SERVICEABILITY REQUIREMENTS AISC Chapter L

Transcription

1 SERVICEABILITY REQUIREMENTS AISC Chapter L AISC provides limited guidance (see attached pages from the specification) DEFLECTIONS AISC Deflections... under appropriate service load combinations shall not impair the serviceability of the structure For beams: max o = elastic deflection computed under service loads = allowable deflection; depends on type of element supported by the beam = L/360 to L/400 are common values Current practice: camber beams for dead load deflections check additional deflections due to live load only

2 CHAPTER L DESIGN FOR SERVICEABILITY This chapter addresses serviceability design requirements. The chapter is organized as follows: L1. General Provisions L2. Camber L3. Deflections L4. Drift L5. Vibration L6. Wind-Induced Motion L7. Expansion and Contraction L8. Connection Slip L1. GENERAL PROVISIONS Serviceability is a state in which the function of a building, its appearance, maintainability, durability and comfort of its occupants are preserved under normal usage. Limiting values of structural behavior for serviceability (such as maximum deflections and accelerations) shall be chosen with due regard to the intended function of the structure. Serviceability shall be evaluated using appropriate load combinations for the serviceability limit states identified. User Note: Serviceability limit states, service loads, and appropriate load combinations for serviceability requirements can be found in ASCE/SEI 7, Appendix C and Commentary to Appendix C. The performance requirements for serviceability in this chapter are consistent with those requirements. Service loads, as stipulated herein, are those that act on the structure at an arbitrary point in time and are not usually taken as the nominal loads. L2. CAMBER Where camber is used to achieve proper position and location of the structure, the magnitude, direction and location of camber shall be specified in the structural drawings. L3. DEFLECTIONS Deflections in structural members and structural systems under appropriate service load combinations shall not impair the serviceability of the structure. Specification for Structural Steel Buildings, June 22, 2010 AMERICAN INSTITUTE OF STEEL CONSTRUCTION

3 DEFLECTIONS [Sect. L3. User Note: Conditions to be considered include levelness of floors, alignment of structural members, integrity of building finishes, and other factors that affect the normal usage and function of the structure. For composite members, the additional deflections due to the shrinkage and creep of the concrete should be considered. L4. DRIFT Drift of a structure shall be evaluated under service loads to provide for serviceability of the structure, including the integrity of interior partitions and exterior cladding. Drift under strength load combinations shall not cause collision with adjacent structures or exceed the limiting values of such drifts that may be specified by the applicable building code. L5. VIBRATION The effect of vibration on the comfort of the occupants and the function of the structure shall be considered. The sources of vibration to be considered include pedestrian loading, vibrating machinery and others identified for the structure. L6. WIND-INDUCED MOTION The effect of wind-induced motion of buildings on the comfort of occupants shall be considered. L7. EXPANSION AND CONTRACTION The effects of thermal expansion and contraction of a building shall be considered. Damage to building cladding can cause water penetration and may lead to corrosion. L8. CONNECTION SLIP The effects of connection slip shall be included in the design where slip at bolted connections may cause deformations that impair the serviceability of the structure. Where appropriate, the connection shall be designed to preclude slip. User Note: For the design of slip-critical connections, see Sections J3.8 and J3.9. For more information on connection slip, refer to the RCSC Specification for Structural Joints Using High-Strength Bolts. Specification for Structural Steel Buildings, June 22, 2010 AMERICAN INSTITUTE OF STEEL CONSTRUCTION

4 CAMBERING OF BEAMS DEFINITION: An upward deflection (usually provided during fabrication) to compensate for part of the total downward deflection anticipated for the beam. Camber in a beam can be designed to compensate for either: A certain percentage of the dead load deflection The full dead load deflection The full dead load deflection as well as a percentage of the live load deflection In practice, camber is usually designed to compensate for the dead loads. In such a case, the beam should approximately deflect to a straight line under dead loads.

5 NOTES: Cold cambering is a common method used in the shop to camber beams. The amount of cambering will depend on the beam size and yield strength. Common values are ¾ to 1-1/2 for spans up to about 40 ft. Larger values may be difficult to achieve without damaging (local buckling) the beam (consult with the fabricator).