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D2.B.1 General

The design philosophy embodied in this specification is based on the concept of limit state design. Structures are designed and proportioned taking into consideration the limit states at which they would become unfit for their intended use. Two major categories of limit-state are recognized - ultimate and serviceability. The primary considerations in ultimate limit state design are strength and stability, while that in serviceability is deflection. Appropriate load and resistance factors are used so that a uniform reliability is achieved for all steel structures under various loading conditions and at the same time the chances of limits being surpassed are acceptably remote.

In the STAAD implementation, members are proportioned to resist the design loads without exceeding the limit states of strength, stability, and serviceability. Accordingly, the most economic section is selected on the basis of the least weight criteria as augmented by the designer in specification of allowable member depths, desired section type, or other such parameters. The code checking portion of the program checks whether code requirements for each selected section are met and identifies the governing criteria.

The following sections describe the salient features of the STAAD implementation of AS 4100. A detailed description of the design process along with its underlying concepts and assumptions is available in the specification document.

D2.B.1.1 Strength Limit States

Strength design capacities (ϕRu) are calculated and compared to user-defined design action effects (S*), so as to ensure that S* ≤ ϕRu in accordance with AS 4100 3.4. Details for design capacity calculations are outlined in the sections that follow.

D2.B.1.2 Deflection Limit States

STAAD.Pro’s AS 4100 implementation does not generally check deflections. It is left to the user to check that both local member and frame deflections are within acceptable limits.

Note: Local member deflections parallel to the local member y-axis can be checked against a user-defined maximum “span / deflection” ratio. This can be performed using the DFF, DJ1, and DJ2 design parameters, however this is only available for MEMBER Design. Details are provided in the sections that follow.

D2.B.1.3 Eccentric Beam Reactions

STAAD.Pro does not automatically account for minimum eccentricity distances for beam reactions being transferred to columns as per AS 4100 4.3.4. However member offsets can be used to model these eccentricities.

Refer to TR.25 Member Offset Specification for further information on the Member Offset feature.

D2.B.1.4 Limit States Not Considered

The following limit states are not directly considered in STAAD.Pro’s implementation of AS 4100.

Table 1. Limit States Not Considered in STAAD.Pro AS 4100 Design
Limit State Code Reference
Stability AS 4100 3.3
Serviceability AS 4100 3.5
Brittle Fracture AS 4100 3.7
Fire AS 4100 3.9
Other Design Requirements AS 4100 3.11

D2.B.1.5 Connection Design

STAAD.Pro and Bentley’s RAM Connection program currently do not support design of connections in accordance with AS 4100. In some cases connection design may govern the size of members. Such considerations are not considered in STAAD.Pro’s AS 4100 and should be checked by separately.

D2.B.1.6 Bolts and Welds

Bolt holes and welds are not generally considered in STAAD.Pro’s AS 4100 member design.

Note: NSC and NSF design parameters are used to manually specify a reduction in net section area for compression or tension capacity calculations. These can be used to account for bolt hole area reductions. Further details are provided in the sections that follow.