Presentation on theme: "Serviceability Considerations"— Presentation transcript:
1Serviceability Considerations November 6, 2014Emily GuglielmoDEFINED LIMITIMPOSED LOADPREDICTED BEHAVIOR**** Imposed Loads: Conservative loadings (extreme events) will yield inefficient, overdesigned, and expensive structures.**** Predicted Behavior: Predictions of behavior using inelastic behavior and cracked sections for loading levels that below the elastic range will yield overly conservative evaluations.**** Defined Limit: The limits on behavior must be matched to the expectations of the building owner and occupants.
2Topics: What is serviceability? Serviceability load and limit states Serviceability in concrete, steel, woodServiceability lessons learnedServiceability and building codes. Where do we go from here?
3What is structural failure? When a structure can no longer support the loads imposed by the intended use of the structure.strength limit statePhoto credit:
4What is structural failure? A significant reduction in the quality of the space caused by the movement or deterioration of the structural system.Service limit statePhoto credit:
5What is structural failure to a lab technician? The transient vibrations in the floor do not allow him to complete necessary experiments.Photo credit:
6What is structural failure to an office building owner? The floor deflections result in unlevel floors, which prevent the sale or lease of the building at a competitive price.Photo credit:
7What is structural failure to an apartment building owner? The number and size of cracks in the floors and walls of the apartment rooms result in lower occupancies and rent collected.Photo credit:
8DEFINED LIMIT IMPOSED LOAD PREDICTED BEHAVIOR **** Imposed Loads: Conservative loadings (extreme events) will yield inefficient, overdesigned, and expensive structures.**** Predicted Behavior: Predictions of behavior using inelastic behavior and cracked sections for loading levels that below the elastic range will yield overly conservative evaluations.**** Defined Limit: The limits on behavior must be matched to the expectations of the building owner and occupants.
9Service loads that may require consideration: Static loads from the occupants and their possessionsSnow or rain on roofsTemperature fluctuationsDynamic loads from human activities,wind-induced effects, or the operation ofbuilding service equipment.
103 Limit States Loads Ultimate Loads Nominal Loads Service Loads When considering the appropriate loads to use in serviceability design there are 3 levels of loadings related to limit states—ultimate, nominal and service.
11Ultimate Loads Utilized for strength design in the IBC. Intended to be maximum loads imposed on the structure during its life.Typically use a load factor of greater than 1.0.Wind and seismic already at ultimate level in IBC 2012.
12Nominal Loads Loads specified in Chapter 16 (except wind and seismic) Live loads: Panel of expertsSnow loads: 50-year mean recurrence intervalRain loads: Amount of rain water retained if the primary drainage system fails
13Service LoadsThe service loads are those loads that act on the structure at an arbitrary point in time.Not defined or used in IBC.See ASCE 7-10, Appendix C Commentary
14Ultimate Nominal Service Small Probability of Being Exceeded in 50 YearsSmall Probability of Being Exceeded in 1 YearLoads Act on the Structure at an Arbitrary Point in Time
15Load Combinations: Vertical Deflections For serviceability limit states involving short-term effects, the suggested load combinations are:D + LD + 0.5SFor serviceability limit states involving creep, settlement, or similar long-term or permanent effects, the suggested load combination is:D + 0.5L** Dead Load “D” may be portion of dead load that occurs after attachment of nonstructural elements. **First order reliability analysis annual probability of 0.05 of being exceeded.For example, in composite construction, the dead load effects frequently are taken as those imposed after the concrete has cured; in ceilings, the dead load effects may include only those loads placed after the ceiling structure is in place.Load combinations for checking static deflections can be developed using first-order reliability analysis (Galambos and Ellingwood 1986). A combined load with an annual probability of 0.05 of being exceeded would be appropriate in most instances.
16Serviceability Considerations Short Term:Vertical DeflectionsDrift of Walls and FramesVibrationsLong-Term DeflectionsExpansion and ContractionDurability
17Deflection Limits: Vertical Deflections Limiting values of deformations depend on the type of structure, detailing, and intended use.L/300 are visible and may lead to architectural damage or cladding leakage.L/200 may impair operation of doors and windows.
