298

energy, with 76%-81% occurring over the brace diagonal. The specimen with CJP welded

beam- to-column connections (HSS-05) dissipated significantly more total energy, 6941 kip-in, with

similar distribution between brace diagonal and frame as the current specimens.

All of the TRGP specimens showed significant improvement in energy dissipation compared to

the two AISC reference specimens (HSS-01 and HSS-12). The AISC designed reference

specimen with fillet interface welds (HSS-01) dissipated 3334 kip-in of total energy with 95%

over the brace diagonal, whereas the TRGP specimen with CJP welded beam-to-column

connections (HSS-05) showed a 108% improvement in energy dissipation with the more

compact, thinner gusset plate using the Balance Design Procedure resulting in a more ductile

response of the system. The TRGP specimen with bolted web CJP welded flange connections

(HSS-24) resulted in 24.5% decrease in total energy dissipation compared the CJP welded web

and flange connection, 23.7% decrease from the brace diagonal and most notably 34.8 % less

energy dissipated through the frame. The bolted shear plate beam-to-column connection (HSS-

18) saw 28.0% less total energy dissipated by the system than the CJP welded connection. The

brace diagonal and frame dissipated 29.4% and 33.1% less energy, respectively.

The 18 bolt and 14 bolt beam end plate connections (HSS-20 and HSS-21) resulted in 50.5%

and 44.7% improvements in energy dissipated compared to the AISC reference specimen with

fillet interface welds (HSS-01), but could not achieve similar levels as the TRGP specimen with

CJP welded beam-to-column connections. The 18 bolt configuration saw a 27.7% loss in total

energy dissipation and the 14 bolt configuration lost 30.5%.

7.2.5 TRGP Summary

This comparison has shown that modifications from the CJP welded beam-to-column

connection utilized in HSS-05 resulted in reductions in ultimate drift capacity and energy

dissipation, but not necessarily in total resistance. All TRGP specimen is this test series achieve

larger total drift ranges and more desirable brace behavior that either AISC reference

specimens. The specimen with the bolted web CJP welded flange connection (HSS-24) showed

the closest similarity in response to full CJP welded beam-to-column connection (HSS-05) by

achieving the larger total drift range and dissipating more energy than the others but the

decrease in these important performance parameters was significant by only modifying the

beam web connection to the column. The bolted shear plate beam-to-column connection (HSS-

18) resulted in more significant reduction in ultimate drift capacity and total resistance of the

system, which can be attributed to the decrease in frame stiffness and early brace fracture.

2

The stiffness of the beam-to-column section also appears to influence the level of residual

deformation of the brace and gusset plate out-of-plane. Under cyclic loading, larger strain

demands are required to straighten the brace in tension once plastic deformation has occurred,

increasing the total plastic strain accumulated at the brace center. This could potentially lead to

early brace fracture at smaller drift ranges as illustrated for the bolted shear plate connection

(HSS-18), which saw larger residual out-of-plane displacements than the bolted web CJP flange

connection (HSS-24) and failed at a lower drift range, 4.19% compared to 4.44%.

Both bolted end plate connection (HSS-20 and HSS-21) exhibited decreases in ultimate drift

capacity and energy dissipation compared to the CJP welded beam-to-column connection (HSS-

05), but exhibited increased stiffness and maximum resistance. The response of the frame with

the bolted connection, resistance verses drift ratio, was also comparable to that of HSS-05 and

achieved larger resistances at a given drift level.

7.3 Beam-to-Column Connections for Thin Tapered Gusset Plates

Thin tapered gusset plates (TTGP) have shown the ability to achieve large ultimate drift

capacities and also reduce the amount of material and welding time during construction. The

geometry of the tapered plates allows for free rotations as the brace buckles out-of-plane but

also reduces the buckling capacity of the brace by increasing the effective slenderness ratio.

