Element Behavior

The effective section properties and allowable loads are computed using the AISI and ASCE specifications, as selected in the application. However, since CFS allows you to define any general shape including holes, there are some situations that are not covered by these specifications. Therefore, some assumptions have been made as described below. These assumptions are made with reasonable engineering judgment but might not always be appropriate for your design. It is the responsibility of the engineer to evaluate these assumptions and determine the validity of the results. For more information about specific effective width calculations, utilize the Trace feature to get the calculation details.

Unstiffened Elements

The first and last elements of an open part are considered unstiffened. However if the free edge is in tension, the element is considered stiffened.

If an unstiffened element contains a hole, any ineffective area exclusive of the hole is removed from the section. Thus if a hole is entirely within the ineffective portion, the element has the same capacity with or without the hole.

Partially Stiffened Elements

The second and second to last elements of an open part are considered partially stiffened, if it is entirely in compression and the adjacent edge element is treated as unstiffened. Otherwise it is treated as stiffened.

If a partially stiffened element contains a hole, the two flat portions of the element on each side of the hole are treated as unstiffened elements. Further, the plate buckling coefficients for these two elements are reduced below the normal value for unstiffened elements, just as a partially stiffened element has a reduced plate buckling coefficient. These plate buckling coefficients are proportioned by the same ratio as the width reduction factor for the partially stiffened element.

Stiffened Elements

All elements between the third and third to last element of an open part and all elements of a closed part are considered stiffened, except for cases where elements are identified by CFS as having intermediate stiffeners (see Intermediate Stiffeners below). The plate buckling coefficient (k) for stiffened elements is computed based on the stress gradient per AISI and ASCE.

Three Element Parts

If a part has three elements, the middle element is considered fully stiffened.

One Element Parts

If a part has only one element, it is treated as an unstiffened element. If both edges are in compression, the ineffective area is removed from each edge, proportioned by the relative stresses at the edges.

Intermediate Stiffeners

The AISI Specification has provisions for elements with multiple intermediate stiffeners. CFS applies this new provision by first searching for elements that may contain intermediate stiffeners. Starting from the second element and continuing through to the second-to-last element, intermediate stiffeners are identified when a set of two or more elements are found to be collinear (parallel and along the same line). There are some conditions that disqualify all or part of the section as having intermediate stiffeners:

CFS uses this method even if there is only one intermediate stiffener. If a stress gradient exists along the stiffened element, the method is still applied using the average compressive stress in the element.

Earlier editions of the AISI Specification contained different provisions for multiple intermediate stiffeners, but CFS uses the above method for all cases, including stainless steel design.

Stiffness Override

If you input an element stiffness (k), it is used with the average compressive stress in the element, and the width of the compressive portion of the element to determine the effective width. The position of the ineffective portion is determined using the same method as for stiffened elements with a stress gradient, even if the override is applied to an edge element. Elements with holes cannot have a stiffness override.

Cover Plate with Intermittent Fasteners

The AISI Specification has special provisions for determining the effective width of cover plate segments connected to fluted sections with intermittent fasteners. To have CFS perform these calculations, define the cover plate as consecutive elements in the same direction, where the beginning and end of each element defines the fastener location. Then enter a non-zero plate buckling coefficient (k>0) for those elements connected by intermittent fasteners (including edge elements) and define the fastener spacing as the Connector Spacing on the Section tab of the Section Inputs.

These provisions only apply to the flexural strength of the cover-plated member with the cover plate under uniform compression. For axial load or cover plates under non-uniform compression, CFS will use the Stiffness Override logic stated above with the k value provided.

For interior cover plate elements, CFS uses k=4 regardless of the k value provided. For edge cover plate elements, CFS conservatively uses k=1.25 rather than applying the edge stiffener provisions to determine k. The cover plate may contain any type of elements beyond the flat portion. These elements are treated as described elsewhere in this help topic. If the cover plate has a simple edge stiffener element, CFS does not use the modified edge stiffener width and stress as defined by AISI. This is an approximation which typically has little impact on the overall flexural strength calculation.

CFS does not treat the cover plate effective width calculation as a rational analysis using more conservative safety and resistance factors, unless the section parameters do not comply with the limits stated in the AISI provisions.

Large Radius Arc Segments

The 2012 and later editions of the AISI Specification limited R/t to 10. The Commentary provides a rational analysis for reducing the plate buckling coefficients for elements adjacent to large radius bends. This rational analysis is employed by CFS for 10 < R/t < 30.

For bends with larger R/t, CFS may reduce the effective area in the bend itself. The effective area calculations for cylindrical tubular members under compression have been extended in CFS for use on arc segments of non-cylindrical sections. This provides continuity in member strength for sections that are nearly cylindrical.

Extreme Fiber

If the extreme compression fiber of the section is the tip of an unstiffened element, application of the AISI and ASCE Specifications might produce conservative allowable moments. If the element is not fully effective, the depth of the section may be reduced dramatically. CFS uses the reduced depth for the effective moment of inertia and effective section modulus.

Shear Distribution

For sections with multiple webs, the shear strength is determined as the sum of the shear strengths for each web. The shear strengths are calculated for unreinforced webs. For angled webs, the shear strength is multiplied by the cosine of the angle between the load direction and element direction. If all web elements are perpendicular to the load direction, the shear strength is zero in that direction.

In computing the shear strength of webs with holes, CFS assumes the holes to be non-circular, which results in a capacity slightly less than for circular holes. Although the AISI provisions only apply to C-sections with centered holes, CFS uses the same calculations for other conditions.