18Load Combinations: Drift of Walls and Frames For serviceability limit states involving wind the suggested load combinations are: D + 0.5L + WaWhat is Wa?
20APPENDIX TO COMMENTARY C Occupancy/RiskCategory IOccupancy/ RiskCategory IIOccupancy/ RiskCategory III, IVRatio drift from ultimate map to 10-year map.Some designers have used a 10-year MRI (annual probability of 0.1) for checking drift under wind loads for typical buildings (Griffis 1993), whereas others have used a 50-year MRI (annual probability of 0.02) or a 100-year MRI (annual probability of 0.01) for more drift-sensitive buildings. The selection of the MRI for serviceability evaluation is a matter of engineering judgment that should be exercised in consultation with the building client.The maps included in this appendix are appropriate for use with serviceability limit states and should not be used for strength limit states.Because of its transient nature, wind load need not be considered in analyzing the effects of creep or other long-term actions.Deformation limits should apply to the structural assembly as a whole.The stiffening effect of nonstructural walls and partitions may be taken into account in the analysis of drift if substantiating information regarding their effect is available.Where load cycling occurs, consideration should be given to the possibility that increases in residual deformations may lead to incremental structural collapse.(MRI) mean recurrence intervalGriffis, L. G. (1993). “Serviceability limit states under wind load.” Engineering J., 30(1), 1–16.10-year returnperiod25-year returnperiod50-year returnperiod100-year returnperiodAPPENDIX TO COMMENTARY C
21Deflection Limits: Drift Drift limits in common usage for building design are on the order of 1/600 to 1/400 of the building or story height.An absolute limit on story drift may also be needed due to reduce damage (nonstructural partitions, cladding, and glazing) which may occur if the story drift exceeds 3/8 in.Story height/ 400Building height/ 500Floor-to-floor greater than 12.5’ 3/8” would control.ASCE Task Committee on Drift Control of Steel Building Structures 1988 and Griffis 1993These limits generally are sufficient to minimize damage to cladding and nonstructural walls and partitions.Smaller drift limits may be appropriate if the cladding is brittle. West and Fisher (2003) contains recommendations for higher drift limits that havesuccessfully been used in low-rise buildings with various cladding types. It also contains recommendations for buildings containing cranes.Absolute limit on story drift (Freeman 1977 and Cooney and King 1988).
22Deflection Limits: Drift C&C Note f., includes adjustment from factored to service loadsASCE Task Committee on Drift Control of Steel Building Structures 1988 and Griffis 1993These limits generally are sufficient to minimize damage to cladding and nonstructural walls and partitions.Smaller drift limits may be appropriate if the cladding is brittle. West and Fisher (2003) contains recommendations for higher drift limits that havesuccessfully been used in low-rise buildings with various cladding types. It also contains recommendations for buildings containing cranes.Absolute limit on story drift (Freeman 1977 and Cooney and King 1988).“Wind load 0.42 C&C for deflection”
23Vibration In recent years vibration complaints have increased. Traditional static deflectionchecks do not cover vibrationconcerns.More flexible structures that results from modern construction practices.Ways to control vibrations:Increase stiffnessMass distributionDamping
24Vibration VIBRATION PERCEPTION Continuous 0.05g to 0.1g Transient Individuals find continuous vibrations more objectionable than transient vibrations.Continuous vibrations (minutes) with accelerations on the order of 0.005g to 0.01g are annoying to people engaged in quiet activities. People engaged in physical activities or spectator events may tolerate steady-state accelerations on the order of 0.02g to 0.05g (5 TIMES).Thresholds for transient vibrations (seconds) are considerably higher and depend on the structural damping present (Murray 1991). For a finished floor with 5% damping peak transient accelerations of 0.05g to 0.1g (2-10 TIMES) may be tolerated.Quiet Activity0.005g to 0.01gSpectator Event0.02g to 0.05g
25VibrationHuman activity imparts dynamic forces at frequencies of 2-6 Hz.If the fundamental frequency of the floor matches this range and the activity is rhythmic (dancing, aerobic exercise, cheering at spectator events) resonance may occur. Tune the system away from this range.As a general rule of thumb, the natural frequency of structural elements should be at least 2 times the frequency of the steady state excitation.
