P

Figure 3.56. Intermediate web stiffener details for link beams. The required strength of welds, Wf, connecting the stiffener to link flanges is Ast Fy, whereas the strength Ww of welds connecting the stiffener to the link web is Ast Fy. Ast st A y'

is the area of the stiffener.

where

Ast = area of stiffener

"srst

The weld between the stiffener and the flange is necessary to develop the rigidity of the stiffener and restrain flange buckling. The weld force is given by

For a shear link with e < 1.6Mp/Vp, intermediate stiffeners are required, at a spacing of

5 < 30tw - d/5 ... for gp = 0.08 radian 5 < 52tw - d/5 ... for gp < 0.02 radian

Linear interpolation may be used for intermediate link rotations.

For 2.6Mp/Vp < e < 5Mp/Vp, intermediate stiffeners are required at a distance of 1.5bf from each end of the link.

For 1.6Mp/Vp < e < 2.6Mp/Vp, intermediate stiffeners are required to satisfy both the aforementioned requirements.

For e > 5Mp/Vp, intermediate stiffeners are not required.

Single-sided, full-depth web intermediate stiffeners are permitted, provided the link depth is less than 24 in. The required width is given by bst > bf /2 - tw and the thickness by tst = tw

• As specified in AISC-Seismic Section 15.5, lateral bracing to the top and bottom flanges is necessary at each end of the link to prevent instability and restrain the link from twisting out-of-plane. Lateral support shall be provided at the ends of the link with a design strength of

Pbl = 0.06RyFybftf where

Ry = ratio of the expected yield stress to the minimum specified yield strength as given in Table 3.1.

3.11.2.2. Beam Design

AISC-Seismic specifies the following design requirements for the beam outside the link:

• The nominal required axial and flexural capacity of the beam shall be determined from the forces generated by 1.1 times the nominal shear capacity of the link defined as

where

Ry = ratio of the expected yield stress to the minimum specified yield strength of the link

Vn = nominal required shear capacity of the link

In determining the design capacity of the beam, the design capacity determined using the procedures from LRFD Sections C, E, F, and H may be multiplied by Ry. For Grade 50 steel, Ry = 1.1 and the enhanced design capacity becomes

• Where required, the beam shall be provided with lateral support at both the top and bottom flanges. Each support shall have a design capacity of

Pb = 0.02FyBftf where

Fy = specified yield strength of the beam bf = width-of-beam flange tf = thickness-of-beam flange

• In accordance with AISC-Seismic, Section 15.7, beam-to-column connections away from the link may be designed as pinned in the plane of the web. The connection shall have a design capacity to resist a torsional moment about the longitudinal axis of the beam, with a magnitude of

Mt = 0.02Fybftfd 3.11.2.3. Brace Design

AISC-Seismic specifies the following design requirements for the diagonal brace:

• To allow for strain hardening in the link, the nominal required axial and flexural capacity of the brace shall be determined from the forces generated by the amplified nominal shear capacity of the link defined as

1.25RyVn where

Ry = ratio of the expected yield stress to the minimum specified yield strength of the link

Vn = nominal required shear capacity of the link

• The width-thickness ratios of the brace shall satisfy the requirements of LRFD Table B5.1.

• As shown in Fig. 3.14, the intersection of the brace and beam centerlines shall be at the end of the link or within the link. In accordance with AISC-Seismic, Commentary Section C15.6c, the intersection of the brace and beam centerlines should not be located outside the link because the eccentricity creates additional moment in the beam.

• The required strength of the brace-to-beam connection shall not be less than the nominal strength of the brace. No part of the connection shall extend over the length of the link. If the brace resists a portion of the link end moment, the connection shall be designed as a fully restrained moment connection.

3.11.2.4 Column Design

To ensure that link yielding is the predominant inelastic behavior, AISC-Seismic specifies the following loading combinations for the design of the column:

Ql = forces generated by 1.1 times the nominal strength of the link. This is equal to 1.1RyVn.

Ry = ratio of the expected yield stress to the minimum specified yield strength of the link

Vn = nominal required shear capacity of the link

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