Presentation on theme: "Introduction and Building Loads"— Presentation transcript:
1 Introduction and Building Loads CE Design of Multi-Story StructuresT. B. QuimbyUAA School of Engineering
2 Course ObjectiveThe objective of the course is to give entry level structural engineers an understanding of the principles associated with the structural design of building systems.
3 Expected OutcomesAt the conclusion of this course, the students will have:an understanding of the engineering design process as it relates to building structural design, including an appreciation for:the iterative nature of the design processthe concept that there are more than one way to solve most engineering problemsan understanding of structural loads and their determination.a basic understanding of the behavior and use of various structural systemsa basic understanding of what is required in a set of construction drawingsa basic understanding of what is required in a set of construction specificationsa recognition of the need for continual learning as a professionalan understanding of the need for professional registrationan understanding of professional and ethical responsibilitythe basic ability to:identify, formulate, and solve building structural design problemsproduce a set of construction drawings.produce a set of construction specifications
4 Course ContentThe emphasis of the course will be slightly different than the text. We will be considering all multi-story structures, not just “Tall” buildings.Load computationsPreliminary calculation methodsComputer modelingDifferent GFRS and LFRSCalculations and Contract Documents
5 Tall BuildingsAuthor: “A tall building .... is one that, because of its height, is affected by lateral forces due to wind or earthquake actions to an extent that they play an important role in the structural design.HistoryDefenseEcclesiasticalCommercial (from 1880 to current)Residential (from 1880 to current)Maximize use of high cost land
6 Factors Affecting Development MaterialsTimber & Masonry limit to ~ 5 storiesWrought Iron & Steel in mid 1880sStructural Concrete after 1900The ElevatorMade upper stories attractive to rentMade tall buildings financially viableConstruction TechnologyIncrease SpeedMore efficient equipmentImproved methods
7 Office vs Residential Office/Commercial buildings Large entrances and open lobbiesReconfigurable space (large column free open areas)Residential buildingsPartitions are frequent and the same from story to story
8 The Design Team Consists of: OwnerArchitectStructural EngineerServices Engineer (Mechanical & Electrical)Team should collaborate EARLY to agree on a form of structure to satisfying the conflicting requirements.Structural system is subservient to the architectural requirements.Compromise is inevitable.
9 The Design Process Design is an evolutionary (iterative) process. Do preliminary sizing of members for gravity loads using approximate analysis.Check lateral strength and deflections, adjust members sizes and configuration as necessary.Make alterations to original layout as owner and architect refine the design. May require radical rearrangement and complete review of structure.Make a rigorous final analysis using a refined analytical model and verify deflections and member strengths.Include the effects of movements due to creep, shrinkage, temperature differentials, and foundation settlement.Complete Construction Documents
10 Design Criteria Architectural Structural Services Internal layout to meet functional requirementsAesthetic qualitiesStructuralStrength (Elastic vs. Plastic)Serviceability (deflections, vibrations, etc....)ServicesPowerVentilation
11 Limit State Design A probabilistic approach Structural propertiesLoading conditionsWhen a LIMIT STATE is reached, the structures said to have failed.Strength Limit StatesExceedance of these limit states endanger lives and/or cause serious financial loss.Probability of material failure and instability must be low.Serviceability Limit StatesFitness of the building for normal useProbability of failure may be higher since failure is not catastrophic.
12 LoadingBuildings are designed to carry all gravity loads and lateral loads to be seen during construction and service.Must consider sequential loading (particularly during construction) in buildings where the sequence is important.Types of Loading:DeadOccupancy (Live)ImpactSnowWindSeismic1997 UBC Chapter 16
13 Strength & Stability“the building structure should have adequate strength to resist, and to remain stable under, the worst probable load actions that may occur during the lifetime of the building, including the period of construction.”Consider probable load combinations (1997 UBC 1612)Second order affectsProgressive collapseDifferential movement (shrinkage, creep, settlement, temperature)Overturning
14 Stiffness and Drift Limitations Deflections under gravity loads must be with in tolerable limits for the occupancy.Deflections under lateral load must be small enough to satisfySecond order effects (P-delta)Avoid distress to the structure (cracking, redistribution of loads to partitions, etc....)Human comfort (acceleration, period, amplitude, visual and acoustical cues, past experience)ServiceabilityLateral drift requirements (1997 UBC )
15 Tributary AreasUseful for determining member forces due to UNIFORMLY APPLIED loads (dead, live, pressure, etc....) on SIMPLY SUPPORTED members.Use structural analysis theory to find the “path” that loads take as they “find their way” down to the foundation through the structural members.
16 Example #1Applied load is uniformly distributed.
17 Tributary widths of beams supporting joists coming in at odd angles
20 Column Tributary AreaFor a triangular load, the reaction at B is 1/3 of the total load on the beam. This means that the column supports 1/3 of the area.For a triangular load, this means that the column at B support L/sqrt(3) of the length of the beam.
21 Example #2 Identify the Tributary Areas for: For each beam For each column
26 Areas Trib. to column at C2 These columns probably support exterior wall sections as well. Depends on details.Gravity loads tend to accumulate linearly as you go down the building.Live loads may be reduced.
27 Example #4 Have fun with this one! Find area supported by beams on radial grids.
28 Dead Load Calculations Dead loads are the weights of all items permanently attached to the structure.Roof, Floor, and Wall dead loads are typically expressed in terms of “unit” loads (the weight per unit of surface area).Permanently attached equipment and machinery are generally treated as point loads or uniform loads over a limited area.
29 Unit Load Calculations All unit load calculations should be accompanied by a sketch or reference a drawing showing a typical calculation.Each item is expressed in terms of its weight per unit surface area.Must compensate for slopes over 4:12.Final result should be not include decimals! (your overall estimate is not any more accurate than three significant figures (if that!)Should add an appropriate “Misc..” amount for minor items not specifically accounted for in itemized calculation.
32 Live LoadsLive loads are any loads that are not permanently attached to the structure.Live loads may be expressed in term of area loads or point loads.Live loads are placed for maximum effect.Tabulated code values result from experience and typical field surveys.See 1997 UBC 1606 & 1607Live loads may be reduce for design of members that have large tributary areas UBC
33 UBC Floor Live Load Reduction Use when:Member supports more than 150 ft2Live load not greater than 100 psfMember does not support a place of public assemblyUse the lessor of:R = 0.08(A-150)R = 23.1(1+DL/FLL)R = 40% for members receiving load from one level only, or 60% for members receiving load from more than one level.
34 Alternate Floor Live Load Reduction As an alternative, the following equation may be used for member with an “influence area” greater than 400 ft2. (New with the 1994 UBC)L = L0(.25+15/sqrt(AI))Maximum reduction is 50% for members supporting one level and 60% for members supporting multiple levels.AI is the influence area. For a column AI is four times the trib. area. For a beam, AI is two times the trib. area. For a 2-way slab, AI equals the panel area. For a precast T-beam, AI is the span times the full flange width.
35 UBC Roof Live Loads 1997 UBC 1607.4 & Table 16-C If unbalanced loading causes maximum effects, it must be considered.Snow loads must be considered where they exceed the values for the roof live loads. (See 1997 UBC Appendix to chapter 16, Div. I - Snow Load Design)When analyzing for snow loads, must consider unbalanced loading and drifting.Snow Loads may be reduced with increasing roof slope.RS = S/RS = snow load reduction (psf) per degree slope over 20S = total snow load (psf)