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

2. 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.

3. 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

4. Balance Equations are Balancing Yield Mechanisms and Failure Modes

5. 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

6. Why is b Equal to 0.95 for Brace Net Section?

7. Why is b Equal to 0.85 for Block Shear?

8. 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

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

10. Recommended 6tp Horizontal Clearance Band for Midspan GP

11. 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 (Bolts in GP?)
Ignore edge buckling

12. Why b of 1.0 for Tensile Yielding of Whitmore Section?

13. Why b of 0.85 for Buckling of Gusset?

14. Why Ignore Edge Buckling?

15. 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

16. 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%)

17. 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

18. 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

20. 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

21. 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

22. Changes Whitmore Yielding
Significant change but consistent with ductile and nonductile f of AISC Art 2.4.1
Brace Net Section Fracture
b changed from 0.75 to 0.95 but already accomplished AISC 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

23. 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 AISC Art 13.3a

24. 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

25. Other Braced Frame Issues - Fragility Curves

26. 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

27. 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

28. Description of Damage States

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

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

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

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

33. 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

34. DS4 Damage State for Various Conditions

35. 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
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

36. Thank You