Product Development and DSM

Rational Analysis vs. Chapter F For a new section, outside the boundaries of the pre-qualified limits of DSM you begin with two options for the resistance (safety) factor A1.1(b): F:

Rational analysis incentive

New pre-qualified members, or extending the bounds on pre-qualified members “No formal method yet exists in the Specification for extending the Direct Strength Method to additional (new) pre-qualifed members, or extending the bounds for existing pre-qualified members. Currently, to extend the bounds or add an additional cross-section, a formal ballot vote of the AISI Committee on Specifications is required.” (DSM Guide page 75)

New Cross-section Consider the case where –one is interested in pre-qualifying a new cross-section. Suggested procedure –Do three or more tests. –Generate a  using Chapter F, but use the test-to-predicted ratio with DSM as the prediction to generate the professional factor P m, and the C.O.V. V P. 1 –If  is greater than a DSM pre-qualified cross-section (  =0.9 for beams,  =0.85 for columns) this is strong evidence that the cross-section should be pre-qualified. –The proposed bounds for the new cross-section would be the bounds of the testing. (1) (In a method relying solely on testing P M is 1.0 and V P is the scatter in the test results, here P m is the mean accuracy of the predicting method, and V P is the scatter in the predicting method).

Extending the bounds C onsider the case where –engineering judgment makes it clear that a cross-section fits in one of the pre-qualified categories, but geometric bounds are violated. –a large battery of tests cannot be performed Suggested Procedure –Perform at least three tests and determine the average test-to- predicted ratio for DSM (P m *) 1. –If the P m * of the three tests is equal to or greater 1 than the comparable pre-qualified category (see table on next slide), then the bounds should likely be extended. (1) Estimating the mean with a small number of tests is more reliable than estimating V P, so this method assures that the reliability (in essence,  ) is met, by assuming that the variation (V P ) in the new cross-sections is equal to that of the underlying category. (2) In no way does a lower Pm* preclude that the cross-section should be pre-qualified, but additional testing will likely be required.

The existing statistics

Rational analysis incentive

Pilot tests for a new cross-section Consider the case where –Pilot tests on a new cross-section are being considered –A large battery of tests cannot be performed at the time. Suggested procedure –Perform at least 3 tests –get average test-to-predicted ratio for DSM (P m *). –For the C.O.V. (V P ) assume the worst V P observed, 15% 1. –Set the correction for sample size, C p to 1.0. –Find  via Chapter F. If the  produced is greater than or equal to DSM pre-qualified (  =0.9 for beams,  =0.85 for columns) this is evidence that the cross-section should be pre-qualified. 2 (1) Estimating the mean with a small number of tests is more reliable than estimating V P, so this method attempts to assure that the reliability is met, by assuming that V P is the most conservative observed to date. (2) A lower  is not cause for immediate rejection, but does suggest more tests may be needed.

Notes : The preceding procedures do not guarantee the cross-section will be pre-qualified; they are an attempt to provide manufacturers with the best current advice. Currently, testing and calculation evidence would need to be taken to the AISI Committee on Specifications in the form of a ballot for consideration to extend pre- qualified cross-sections. All test results would need to be available to the public. All quantities should be measured, not nominal. The thickness should be the measured base metal thickness, the yield stress should be based on tensile coupons from the as-formed cross- section, and the dimensions should be based on direct measurement.

Other bugaboos Shear –If the webs are flat existing AISI main Spec. will do, DSM Design Guide gives rational analysis options for avoiding testing, but it is more involved than just the finite strip method at this point Web crippling –Depends greatly on the details you use, but the AISI Spec. method is highly empirical and likely not readily extended – may have to perform tests System issues (how you brace etc.) –How you brace, how you ship, etc. etc. all have a large impact on what you do and what you want – many of which clearly drive things more than just what we discussed today