Specimen HSS-17 and HSS-22 utilized the identical gusset plate designs, but HSS-17 consisted

of CJP welded web and flanges, rather than bolted shear plate connection in HSS-22, adjacent

to the gusset plates. The specimen with CJP welded beam-to-column connections (HSS-17)

achieved an ultimate drift range of 4.94% and is considered the upper bound performance for

specimen with tapered gusset plates. This section evaluates the performance of HSS-22 having

the bolted shear plate beam-to-column connection by comparing it to HSS-17. The two AISC

reference specimens (HSS-01 and HSS-12) are also included in the some of the comparisons to

illustrate improved behavior of the system by utilizing the Balance Design Procedure for TTGP

specimens.

Comparison of the system response is discussed first in Section 7.3.1 followed by a

comparison of the response of the brace and gusset plates in Section 7.3.2. The response of the

framing elements are compared in Section 7.3.3 followed by a comparison of energy

dissipation for TTGP specimen in Section 7.3.4.

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7.3.1 TTGP System Response Comparison

The system responses were evaluated by comparing the lateral load verses drift response for

the two TTGP specimens and the two AISC reference specimens. Table 7.3.1 summarizes the

peak values of drift and maximum lateral load resisted, and includes a description of the

controlling failure mechanism and design parameter for each specimen. The resistance is

normalized by the theoretical lateral force associated with brace yielding calculated in Equation

(1.2.1).

Table 7.3.1: Peak Performance Summary for TTGP

SpecimenDrift Ratio, % Resistance Failure

Mechanism

DiscriptionRange Min Max P/Py

HSS-01 2.65 -1.64 1.01 -0.95 1.64Interface

Weld Fracture

Simulate AISC Design with Fillet Welds

HSS-12 3.49 -2.10 1.39 -0.90 1.86Brace

Fracture

Simulate AISC Design with CJP Interface Welds

HSS-17 4.94 -2.79 2.15 -0.79 1.77Brace

Fracture

CJP Welded Bm/Col Thin Tapered GP

HSS-22 3.98 -2.48 1.50 -0.66 1.50Brace

Fracture

Bolted Shear Plate Conn Thin Tapered GP

Both TTGP specimens achieved larger drift ratios than either AISC reference specimen. The

TTGP specimen with CJP welded beam-to-column connections (HSS-17) achieved a total drift

range of 4.94%, which is 41.5% greater than the AISC reference specimen with CJP interface

welds (HSS-12). The total drift range of the bolted shear plate connection specimen (HSS-22),

3.98%, increased by only 14.0% compared to HSS-12. Both TTGP specimen exhibited smaller

maximum negative resistance than the AISC reference specimens and the specimen with the

bolted shear plate connections (HSS-22) also exhibited lower maximum positive resistance than

any test in this comparison.

The enveloped values for the lateral load verses drift ratio response were used to compare the

global response of the system. Figure 7.3.1 compares the global responses of the two TTGP

specimens and the two AISC reference specimens in the positive direction. The responses in the

negative direction are shown in Figure 7.3.2.

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Figure 7.3.1: TTGP Load vs. Drift (Positive Envelope)

The TTGP specimens both exhibit initial tensile yielding at relatively small positive drift levels,

approximately 0.5%, but continued to increase resistance as drift increased. The AISC

reference specimens, especially the specimen with fillet interface welds (HSS-01), retained

stiffness until a more clearly defined point of yielding occurred at approximately 1.30%. The

TTGP specimen with CJP welded beam-to-column connections.

Figure 7.3.2: TTGP Load vs. Drift (Negative Envelope)

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In the negative direction, the point of brace buckling can be seen followed by the nonlinear

response of the system with the brace in compression. Both TTGP specimens exhibited

similar behavior post buckling, maintaining or slightly decreasing resistance at larger drift

levels. The CJP welded beam-to-column connection (HSS-17) exhibited both a greater

resistance at buckling and maintained a larger resistance until failure than the bolted shear

plate connection (HSS-22). The AISC reference specimens exhibited greater buckling

capacity and compressive resistance over the nonlinear response than either TTGP specimen,

which is expected considering the larger gusset plates and stiffer brace end supports.

