Erection Bracing of Low-Rise Structural Steel Buildings phần 5 pps

Column: 1(40)25 = 1,000 lbs.

Beams: 2(35)40(0.5) = 1,400 lbs.

Girders: 2(68)40(0.5) = 2,720 lbs.

Roof framing (40)40(5) = 8.000 lbs.

Total = 13,120 lbs. = 13.1 kips

Gravity load: 13.1 kips lbs.

Wind vertical component: 5.9 kips

Net compression on anchor rods: 7.2 kips

Using load factors per the AISC LRFD Specification:

Pu

= 0.9D 1.3W=0.9 (13.1)-1.3 (5.9) = 4.1 kips

(compression)

Pu

= 1.2D-1.3W= 1.2 (13.1) -1.3 (5.9) = 8.1 kips

(compression)

Vu = 1.3(W) = 1.3 (9.4) = 12.2 kips

Check resistance of (4) 1 in. diameter anchor rods.

Grout thickness is 3 in. Anchor rods have heavy hex lev￾eling nuts and 3/8 in. plate washers. Anchors are spaced

at 10 in. centers and are embedded 12 in.

Anchor rods: ASTM A36

Concrete: f' c

= 3500 psi

Force to each anchor rod:

Axial: 8.1 ÷ 4 = 2.0 kips (compression)

Shear: 12.2 ÷ 4 = 3.1 kips

Using procedure from Section 4.2.4 for axial load:

k = 1.0

Ab

= 0.7854in.2

= 3-(0.375+1)= 1.625 in.

r = 0.25 (d) = 0.25(1) = 0.25 in.

kL/r = 1(1.625)70.25 = 6.5

= 30.53 ksi per LRFD Table 3-36

Bending:

Moment arm = 0.5 (3 - (0.375 + 1)) = 0.81 in.

Mu = 3.1 (0.81) = 2478 in.-lb. = 2.5 in.-kip

= 0.9 (36) 0.167 = 5.4 in.-kip

where

Zx = d3

/6 = (1)3

/6 = 0.167 in.3

Fy = 36 ksi

= 0.9

Using LRFD Eq. H1-16

0.50 < 1.0 o.k.

It should be noted that the anchor rods must be adequate￾ly developed to resist a punch through failure per Sec￾tion 4.2.5.

Design strength in shear using the procedure and nota￾tion in UBC-94:

Vss = 0.75 Ab

f' s

Vss = 0.75(0.785)58 = 34.1 kips

0.85(800)(0.785)(1)(3500)1/2 (1/1000)

=31.5 kips

Vu = 3.1 kips

3.1 <31.5 o.k.

In this example the loads, load factors and load com￾binations resulted in a net compressive force on the an￾chor rods. To illustrate the calculation procedure, using

a net tension force the example continues using a Pu

=

8.1 kips tension. All other design parameters remain un￾changed.

Force to each anchor rod:

Axial: 8.1 ÷ 4 = 2.0 kips (tension)

Shear: 12.2 ÷ 4 = 3.1 kips

Using the procedure and notation in UBC-94

Design strength in tension:

where

1.0 for normal weight concrete

(2.8 As

+ 4At

) represents the surface of a truncated fail￾ure surface cone as presented elsewhere in this guide as:

where

© 2003 by American Institute of Steel Construction, Inc. All rights reserved. 37

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