26VibrationIncrease Stiffness: Minimize vibrations by controlling the maximum deflection (independent of span).Fundamental frequencyDeflectionWays to minimize vibration.Add mass.Add damping.Increase stiffness.Therefore, limit the deflection to an absolute value rather than a fraction of the span (L/??)
27Vibration Human activity: Assume 4 Hz Floor system to be 2 times excitation frequencyPrefer a floor system with a frequency greater than 8 Hz.Human activity 2-6 Hz.Many floors not meeting this guideline are perfectly serviceable.However this guideline provides a simple means for identifying potentially troublesome situations where additional consideration is warranted.Increasing stiffness if one way of minimizing vibrations. Another is to add damping because studies have shown individuals to be more tolerant of vibrations that damp out quickly.The static deflection due to uniform load must be limited to about 5mm (0.20 in.) if the frequency of the floor system is to be kept above 8 Hz.
28Long Term DeflectionUnder sustained loads, structural members may experience time-dependent deformations due to creep.Use a multiplier on immediate deflections.Provided in material standards (1.5 to 2.0.)Use load combination D + 0.5L
29Expansion and Contraction Designs should consider dimensional changes due to expansion or contraction.Provide for such effects through relief joints or controlling crack widths.Restraint of thermal, shrinkage, prestressing.
30Durability Buildings may deteriorate in certain service environments. May be visible upon inspection (weathering, corrosion, staining) or may be undetectable visually.Designer should provide a specific amount of damage tolerance in design or specify adequate protection system or planned maintenance.
31Concrete: Immediate Deflections Two Options (One-Way, Non-Prestressed):Minimum thickness from tableCalculate deflections per equations.Advantages, disadvantagesFy, lightweightLoad Stage #1 shows a load that creates a maximum moment that is less than the cracking moment. In this case no cracking occurs in the beam. The gross moment of inertia can be used to compute the deflection.In Load Stage #2 the load P is increased until the maximum internal moment in the beam reaches the cracking moment. At this time the first flexural crack occurs in the beam. The moment of inertia of the beam right at the crack equals the cracked moment of inertia. The moment of inertia over the rest of the beam equals the gross moment of inertia. End effect is that the beam deflection exceeds that would be found using the gross moment of inertia, but not by much. So the effective moment of inertia is a little less than the gross moment of inertia.As the load P is increased to the value at Load Stage #3 additional cracks form as the stress in the concrete due to bending exceeds the tensile stress, fr, of the concrete. The beam becomes more flexible as the more cracks form, meaning that the deflections become much larger than would be predicted by computing deflections using the gross moment of inertia. The effective moment of inertia continues to decrease as the load increases and more cracks form. This process illustrates why the effective moment of inertia changes with the applied moment.Separate equations for prestressed.
32Concrete: Creep and Shrinkage Time Effects: Creek and shrinkageShrinkage and creep due to sustained loads cause additional long-term deflections. Such deflections are influenced by temperature, humidity, curing conditions, age at time of loading, quantity of compression reinforcement, and magnitude of the sustained load.The deflection is the additional long-term deflection due to the dead load and that portion of the live load that will be sustained for a sufficient period to cause significant time-dependent deflections.
33Concrete: Lateral Drift Section : Elastic second order analysis related to driftProblem: “EI” for strength design (assume cracked sections)Very conservative for wind driftUnless a more accurate estimate of the degree of cracking at service load level is available, it is satisfactory to use I= 1.43Icr.The moments of inertia for service load analyses should represent the degree of cracking at the various service load levels investigated.Unless a more accurate estimate of the degree of cracking at service load level is available, it is satisfactory to use 1.0/0.70 = 1.43 times the moments of inertia given here for service load analyses.
35Steel: Non-Composite Deflection Vibration No significant time effects Assumed to act elastically at service level
36Steel: Composite Similar to non-composite design Camber: Counteract deflections due to self-weight of structureAISC 360 Section L.3: “additional deflections due to shrinkage and creep of the concrete should be considered.”AISC 360 Commentary L3.2: “in the US shrinkage and creep of the concrete flange is generally not considered.”
