Proposed Balanced Design Procedure There are clear advantages in using balanced design for gusset plate connections. How do we achieve it?

Proposed Design Method Design beams, columns and braces for factored design loads as current approach Establish expected plastic capacity of brace in tension (RyAgFy) and compression (1.1RyAgFcr) as currently. Effective length of brace is true brace length For connection design, propose a balance procedure to assure good seismic performance rather than current forced based method.

Proposed Design Method (2) Expected Brace Capacity < yield,1RyRyield,1 ...... < yield,iRyRyield,i And Expected Brace Capacity < fail,1Rfail,1 < fail,2Rfail,2 … and yield < fail

Balance Equations are Balancing Yield Mechanisms and Failure Modes

Proposed Design Method (3) Size weld joining the tube for the expected tensile force as with current method Compare the expected tensile yield force of the brace and the tensile fracture capacity of the brace net section with  of 0.95. Based upon the weld length and tube diameter check block shear of the gusset plate with  of .85

Why is b Equal to 0.95 for Brace Net Section?

Why is b Equal to 0.85 for Block Shear?

Proposed Design Method (4) Establish the Whitmore width by the 30o projected angle method as currently used. Establish the dimensions of corner GPs with the 8tp elliptical clearance model This can be done graphically or by an approximate equation developed in research Establish the dimensions of midspan GPs with 6tp linear (horizontal) clearance

Recommend Elliptical Clearance for Corner GP 8tp elliptical clearance Can be solved graphically or –

Recommended 6tp Horizontal Clearance Band for Midspan GP

Proposed Design Method (5) Use Whitmore width to check GP for buckling, tensile yield and tensile net section fracture. Use average length and K of 0.65 for corner gussets but K of 1.4 for midspan gussets For tensile yield compare expected tensile yield of GP to the expected tensile capacity of the brace with a  of 1.0 For tensile fracture compare the nominal ultimate tensile capacity of the plate to the expected yield capacity of the brace with a  of 0.85. (Bolts in GP?) Ignore edge buckling

Why b of 1.0 for Tensile Yielding of Whitmore Section?

Why b of 0.85 for Buckling of Gusset?

Why Ignore Edge Buckling?

Proposed Design Method (6) Size the welds joining GP to the beam and column to develop the full plastic capacity of GP CJP welds (or fillet welds on both sides slightly larger than tp ) of matching metal CJP welds to join the beam flanges to the column at beam-column connection Resulting GP must be stiff and strong enough to support full loads but should have no extra stiffness or resistance

Why Size Welds to GP Rather than UFM? Inelastic action included Brace yielding and buckling Overall failure mode Fracture of the gusset plate-to-frame welds Drift Capacities: -1.3% to 1.6% (2.9%)

Recommendations. What is Different? What is same? Different clearance models Avoid use of Uniform Force Method Size the gusset welds for plastic capacity of GP Use b factors rather than f factors Avoid overly conservative GP design Strive for thin, compact GP Avoid edge buckling check

Recommendations. What is Different? What is same? Same seismic design forces (ie expected plastic and buckling capacity of brace) Same limit state or failure mode checks Many of b factors are same as current f factors Use Whitmore width for GP evaluation Use Thornton model for GP buckling

While the proposed changes may appear large, the actual changes are quite modest and there is substantial precedent for such changes in the AISC Seismic Provisions

Precedents for Change While b values are increased above current resistance factors the actual design forces used in the evaluation are much, much larger than factored loads

Changes Whitmore Yielding Significant change but consistent with ductile and nonductile f of AISC 358-05 Art 2.4.1 Brace Net Section Fracture b changed from 0.75 to 0.95 but already accomplished AISC341-05 Art 6.2 User note Brace to Gusset Weld No change Block Shear b increased from 0.75 to 0.85 but also consistent with ductile and nonductile f Whitmore Fracture Gusset Plate Buckling No real change. Gusset to Beam-Column Welds The only change is demand or the force we design for

Changes Elliptical clearance method could be introduced in commentary as is the current 2tp Requirement of using the demand of the plate rather than the demand of the brace for the gusset plate weld requires specific mention possibly in AISC341-05 Art 13.3a

What are the Benefits of Proposed Design Approach Obtain 46% greater inelastic deformation capacity prior to braced fracture Result in less yielding and local damage to the beams and columns Result in thinner, smaller and more economical gusset plates

Other Braced Frame Issues - Fragility Curves

Fragility Curves are a PBSD Method Used to Predict Expected Earthquake Damage Statistically based curves derived from experimental observations estimating probability of damage given a definable engineering demand parameter Curves depend upon brace cross section, local and global slenderness, and bracing configuration

Engineering Demand Parameter Require consideration of the entire range of brace deformation Average Maximum Normalized Story Drift – AMNSD Important because the full deformation range of the brace affects the elongation and buckling

Description of Damage States

Based upon Extensive Experimental Data Considering all possible yield mechanisms and failure modes Using test results of frames with little emphasis upon component tests

SCBF Fragility Curves Current SCBFs with HSS braces Balanced Design SCBFs with HSS Braces and Rect Gussets

Rectangular vs Tapered Gussets HSS Brace Balanced Design Rectangular Gussets HSS Brace Balanced Design Tapered Gussets

HSS Brace vs Wide Flange Brace HSS Brace Balanced Design Rectangular Gussets Wide Flange Brace Balanced Design Rectangular Gussets

Other Brace Configuration and Cross Section Insufficient data for many brace systems Older tests of double angle braces shown Single Story X-bracing provides reduced deformation capacity

DS4 Damage State for Various Conditions

References Roeder, C.W., Lumpkin, E.J., and Lehman, D.E. (2011) “Balanced Design Procedure for Special Concentrically Braced Frame Connections,” Elsevier, Journal of Constructional Steel Research, Vol 67 No 11, pgs 1760-72. Roeder, C.W., Lumpkin, E., and Lehman, D.E. (2011) "Seismic Performance Assessment of Concentrically Braced Steel Frames," approved for publication, Earthquake Spectra, EERI, Oakland, CA

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