Steel member design/check assumptions

This section lists the main assumptions that are made in the steel member design module. Some of these assumptions are also described in the previous sections of this chapter and others are listed only in this section. It is up to you to check that these assumptions are suitable for your situation.

 

Note that some of the following general assumptions may be overridden by the code specific items listed in the sections immediately following this one.

  1. Frame imperfections are not automatically allowed for during the design phase. When applicable (usually for multi-storey frames), you should apply notional horizontal forces or initial deformations to the analysis model in accordance with the requirements of the design code.
     

  2. The top flange of a member is the flange on the positive local y-axis (or z-axis if the section has been flipped) side of the member. The top flange of a member can be easily determined by displaying the member local axes graphically and observing the direction of the local y-axis (or z-axis if flipped).
     

  3. The top flange of a group as a whole is defined such that it is the same as the top flange of the first member in the group.
     

  4. The critical flange at any point within a segment is assumed to be the compression flange unless either end of the segment is laterally unrestrained in which case it is assumed to be the tension flange. SPACE GASS is not able to determine whether a loading condition is predominantly due to gravity or wind and you should therefore check that the above rule is valid for your situation.
     

  5. All section and member capacities are calculated assuming that stiffeners do not exist.
     

  6. The AS4100, AISC-LRFD, BS5950, EUROCODE 3, HK CP2011, IS800 and NZS3404 modules assume that second order effects have been taken into account by a second order elastic analysis. Moment magnification is not considered.
     

  7. The AS4100, AISC-LRFD, BS5950, EUROCODE 3, HK CP2011, IS800 and NZS3404 modules assume that the design load cases are factored (ultimate).
     

  8. For single angle sections, the effective lengths and flange restraints must be input relative to the non-principal axes. For all other sections, they must be input relative to the principal axes.
     

  9. The compression effective lengths Lmx and Lmy, used by AS4100 and NZS3404 in clause 8.4.2.2 for the calculation of Nc when ke=1.0, are assumed to be equal to the lesser of the total design group length and the normal compression effective lengths for the segment under consideration. Lmx = MIN(Ltot,Lcmajor) and Lmy = MIN(Ltot,Lcminor), where Ltot is the total design group length and Lcmajor and Lcminor are the normal compression effective lengths.
     

  10. The torsion effective length used by AS4100 and NZS3404 is assumed to be equal to the distance between adjacent full or partial restraints.
     

  11. A C (continuous) flange restraint is assumed to be equivalent to a series of L (lateral) flange restraints spaced at increments of 1mm for the length of the continuous restraint.
     

  12. If C (continuous) or I (ignore) flange restraints are repeated without R, S, F, P or L restraints inbetween (eg. CCC, III or CI) then the last C or I restraint is used and the previous repeated ones are discarded.
     

  13. If an intermediate flange restraint is positioned at the beginning or end of a design group then it is ignored in favour of the appropriate end flange restraint.
     

  14. Member offsets are automatically ignored (skipped over) during a steel member design/check provided that they occur at the ends of a design group. They are treated the same as I (ignore) flange restraints.
     

  15. The extra restraint condition in AS1250, SABS0162, BS5950, HK CP2011 and AS3990 that provides partial restraint against lateral rotation (about the cross section minor axis) of the critical flange is not supported.
     

  16. Because it is difficult for SPACE GASS to determine whether a member is a true cantilever or not, AS1250 clause 5.9.4, SABS0162 clause 7.2.3(b), BS5950 clause 4.3.5.4/4.3.5.5, HK CP2011 clause 8.3.4.3, IS800 clause 8.3.3 and AS3990 clause 5.9.4 have not been considered. This may cause the bending effective length for cantilevers to be underestimated and you should therefore check the bending effective length for cantilevers calculated by the AS1250, SABS0162, BS5950, HK CP2011 and AS3990 modules.
     

  17. When calculating kt for AS4100 or NZS3404, if the critical flange switches from top to bottom within the segment, the critical flange thickness is assumed to be the thickness of the flange at the end of the segment.
     

  18. When calculating kl, SPACE GASS assumes conservatively that top flange loads always occur within the segment rather than at the segment end(s).
     

  19. kl is calculated for "downwards" loads regardless of the member orientation and flange positions. A "downwards" load is assumed to act in the direction from the top flange to the bottom flange. If you want kl=1.0 for columns, sloping beams or beams on their side then you should set the load height position to "Shear centre" regardless of the loaded flange or the load direction.
     

  20. The direction of the transverse load acting on a segment is determined by the sign of the difference in shear force between the two segment ends.
     

  21. AS1250, SABS0162, BS5950, HK CP2011, IS800 and AS3990 do not give specific rules for calculating kr for all combinations of flange restraints at the ends of the segment. In such cases interpolation has been used to calculate some of the values of kr.
     

  22. Eccentric end connection effects (if not suppressed) are taken into account in different ways depending on the design code being used. In most cases, the eccentric end moments are simply added to the normal design moments for the entire design group. Exceptions are BS5950 which optionally uses the provisions of clauses 4.6.3 (tension) or 4.7.10 (compression) and AS4100 and NZS3404 which use a Kt factor for tension members (if activated).
     

