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Serviceability Considerations

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1 Serviceability Considerations
November 6, 2014 Emily Guglielmo DEFINED 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.

2 Topics: What is serviceability? Serviceability load and limit states
Serviceability in concrete, steel, wood Serviceability lessons learned Serviceability and building codes. Where do we go from here?

3 What is structural failure?
When a structure can no longer support the loads imposed by the intended use of the structure. strength limit state Photo credit:

4 What is structural failure?
A significant reduction in the quality of the space caused by the movement or deterioration of the structural system. Service limit state Photo credit:

5 What is structural failure to a lab technician?
The transient vibrations in the floor do not allow him to complete necessary experiments. Photo credit:

6 What 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:

7 What 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:

8 DEFINED 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.

9 Service loads that may require consideration:
Static loads from the occupants and their possessions Snow or rain on roofs Temperature fluctuations Dynamic loads from human activities, wind-induced effects, or the operation of building service equipment.

10 3 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.

11 Ultimate 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.

12 Nominal Loads Loads specified in Chapter 16 (except wind and seismic)
Live loads: Panel of experts Snow loads: 50-year mean recurrence interval Rain loads: Amount of rain water retained if the primary drainage system fails

13 Service Loads The 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

14 Ultimate Nominal Service
Small Probability of Being Exceeded in 50 Years Small Probability of Being Exceeded in 1 Year Loads Act on the Structure at an Arbitrary Point in Time

15 Load Combinations: Vertical Deflections
For serviceability limit states involving short-term effects, the suggested load combinations are: D + L D + 0.5S For 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.

16 Serviceability Considerations
Short Term: Vertical Deflections Drift of Walls and Frames Vibrations Long-Term Deflections Expansion and Contraction Durability

17 Deflection 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.

18 Load Combinations: Drift of Walls and Frames
For serviceability limit states involving wind the suggested load combinations are: D + 0.5L + Wa What is Wa?

19 Occupancy/Risk Category I Occupancy/ Risk Category II Occupancy/ Risk Category III, IV 300-year return period 700-year return period 1700-year return period ASCE 7-05 I=0.87 or 0.77 ASCE 7-05 I=1.0 ASCE 7-05 I=1.15

20 APPENDIX TO COMMENTARY C
Occupancy/Risk Category I Occupancy/ Risk Category II Occupancy/ Risk Category III, IV Ratio 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 interval Griffis, L. G. (1993). “Serviceability limit states under wind load.” Engineering J., 30(1), 1–16. 10-year return period 25-year return period 50-year return period 100-year return period APPENDIX TO COMMENTARY C

21 Deflection 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/ 400 Building height/ 500 Floor-to-floor greater than 12.5’ 3/8” would control. ASCE Task Committee on Drift Control of Steel Building Structures 1988 and Griffis 1993 These 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 have successfully 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).

22 Deflection Limits: Drift C&C
Note f., includes adjustment from factored to service loads ASCE Task Committee on Drift Control of Steel Building Structures 1988 and Griffis 1993 These 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 have successfully 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”

23 Vibration In recent years vibration complaints have increased.
Traditional static deflection checks do not cover vibration concerns. More flexible structures that results from modern construction practices. Ways to control vibrations: Increase stiffness Mass distribution Damping

24 Vibration 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 Activity 0.005g to 0.01g Spectator Event 0.02g to 0.05g

25 Vibration Human 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.

26 Vibration Increase Stiffness: Minimize vibrations by controlling the maximum deflection (independent of span). Fundamental frequency Deflection Ways 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/??)

27 Vibration Human activity: Assume 4 Hz
Floor system to be 2 times excitation frequency Prefer 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.

28 Long Term Deflection Under 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

29 Expansion 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.

30 Durability 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.

31 Concrete: Immediate Deflections
Two Options (One-Way, Non-Prestressed): Minimum thickness from table Calculate deflections per equations. Advantages, disadvantages Fy, lightweight Load 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.

32 Concrete: Creep and Shrinkage
Time Effects: Creek and shrinkage Shrinkage 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.

33 Concrete: Lateral Drift
Section : Elastic second order analysis related to drift Problem: “EI” for strength design (assume cracked sections) Very conservative for wind drift Unless 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.

34 Steel Rolled structural steel Open-web joists Composite steel
Fabricated trusses Cold-formed steel

35 Steel: Non-Composite Deflection Vibration No significant time effects
Assumed to act elastically at service level

36 Steel: Composite Similar to non-composite design
Camber: Counteract deflections due to self-weight of structure AISC 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.”

37 Steel: Composite Long-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., 1997 Robinson (1986), research on shrinkage in composite beams Viest (1958), shrinkage calculations for composite

38 Steel: “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.

39 Open-Web Joists Vibration: High stiffness to mass ratio Deflection
See SJI, AISC references Deflection Connection flexibility: Pinned Camber Joists 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.

40 Open-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?

41 Wood Natural Wood Engineered Wood
Compatibility of Wood with Other Systems

42 Natural Wood Wood is not isotropic and not stable over time
Shrinkage and creep depend on moisture content and drying of material Typical creep about equal to initial deflection NDS 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.

43 Natural Wood IBC : Prescriptive requirements to limit differential shrinkage.

44 Natural Wood Wood Trusses: Time dependent effect- “set”.
Experience, engineering judgment Set is the deflection due to inelastic deformation between wood fibers and slip in the truss joints.

45 Engineered Wood Glulams, parallams, LVLs, plywood, I-joists, composite lumber, wood panels. Serviceability design: See product data along with engineering judgment.

46 Compatibility 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.

47 Commonly Missed Serviceability Issues

48 Commonly Missed Serviceability Issues

49 Commonly Missed Serviceability Issues

50 3. 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. 1 2 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. 3 4

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. 1 2 3 4 1. 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. 5 6

52 Serviceability considerations vary by building type, construction material, client, usage, analysis method. Discuss early with client, PM/ PIC Many 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!


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