serviceability design considerations for steel buildings

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					Abstract Serviceability Design Considerations for Steel Buildings AISC Design Guide 3, 2nd Edition This design guide, originally titled “Serviceability Design Considerations for Low-Rise Buildings”, was published in 1990, and as the title indicates, was limited to low-rise buildings. The original design guide was jointly sponsored by AISC and MBMA. This new edition brings the guide up to current standards, and is expanded in scope to address steel buildings in general. The principle addition will be considerations related to mid-rise and high-rise buildings. In this area, in particular, Mr. Larry Griffis was the lead contributor. The major areas for revisions and additions are: 1. Addition of a review of steel building types, occupancies and serviceability design considerations related to each, as applicable. 2. Addition of material specific to mid-rise and high-rise steel buildings. 3. Revision to current editions of references, ASCE 7, IBC, etc. 4. Inclusion of AAMA guidelines for skylights and curtain walls. 5. Editorial rewrite of the section on cladding, adding a few missing criteria. 6. Addition of material on ponding for roof design. 7. Addition of material on floors, including additional discussion regarding cambering beams and how deflection issues relate to the casting of concrete slabs. 8. Revision of floor vibrations following Design Guide No. 11. 9. Revision of Tables consistent with revisions to text. As in the first edition, the Guide emphases that these are design considerations not codified requirements. While the AISC Specification and Building Codes address serviceability to a degree, this Design Guide more fully addresses this topic as a design guide. Prepared by James M. Fisher and Michael A. West

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SERVICEABILITY DESIGN CONSIDERATIONS FOR STEEL BUILDINGS AISC DESIGN GUIDE 3, 2ND EDITION James M. Fisher, Ph.D., P.E. Michael A. West, P.E. Larry G. Griffis, P.E. This paper is written to present a brief synopsis of the second edition of AISC Design Guide 3, “Serviceability Design Considerations for Steel Buildings”. The Design Guide, originally titled “Serviceability Design Considerations for Low-Rise Buildings”, was published in 1990. As the title indicates, it was limited to low-rise buildings. Serviceability is defined in the AISC Specification as “a state in which the function of a building, its appearance, maintainability, durability, and comfort of its occupants are preserved under normal usage”. Although serviceability issues have always been a design consideration, changes in codes and materials have heightened attention to these matters. Numerous serviceability design criteria exist, but they are spread diversely through codes, journal articles, technical committee reports, manufacturers’ literature, office standards and the preferences of individual engineers. As in the case of the first edition, this Design Guide gathers these criteria for use in establishing serviceability design criteria for a project. The new title “Serviceability Design Considerations for Steel Buildings” reflects the addition of information on tall buildings and the following more general information: 1. A review of steel building types, occupancies and serviceability design considerations related to each, as applicable. 2. Revision to current editions of references. 3. Information on ponding for roof design. 4. Information on floors, including discussion regarding cambering beams and how deflection issues relate to the construction of concrete slabs. 5. Revision of floor vibration information to follow AISC Design Guide No. 11, Floor Vibrations Due to Human Activity (Murray et al, 1997). Review of Building Types and Occupancies In order to assist the reader in understanding and applying the proposed serviceability design criteria, this edition provides a list of building types and occupancies and presents serviceability issues which normally are associated with these building types and occupancies; namely, Storage/Warehouses, Manufacturing, Heavy Industrial/Mill Buildings, Mercantile/Shopping Malls, Health Care and Laboratory Facilities, Educational, Office Buildings, Parking Structures, Residential/Apartments/Hotels and Assembly/Arenas.