  23. Where applicable (see previous item), moments due to eccentric end connection effects for angles, channels and Tee sections subjected to axial loads are added to the normal design moments only if they don’t cause a net reduction in the final design moment.
     

  24. Eccentric end moments are calculated by multiplying the axial force by the distance from the centroid of the connected plate to the centroid of the cross section.
     

  25. The major axis of a single or double angle section is assumed to be parallel to the short leg(s) of the section.
     

  26. Double angle sections are assumed to have no space between the individual angle sections.
     

  27. The AS1250, SABS0162 and AS3990 modules assume that double angles are connected together at intermediate points sufficient to ensure that half of the design axial compressive force for the combined section does not exceed the compressive capacity of each angle section considered individually using an effective length (for buckling of the sections away from each other) equal to the distance between connection points.
     

  28. The AS1250, SABS0162 and AS3990 modules consider only axial forces and shears for single or double angle sections. Bending moments are not considered. Eccentric end moments are considered where applicable. The AS4100, BS5950, HK CP2011, EUROCODE 3 and NZS3404 modules consider axial forces, shears (along minor axis) and bending moments (about both axes) for single or double angle sections.
     

  29. The AS4100, NZS3404, AISC-ASD, AISC-LRFD, EUROCODE 3, BS5950, IS800 and HK CP2011 modules convert double angle sections into the equivalent Tee section and then treat them as a solid Tee shape. The AS4100, NZS3404 and HK CP2011 modules do not support double starred angles.
     

  30. Beam Tees have the major axis parallel to the flange and are therefore assumed to have their web vertical (assuming a zero direction angle and no flipping).
     

  31. Column Tees have the major axis parallel to the web and are assumed to be lying on their side with their flange vertical (assuming a zero direction angle and no flipping).
     

  32. The AS1250, SABS0162 and AS3990 modules do not support column Tee sections.
     

  33. The AS4100 and NZS3404 modules do not support welded Tee sections unless they are beam Tees with d/t<15 (lightly welded longitudinally) or d/t<14 (heavily welded longitudinally).
     

  34. The AS4100 and NZS3404 modules assume that heavily welded (longitudinally) I and H sections with equal flanges are flame cut. Lightly welded (longitudinally) or unequal flanged I and H sections and all plate web girders are assumed to be welded "as rolled".
     

  35. The AS1250, SABS0162 and AS3990 modules do not support welded box sections.
     

  36. The AS4100 and NZS3404 modules do not support welded circular hollow sections, channels or angles.
     

  37. The AS4100, BS5950, HK CP2011, IS800 and NZS3404 modules assume that channel sections have equal flanges.
     

  38. The AS4100, BS5950, HK CP2011, IS800 and NZS3404 modules assume that angle sections have uniform plate thicknesses throughout the section.
     

  39. The AS4100 and NZS3404 modules do not support solid sections.
     

  40. The BS5950 and HK CP2011 modules assume that solid sections are class 1.
     

  41. When calculating the area removed from the section due to a bolted end connection, SPACE GASS assumes that the bolts are through the web(s) unless the end connection type is specified as "F", in which case the bolts are assumed to be through the flange(s).
     

  42. The area removed from the section due to a bolted end connection is assumed to apply for a distance of 250mm from each end of the design group.
     

  43. The BS5950 module assumes conservatively that single angle sections are connected with a single fastener for clause 4.7.10.
     

  44. The AS4100 and NZS3404 modules perform a web capacity check in accordance with appendix I. If the check fails, SPACE GASS treats it as a warning rather than a failure condition.
     

  45. Serviceability requirements are not considered automatically. They must be checked manually by direct inspection of displacement diagrams.
     

  46. Torsional effects are not considered.
     

  47. Member end bearing capacity is not considered.
     

  48. For the AS4100 and NZS3404 modules, am is calculated using the formula in clause 5.6.1.1(a)(iii) when the segment is restrained at both ends. If the segment is unrestrained at one end, AS4100 and NZS3404 require the bending moment diagram to be matched to one of the three diagrams shown in table 5.6.2. This is very difficult when the bending moment diagram could be any conceivable shape. Therefore, SPACE GASS uses am = 0.25 if there is a non-zero moment at the unrestrained end, am = 2.25 if the mid-segment moment is less than 25% of the restrained end moment, am = 1.25 if the mid-segment moment is less than 50% of the restrained end moment or otherwise am = 1.0. This is less conservative than the approach adopted in v12.25.334 and earlier versions..
     

  49. Shear force in the major axis direction is not considered.
     

  50. If any term in the steel member design failure equation becomes negative, it is assumed that the section has failed and a value of 9.99 is used in place of the negative value.
     

  51. The brace, purlin, girt or other member that provides full, partial or lateral restraint to the critical flange of a member must be capable of resisting the force required to provide such restraint. This is not automatically allowed for in the analysis or design. If you wish to take this into account then you should add the restraint forces to your applied loads. The restraint forces are code specific and you should refer to the appropriate clauses for the design code you are using.

    This effect is particularly important for deep beams where the forces required to restrain the critical flange can be quite high. You should check that your model is capable of withstanding these forces.
     

  52. Built-up, haunched, tapered, non-standard, mirrored or rotated sections cannot be used in the design/check modules.