Basic Allowable Stresses for Bridges

Table 11.16 lists the basic allowable stresses for highway bridges recommended in AASHTO  Standard Specifications for Highway Bridges for ASD. The stresses are related to the minimum yield strength Fy , ksi, or minimum tensile strength Fu, ksi, of the material in all cases except those for which stresses are independent of the grade of steel being used.
The basic stresses may be increased for loading combinations (Art. 11.5). They may be superseded by allowable fatigue stresses (Art. 11.10).
Allowable Stresses in Welds. Standard specifications require that weld metal used in bridges conform to the Bridge Welding Code, ANSI/AASHTO/AWS D1.5, American Welding Society.
Yield and tensile strengths of weld metal usually are specified to be equal to or greater than the corresponding strengths of the base metal. The allowable stresses for welds in bridges generally are as follows:
Groove welds are permitted the same stress as the base metal joined. When base metals of different yield strengths are groove-welded, the lower yield strength governs.
Fillet welds are allowed a shear stress of 0.27Fu, where Fu is the tensile strength of the electrode classification or the tensile strength of the connected part, whichever is less. When quenched and tempered steels are joined, an electrode classification with strength less than that of the base metal may be used for fillet welds, but this should be clearly specified in the design drawings.
Plug welds are permitted a shear stress of 12.4 ksi.
These stresses may be superseded by fatigue requirements (Art. 11.10). The basic stresses may be increased for loading combinations as noted in Art. 11.5.
Effective area of groove and fillet welds for computation of stresses equals the effective length times effective throat thickness. The effective shearing area of plug welds equals the nominal cross-sectional area of the hole in the plane of the faying surface.
Effective length of a groove weld is the width of the parts joined, perpendicular to the direction of stress. The effective length of a straight fillet weld is the overall length of the full-sized fillet, including end returns. For a curved fillet weld, the effective length is the length of line generated by the center point of the effective throat thickness. For a fillet weld in a hole or slot, if the weld area computed from this length is greater than the area of the hole in the plane of the faying surface, the latter area should be used as the effective area.
Effective throat thickness of a groove weld is the thickness of the thinner piece of base metal joined. (No increase is permitted for weld reinforcement. It should be removed by grinding to improve fatigue strength.) The effective throat thickness of a fillet weld is the shortest distance from the root to the face, computed as the length of the altitude on the hypotenuse of a right triangle. For a combination partial-penetration groove weld and a fillet weld, the effective throat is the shortest distance from the root to the face minus 1⁄8 in for any groove with an included angle less than 60 at the root of the groove.
In some cases, strength may not govern the design. Standard specifications set maximum and minimum limits on size and spacing of welds. These are discussed in Art. 5.19.
Rollers and Expansion Rockers. The maximum compressive load, Pm, kips, should not exceed the following:
for cylindrical surfaces,

for spherical surfaces,

Allowable Stresses for Bolts. Bolted shear connections are classified as either bearing-type or slip-critical. The latter are required for connections subject to stress reversal, heavy impact, large vibrations, or where joint slippage would be detrimental to the serviceability of the bridge. These connections are discussed in Sec. 5. Bolted bearing-type connections are restricted to members in compression and secondary members.
Fasteners for bearing-type connections may be ASTM A307 carbon-steel bolts or A325 or A490 high-strength bolts. High-strength bolts are required for slip-critical connections and where fasteners are subjected to tension or combined tension and shear.
Bolts for highway bridges are generally 3⁄4 or 7⁄8 in in diameter. Holes for high-strength bolts may be standard, oversize, short-slotted, or long-slotted. Standard holes may be up to 1⁄16 in larger in diameter than the nominal diameters of the bolts. Oversize holes may have a maximum diameter of 15⁄16 in for 3⁄4-in bolts and 11⁄16 in for 7⁄8-in bolts. Minimum diameter of a slotted hole is the same as that of a standard hole. For 3⁄4-in and 7⁄8-in bolts, shortslotted holes may be up to 1 in and 11⁄8 in long, respectively, and long-slotted holes, a
maximum of 17⁄8 and 23⁄16 in long, respectively.
In the computation of allowable loads for shear or tension on bolts, the cross-sectional area should be based on the nominal diameter of the bolts. For bearing, the area should be taken as the product of the nominal diameter of the bolt and the thickness of the metal on which it bears.
Allowable stresses for bolts specified in Standard Specifications for Highway Bridges of the American Association of State Highway and Transportation Officials (AASHTO) are summarized in Tables 11.17 and 11.18. The percentages of stress increase specified for load combinations in Art. 11.5 also apply to high-strength bolts in slip-critical joints, but the percentage may not exceed 133%.

In addition to satisfying these allowable-stress requirements, connections with highstrength bolts should also meet the requirements for combined tension and shear and for fatigue resistance.
Furthermore, the load PS, kips, on a slip-critical connection should be less than

where Fs  allowable stress, ksi, given in Table 11.17 for a high-strength bolt in a slipcritical
Ab  area, in2, based on the nominal bolt diameter
Nb  number of bolts in the connection
Ns  number of slip planes in the connection

Surfaces in slip-critical joints should be Class A, B, or C, as described in Table 11.17, but coatings providing a slip coefficient less than 0.33 may be used if the mean slip coefficient is determined by test. In that case, Fs for use in Eq. (11.14) should be taken as for Class A coatings but reduced in the ratio of the actual slip coefficient to 0.33.
Tension on high-strength bolts may result in prying action on the connected parts. See Art. 5.25.3.
Combined shear and tension on a slip-critical joint with high-strength bolts is limited by the interaction formulas in Eqs. (11.15) and (11.16). The shear Æ’v , ksi (slip load per unit area of bolt), for A325 bolts may not exceed

Fatigue may control design of a bolted connection. To limit fatigue, service-load tensile stress on the area of a bolt based on the nominal diameter, including the effects of prying action, may not exceed the stress in Table 11.19. The prying force may not exceed 80% of the load.

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