ConceptsWe can first assume that we’ll have major girders running in one direction in our one-way systemPlan
21 Framing ConceptsIf we span between girders with our slab, then we have a load path, but if the spans are too long…Plan
22 Framing ConceptsWe will need to shorten up the span with additional beamsPlan
23 Framing ConceptsBut we need to support the load from these new beams, so we will need additional supporting membersPlan
24 Framing Concepts Now we have a viable plan… Let’s think back through our load path now to identify our “heirarchy” of membersPlan
26 Framing ConceptsNote that by running the one-way slab in this EW direction, we are actually making the EW running beams our major girdersThe NS running beams simply transfer the load out to these girders (or directly to a column)Plan
27 Now let’s go back through with a slightly different load path Framing ConceptsNow let’s go back through with a slightly different load pathPlan
28 Framing ConceptsWe again assume that we’ll have major girders running in one direction in our one-way systemPlan
29 Framing ConceptsThis time, let’s think about shortening up the slab span by running beams into our girders.Our one-way slab will transfer our load to the beams.Plan
30 Framing ConceptsWith this approach, we have already established our “heirarchy”The only difference is in the “direction” of our load path90 degree rotationPlan
31 Framing Concepts - Conclusions Either load path will workIn this case, they are identicalWith a rectangular bay (instead of a square) bay, there will be a differenceTradeoff is usually in number of supporting members vs. span of supporting members
33 Framing Concepts - Considerations For your structure:Look for a “natural” load pathIdentify which column lines are best suited to having major framing members (i.e. girders)Assume walls are not there for structural support, but consider that the may help you in construction (forming)
34 Connection / Analysis Issues With continuous reinforced concrete framing systems, connections are a major issue with respect to:Detailing of reinforcement at these congested areasAssumptions regarding fixity of beams and slabs
35 Connection / Analysis Issues Let’s first consider our continuous one-way slab (12” strip shown) framing into an exterior (spandrel) beamPlan
36 Slab-Exterior Beam Connection Slab is a six span continuous systemSome fixity at end of slab due to torsional rigidity of exterior beam, but what happens when beam and slab crack?Do we want to count on fixity?Also, if we design slab for negative moment here, we must develop reinforcement (like a cantilever)
37 Slab-Exterior Beam Connection Typical assumptions:Simple support at endNo moment in slab at endPlace some reinforcement at top of slab to control crackingDesign exterior beam for minimal torsion
39 Beam-Girder Connection Beam is a two span continuous systemSimilar situation: some fixity at end of beam due to torsional rigidity of exterior girder, but what happens when beam and girder crack?Do we want to count on fixity?Also, if we design beam for negative moment here, we must develop reinforcement (like a cantilever)
40 Slab-Exterior Beam Connection Typical assumptions:Simple support at endNo moment in beam at endPlace some reinforcement at top of beam to control crackingDesign exterior girder for minimal torsion
41 Analysis – One-Way Slab & T-Beams For the simple elements just described, where supports are provided by beams and girders,Supporting elements have some stiffness, but it is fairly smallAssumption of treating one-way slabs and T-beams as continuous beams is validA frame analysis is not needed since there are no columns involvedSimple analysis methods can be used if all assumptions are met (i.e. ACI moment coefficients)
42 Connection / Analysis Issues Finally, let’s look at beam-column and girder-column jointsThree situations:Interior columnExterior columnCorner columnPlan
43 Interior Column Connection Girders framing in to a column:Columns will provide some rigidityMoments will depend upon distribution of stiffnessFrame analysis is warranted to determine these momentsUnbalanced loading (patterned live load) must be consideredGoal: Determine moments in girders (they will not necessarily be equal), as well as axial load & moment combinations for columnsBeam/girder reinforcement must be continuous through jointPlanM cuM2M1M cl
44 Exterior Column Connection Same basic situation:Columns will provide some rigidityMoments will depend upon distribution of stiffnessFrame analysis is warranted to determine these momentsUnbalanced loading (patterned live load) must be consideredGoal: Determine moments in girders (they will not necessarily be equal), as well as axial load & moment combinations for columnsBeam/girder reinforcement must be developed for negative momentPlanM cuM1M cl
45 Corner Column Connection This is essentially the same situation as an exterior columnNote that where we have beams (not girders) framing into columns, the same principles applyHowever, these moments are typically very small and will usually not control the designPlanM cuM1M cl
46 Analysis – Girders & Beams Framing Into Columns For these elements, support is provided by columnsColumns have substantial stiffness and will attract some momentsAssumption of treating these girders and beams as continuous beams is not validA frame analysis is needed to determine the appropriate distribution of momentsElastic analysis is recommended (STAAD, PCABeam)
47 Seismic Detailing Requirements for Reinforced Concrete - Introduction IBC Section 1910ACI Chapter 21These two sections, together, identify specific detailing requirements related to seismic design of concrete structuresLevel of detailing required is based on Seismic Design Category
48 CEE 7402 Forensic Engineering Work TasksDetermine final loads on the structureGravity loads (dead, live)Lateral loads (seismic, wind)Truss analysis on roof & design of roof membersDetailing of roof-to-structure connectionDevelop a load path (framing plan) to support the gravity loads associated with the second story chapel
49 CEE 7402 Forensic Engineering Work TasksLook into how the selection of Seismic Design Category D will affect concrete design detailing requirements for your beams, columns, and slabWork on design of one-way slab, beams, and girdersWe will discuss design for shear and torsion next time!
50 Assignment for Tuesday CEE 7402 Forensic EngineeringAssignment for TuesdaySubmit a detailed sketch showing your framing plan (load path for gravity loads) for the second story chapelIdentify all columns, beam, and girder locations, and specify a slab thicknessSummarize on one sheet how the selection of Seismic Design Category D will affect the detailing of your structureUse a bullet item / list format to identify specific detailing requirements for your beams, columns, and slabDon’t consider shear walls for now (they will be masonry)