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Revision to Current Editions of References. In the fourteen years since the publication of the original edition of the Guide, almost all of the referenced publications were revised and re-issued. The Guide reflects these changes. Two significant developments were the publication of the “International Building Code” and the addition of an appendix entitled, ”Serviceability Considerations” to ASCE 7-02, “Minimum Design Loads for Buildings and Other Structures”. In addition to bringing the references up to their current editions, the list of references was also expanded to support the added topic areas, such as ponding, cambering of steel beams and tall buildings. Ponding for Roof Design Although ponding relates to both the strength and stiffness of the roof structure, ponding stability is ultimately a strength design consideration; see AISC Specification Section K2 (LRFD, 1999; ASD 1989). Because of the importance of ponding stability as a design issue, and because ponding instability is a function of load and deflection, the discussion of the topic is included in the Design Guide. Construction of Concrete Slabs and Cambering The most common floor construction in many low-rise and most mid- and high-rise office and other similar structures consists of a cast-in-place concrete slab on composite steel deck supported on composite steel beams and girders. In recent years, situations that have arisen during construction have raised concerns about the flatness and levelness of floors and the means required to achieve these specified conditions. Both the use of higher strengths of steel and the use of camber in the frame have amplified the degree of concern over the topic. The owner/occupant of these structures desires that the floors be flat and level but also expects to receive the project for the most economical price possible. For the sake of economy, composite construction is often employed. By their nature, composite beams provide significantly greater strength and stiffness than the base steel beam in its noncomposite condition. Framing is commonly cambered for the expected dead load with the expectation that the beams will deflect to level during concreting. The deck or framing is rarely shored during concreting operations. While the framing system described above is common and efficient, it is not without its pitfalls in design and construction. For example, using the nominal floor elevation and nominal top of steel as the actual condition, initially the tops of the cambered beams rise above the plane established by the nominal top of steel. In all designs, a nominal thickness of concrete is established over the top of the deck. The slab thickness is generally set by strength requirements and is frequently part of the fire rating of the floor system.

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The successful concreting of floors on steel deck and framing is an art. In addition to the skills required to place and finish concrete, the work is performed on a deflecting platform. It is essential that the concrete contractor be experienced in this type of work. Also, the contractor must be informed as to the basis for the cambers specified and the expectations of the structural engineer with regard to deflections during concreting. The Engineer’s expectations for the behavior of the structure can be conveyed in the construction documents and during a preconstruction meeting. It is in the nature of structural engineering and design to overestimate loads and underestimate resistance. With regard to the calculation of expected deflections during concreting, this rubric will likely result in over cambered beams and the need to have the slab follow the cambered curve. John Ruddy in two papers on this subject emphasizes the need to accurately determine loads and the deflection response. For example, he notes that the deck will deflect during concreting and recommends that the nominal weight of the concrete slab be increased by ten percent to account for this. Additionally, while it is essential to account for the weight of workers and equipment for strength, these loads are transient and should not be overestimated in determining deflection. Perhaps Ruddy’s more significant insight is that the effects of end connection partial restraint should be considered in the calculation of deflections even though the members in question are considered simple span members. Ruddy’s proposal is to reduce the estimated simple span deflections to 80 percent of the calculated values when setting cambers. It is common practice not to camber beams when the indicated camber is 3/4-in. or less. The AISC Code of Standard Practice provides that if no camber is specified, horizontal members are to be fabricated and erect beams with “incidental” camber upward. The AISC Code also provides that beams received by the Fabricator with 75 percent of the specified camber require no further cambering. Because of the provisions, it should be expected that all framing members should have at least some upward camber at the initiation of concreting operations. However, given the limits presented there will be instances of downward deflection below level during concreting. To control the excessive accumulation of concrete in the deflected bay Ruddy (1986), quoting Fisher/West in the first edition of this Guide, recommends that the total accumulated deflection in a bay due to dead load be limited to L/360, not to exceed 1 in. The Guide’s discussion on determining and specifying camber is intended to impress upon the designer of the framing to be judicious in determining cambers and to be proactive in communicating the basis of the camber determinations. Floor Vibrations The Guide has been revised to follow AISC Design Guide No. 11, “Floor Vibrations Due to Human Activity”, by Murray, Allen and Ungar, published in 1997.

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Tall Buildings Mr. Larry Griffis contributed the material on tall buildings to the Guide. The two principal topic areas are tall building drift and the perception to building motion under the action of wind and the acceleration of the building that it induces. While the Guide does not profess to stand alone in its presentation on these topics, it does present a useful overview of what are rather esoteric subjects. Conclusion As in the first edition, the emphasis in the Guide is on design considerations not the codification of requirements. While the AISC Specification and Building Codes address serviceability to a degree, this Design Guide more fully addresses this topic as design guidance.

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