SEISMIC STEEL DESIGN: SMRF
4.12 1997 VERSUS 1994 DESIGN
4.21 SECOND-FLOOR PANEL ZONE
4.20 SHEAR TAB-TO-BEAM WELDED CONNECTION The strength of a––161-in. weld is given as
1 1
–– ––
(0.707)( )(0.6)F16 EXX ⫽0.75(0.707)( )(0.6)(70)16 ⫽1.392 kips / in.
The fillet weld to connect the shear tab to a -in.-thick beam web must be at–58 least –14 in. thick following the minimum weld sizes of AISC Table J2.4. The limit-state equation will be derived assuming a -in. fillet weld with a design–38 strength of 8.352 kips / in. resistance and noting that the internal arm is
d⫺ X⫽26.0 ⫺X
for a fully plasticized tension / compression zone of uniform thickness. The equation of the required length X of the -in. fillet weld applied to top and–38 bottom of the shear tab-to-beam web connection,
X(8.352)(26.0⫺ X)⫽ 0.2Mu ⫽1207 kip-in.
reduces to
X2⫺ 26X⫹ 144.5⫽ 0 giving
X⫽ 8.0 in.
Thus provide an 8-in.-long, ––166-in. weld top and bottom of the shear tab. See Figure 4.10.
The regulation that requires welding in addition to bolts stems from test results at the University of California at Berkeley and the University of Texas at Austin. During cyclic loading the tests exhibited considerable slippage of bolts in shear tab connections not welded to the beam web.
4.21 SECOND-FLOOR PANEL ZONE
The 1997 UBC, CHAP. 22, DIV. IV, Section 8.3.a, mandates load combina-tions (3-5) and (3-6) for the panel zone design. Of these, (3-5) defines the bending moment acting on structural element 3, represented by [10], which yields the largest value for the panel zone design shear.
The moment computed by [10] is
100 SEISMIC STEEL DESIGN: SMRF
Mu⫽ 645.4 kip-ft
coupled with a floor shear component S⫽ 35.3 kips. The load combination formulas
1.2D⫹1.0E⫹ 0.5L⫹ 0.2S (3-5)
0.9D⫺(1.0E or 1.3W) (3-6)
contain E, which is well defined in the 1997 UBC, CHAP. 16, DIV. IV, Section 1630.1.1, under ‘‘Earthquake Loads’’ as
E⫽ Eh ⫹Ev (30-1)
as the sum of all earthquake effects, horizontal and vertical.
Regrettably, Section 2211, rather than keeping the well-organized system of CHAP. 16, adopts AISC ‘‘Seismic Provisions for Structural Steel Build-ings,’’ June 15, 1992, without incorporating it into the body of the 1997 UBC.
In the adoption, E is defined as the ‘‘earthquake load (where the horizontal component is derived from base shear Formula V ⫽ Cs Wg),’’ taking away the clear-cut mathematical definition of CHAP. 16 and leaving the engineer in the uncertainty as to whether to take the vertical component into consid-eration or not.
Formulas (3-5) and (3-6) are just a variation of the 1997 UBC, Section 1612.2.2.1, formulas (12-5) and (12-6), where E is clearly defined under the section ‘‘Notations’’ as the sum of all earthquake effects in the quoted Section 1630.
The Csdoes not appear in the UBC notation or its formulas. It comes from the transcribed notations of the AISC ‘‘Seismic Provisions’’ without being adjusted and properly incorporated into the framework of the Code.
In general, the local building code [NBC by BOCA, SBC by Southern Building Code Congress International (SBCCI), UBC by ICBO] takes prec-edence over a national standard (ACI, AISC, NDS) and often modifies its recommendations. In this text the 1997 UBC well-defined E will be used, including Ev, the vertical effect of earthquake.
It would be unwise to ignore the physics of earthquakes and the strong message sent by the San Fernando earthquake of nearly 100% g vertical component and the Northridge earthquake with nearly 200% g measured at specific locations. Still vivid in the author’s memory are the news of the nearly 100% g vertical acceleration readings by strong-motion seismograph at the Pacoima Dam during the 1971 San Fernando earthquake.
The panel zone shear is expressed by the difference of two vector com-ponents:
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4.21 SECOND-FLOOR PANEL ZONE 101
1. the reaction of the beam flanges H that can be conservatively approximated by Mu/ (d ⫺ tƒ) ignoring any contribution of the beam web,
Mu 645.4⫻ 12
H⫽ ⫽ ⫽221.3 kips
d⫺ tƒ (36.0⫺ 1.0)
less
2. the floor shear of 35.3 kips transferred by the column above the joint.
The resultant shear that intends to shear across the column web and flanges, represented by the second-order term of UBC formula (8-1),
Vu⫽ 221.3⫺ 35.3⫽ 186.0 kips
must be less than or equal to the panel zone shear strength given by UBC (8-1),
3b tcƒ cƒ2
vVn ⫽0.6v yƒ d tc p冋1 ⫹ d d tb c p册⫽0.6(0.75)(36)(16.4)(1.175) 3 ⫻16 ⫻1.892
⫻ 冋1⫹ 36⫻ 16.4⫻ 1.175册⫽ 389 kips
As the computed panel zone shear is less than the shear strength, 186 k ⬍ 389 k, the panel zone meets the 1997 UBC strength requirements. The UBC symbols for formula (8-1) are
tp⫽ total thickness of panel zone including doubler plate(s) dc⫽ overall column depth
db⫽ overall beam depth tcƒ⫽ column flange thickness
Fy⫽ specified yield strength of panel zone steel
In addition, the 1997 UBC 8.3.b, (8-2), requires a minimum panel zone thick-ness
dz ⫹wz 34⫹ 12.6
tz ⱖ ⫽ ⫽ 0.518 in (8-2)
90 90
102 SEISMIC STEEL DESIGN: SMRF
As the column web thickness twc constitutes the panel zone thickness (no doubler plates provided) and
twc⬎ tz
⫽ 1.175 in.⬎0.518 in.
this provision is also met.
For a better load transfer of the rather concentrated beam flange reactions impacting the column flanges during cyclic sway of the frame, we will pro-vide horizontal continuity plates between the column flanges, top and bottom of the panel zone, in line with the centerline of beam flanges. It is good practice to match the size of the continuity plates with the size of beam flanges rather than relying on column flange resistance to compensate for undersized continuity plates. Earthquake damage survey recorded by the author involved badly damaged column flanges torn from the column web. It was a clear manifestation that the column flange-to-web joint, in general, is not strong enough to resist the large impacting forces of a wildly swaying joint. This action delivers an impact reaction of the beam flange during dynamic, earthquake-generated cyclic loading, not quasi-static laboratory cyclic load-ing.
To disperse the concentrated effect of horizontal beam-end reactions, we will provide 5-in.-wide, 1-in.-thick continuity plates closely matching the beam flange sizes. Figure 4.10 shows the horizontal continuity plates snugly fitted and welded to both column flanges and column web.
Observation of the impact of the Northridge earthquake on SMRF struc-tures revealed surprisingly extensive panel zone damage. In a significant number of cases tearing of the column flange occurred, especially where continuity plates had not been provided.
It is common sense to spread the rather large and concentrated beam flange reaction load among the individual column components: the inner and outer column flanges and the column web. It is also a fact that the dynamic impact of the beam flange reaction caused by the cyclic sway of the frame at story levels can be significantly larger than predicted by the static force procedure (Section 1630.2). A critical view and analysis of this problem are presented in Chapters 1 and 12 of this book.