7.3.2 TTGP Brace and Gusset Plate Comparison

The response of the brace and gusset plates for the two TTGP specimens (HSS-17 and HSS-22)

are compared in this section to evaluate the effect the beam-to-column connection between has

on brace and gusset plate behaviors. The two AISC reference specimens are including in some

of the comparisons to illustrate how the behavior of the thin tapered gusset plates differ and are

more beneficial than the gusset plates design to simulate current AISC procedures.

The brace forces and nonlinear brace response are compared in Section 7.3.2.1. Comparison of

the brace out-of-plane displacement and gusset plate rotations are included in Section 7.3.2.2.

Brace and gusset plate elongation in tension are compared in Section 7.3.2.3.

7.3.2.1 TTGP Brace Force Comparison

Comparisons of the brace force and nonlinear brace responses are evaluated in this sub-section.

First, the critical buckling capacity of the brace, , was determined from strain gauge data

directly on the brace. Values obtained from the tests were compared to the design nominal

buckling capacity, , in Table 7.3.2. Also included in this table is

the ratio of the experimental critical buckling load over the nominal design value and the

experimental effective length factor, , and

slenderness ratio, .

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Table 7.3.2: TTGP Experimental Buckling Capacity Comparison

Design ParameterExperimental AISC Exp/

AISC Pcr/Pn

Experimental Effective

Length Factor, K

Experimental Slenderness Ratio (Kl/r)

Pcr Pn=FcrAg

AISC w/ Fillet 209.5 200.7 1.04 0.94 67.4AISC w/ CJP 195.7 200.7 0.98 1.04 74.5CJP Welded

Beam/Col Conn 208.8 176.7 1.18 0.81 67.7

Shear PlateBeam/Col Conn 176.3 176.7 1.00 1.00 84.3

The buckling capacity of the brace for the TTGP specimen with CJP welded beam-to-column

connections (HSS-17) was significantly larger than the nominal design value. The resulting

effective length factor, , equaled 0.81, which is surprising considering the increased flexibility

of the brace end connection because of the thin gusset plates and tapered geometry compared

to the effective length factors observed with the AISC reference specimens. The bolted shear

plate specimen exhibited an experimental buckling capacity equal to the design value and

equal to 1.0.

The nonlinear bracer response is plotted for the two TTGP specimens in Table 7.3.3. The

hysteretic plots were created using the brace strain gauge data and the elongation over the

brace diagonal, work-point to work-point, as described in Chapter 6, Section X 6.3.2.1.

Table 7.3.3: TTGP Brace Response

CJP

Wel

ded

Bea

m/C

ol C

onn

.

Shea

r P

late

Bea

m/C

ol C

onn

.

A comparison of the nonlinear response of the brace in compression was made to evaluate the

degradation for brace capacity post buckling. Figure 7.3.3 shows brace compression over total

drift range and illustrates the level of force degradation exhibited by the two TTGP specimens

with different beam-to-column connection types. The compressive force is normalized by the

design nominal buckling capacity. Again, the nonlinear brace force used for this comparison was

calculated by subtracting the columns shear described in Chapter 3, Section 6.4.2 from the

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applied lateral load to the system. Table 7.3.4 is provided summarizing the compressive response,

including

3

the ratio of minimum compressive capacity prior to failure, , over the critical

buckling capacity.

Figure 7.3.3: TTGP Compressive Degradation Comparison

The ratio of capacity for the TTGP specimen with the bolted shear plate beam-to-column

connection (HSS-22) was significantly less than that observed for the CJP welded connection

(HSS- 17), 0.10 compared to 0.26.