37Steel: CompositeLong-term deflection due to shrinkage significant? L/1200 to L/1000.Simplified method of predicting additional deflection due to beam shrinkage by modeling shrinkage strain as a compressive force in the slab.Suggestion: Long-term deflections due to creep/ shrinkage are usually small and can be ignored.Consideration for long spans, high sustained loads, high composite action.Composite Construction Design for Buildings, Viest, I.M, et al., American Society of Civil Engineers and McGraw-Hill, Inc., 1997Robinson (1986), research on shrinkage in composite beamsViest (1958), shrinkage calculations for composite
38Steel: “Pinned” Connections At ultimate levels, bolted connections behave as pinned.At service levels, bolted connections in WF framing provides restraint to reduce deflections up to 80% of a pin-connection.
39Open-Web Joists Vibration: High stiffness to mass ratio Deflection See SJI, AISC referencesDeflectionConnection flexibility: PinnedCamberJoists have high stiffness to mass ratios compared with concrete/ steel systems. So they tend to have more vibration issues because they lack the damping inherent to heavier systems.Unlike structural steel, joist connections are more nearly pinned unless bottom chords extended.Inherent conservatism in the deflection of wide flange beam systems does not apply to joists.
40Open-Web Joists: Deflection: Camber Supplied with standard camber unless otherwise specified.Camber is related to the dead load deflections based on the capacity of the joist.Uniform depth joists?Heavy live loads?
41Wood Natural Wood Engineered Wood Compatibility of Wood with Other Systems
42Natural Wood Wood is not isotropic and not stable over time Shrinkage and creep depend on moisture content and drying of materialTypical creep about equal to initial deflectionNDS increase short term deflections to account for creep“Green” wood may actually creep as much as 4-5 times the initial deflection as it dries.
43Natural WoodIBC : Prescriptive requirements to limit differential shrinkage.
44Natural Wood Wood Trusses: Time dependent effect- “set”. Experience, engineering judgmentSet is the deflection due to inelastic deformation between wood fibers and slip in the truss joints.
45Engineered WoodGlulams, parallams, LVLs, plywood, I-joists, composite lumber, wood panels.Serviceability design: See product data along with engineering judgment.
46Compatibility of Wood and Other Systems Wood floor/ roof in parallel with steel.Load bearing wood walls adjacent to CMU/ concrete walls.Load bearing wood walls adjacent to steel columns.Adjacent wood walls of varying height due to basement walls.
503. Some designers do not adequately address serviceability. “Why should the building code include a more thorough treatment of serviceability.”1. The current code language can be used in a legal argument against the designer.2. Engineers could “check the box” saying that such minimum serviceability limits are met and therefore, the design is adequate.3. Some designers do not adequately address serviceability.4. Some international standards (The Canadian Code, Eurocodes) have serviceability in the code.121. The current code language can be used in a legal argument against the designer.2. Engineers could “check the box” saying that such minimum serviceability limits are met and therefore, the design is adequate.3. Some designers do not adequately address serviceability.4. Some international standards (The Canadian Code, Eurocodes) have serviceability in the code.34
51“Why should the building code NOT include a more thorough treatment of serviceability.” Serviceability is complex and minimum targets are dependent on the analysis method and assumptions.2. Serviceability requirements vary widely among stakeholders and should be addressed at the beginning of the project.3. Strength limit states are a matter of life safety and serviceability limit states are contractual between the designer and the client.4. Serviceability limit states are more properly addressed in design guides.5. Serviceability requirements will increase the size and complexity of the code.6. Placing more serviceability requirements in the standard seems to contradict the goals of performance based design.12341. Serviceability is complex and minimum targets are dependent on the analysis method and assumptions.2. Serviceability requirements vary widely among stakeholders and should be addressed at the beginning of the project.3. Strength limit states are a matter of life safety and serviceability limit states are a contractual matter between the designer and the client.4. Serviceability limit states are more properly addressed in design guides.5. Serviceability requirements will increase the size and complexity of the code.6. Placing more serviceability requirements in the standard seems to contradict the goals of performance based design.56
52Serviceability considerations vary by building type, construction material, client, usage, analysis method.Discuss early with client, PM/ PICMany of the serviceability requirements are in material standards and/or design guides.Serviceability is only partially addressed in the building code, and the direction of serviceability requirements in the code is evolving.Thank you!