Table 7.3.4: TTGP Brace Compression Capacity Degradation Ratio

Design Parameter Pcr* Pmin Pmin/*Pcr

CJP Welded Beam/ColConn 203.25 53.3 0.26

Shear Plate Beam/ColConn 160.57 15.9 0.10

* critical buckling load calculated by subtracting column shears from applied lateral load for horizontal componet resisted by brace

7.3.2.2 TTGP Brace Displacement and Gusset Plate Rotation Comparisons

The thin tapered gusset plates provide a more flexible end support for the brace and typically

result in brace behavior more closely resembling a pinned-pinned compressive member. This

section evaluates the behaviors of the brace and gusset plates in compression by comparing the

3

brace

3

center displacement and gusset plate rotations for the two TTGP specimens. Also included in

the comparison is the AISC reference specimen with CJP interface welds (HSS-12) to illustrate

behavioral differences between rectangular gusset plates design to simulate current AISC

procedures and thin tapered plates designed following the BDP. Brace out-of-plane

displacement as a percent of the total brace length are compared in Figure 7.3.4.

Figure 7.3.4: TTGP Brace Out-of-Plane Displacement Comparison

Both TTGP specimens exhibited similar out-of-plane behavior at a given drift range compared

to the AISC reference specimen regardless of gusset plate detail. The displacement of the brace

for the AISC reference specimen does jump at approximately 2.5% total drift, where as the

TTGP continue to increase steadily until failure. The specimen with bolted shear plate

connections (HSS-

22) did exhibited slightly larger displacements than the CJP welded connections (HSS-17)

for a given drift range, which achieved a larger maximum out-of-plane displacement prior to

brace fracture at a greater maximum drift range. The gusset plate rotations at the NE

connection are compared in Figure 7.3.5.

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Figure 7.3.5: TTGP NE Gusset Rotation Comparison

As expected, the gusset plate rotations for both TTGP were greater than those achieved the

AISC reference specimen at a given drift range. The specimen with bolted shear plate

connections achieved larger rotations at smaller drift ranges than the CJP welded connection but

the response of the specimens with thin tapered gusset plates was similar at drift levels greater

than1.5%. It should be noted that interface welds at both gusset plates of the specimen with CJP

welded beam-to- column connection exhibited severe tearing of the base material prior to failure,

which would have reduced the rotational stiffness of the connection. The bolted shear plate

connection only experience moderate tearing of the interface welds. The residual brace out-of-

plane displacement and NE gusset plate rotations are compared in Figure 7.3.6 and Figure 7.3.7.

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Figure 7.3.6: TTGP Residual Brace OOP Displacement Comparison

Figure 7.3.7: TTGP Residual NE Gusset Plate Rotation Comparison

Similar to the behavior of the brace with thin rectangular gusset plates, the specimen with

bolted shear plate connections exhibited larger out-of-plane displacement at the brace center

and larger gusset plate rotations. Greater residual out-of-plane displacements and gusset plate

rotations suggest increased the strain demands over the brace diagonal to straighten in tension,

causing local deformation of the brace plastic hinge to occur at smaller drift ranges, shortening

the life of the brace.

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7.3.2.3 TTGP Brace and Gusset Plate Elongation Comparisons

Thicker gusset plates increase yielding in the brace section by concentrating all the elongation

to the brace section. Thin gusset plates connections designed using the Balanced Design

Procedure and a β factor of 1.0, regardless of geometry, have typically exhibited yielding in

both the brace and the gusset plates. Brace elongations for the two TTGP specimens (HSS-17

and HSS-22) and the AISC reference specimen with CJP interface welds (HSS-12) are

compared in Figure 7.3.8.

Figure 7.3.8: TTGP Brace Elongation vs. Total Drift Range

The two TTGP specimens show similar elongation behavior but CJP welded beam-to-column

connection achieved a greater maximum value by reaching a larger total drift range. The AISC

reference specimen exhibited the greatest brace elongation for a given drift range beyond 1.5%

in this comparison because of the increased axial stiffness of the thicker gusset plates.

Gusset plate elongations for TTGP specimens and AISC reference specimen HSS-12 are

compared in Figure 7.3.9. At smaller total drift ranges, the gusset plates for bolted shear plate

beam-to- column showed larger gusset plate elongation but at approximately 2.5% drift range,

gusset plate yielding was capped. The gusset plates for the CJP welded beam-to-column

connections, however, continued to yield as total drift range increased. Both TTGP specimens

experienced greater gusset plate elongation for a given drift range than the ½” AISC designed

gusset plate from HSS-12.