ICE LOAD AND CONCURRENT WIND LOAD

Design code: ASCE 7-02 & 05

Section 10.4: Ice load due to freezing rain

Important factors and equations:

1. ASCE 10.4.1: Ice load shall be determined using the weight of glaze ice form on all expose surfaces of the structural members

2. Cross section of ice on structural shape, prismatic members, and other similar shapes, shall be

A_i = p t_d (D_c + t_d)

Where

A_i is cross section of ice, t_d is thickness of ice, D_c is equivalent diameter of structural shape as

3. For large plate, and large three-Dimentional objects such as domes and sphere, volume of ice shall be determined as

V_i = p t_d A_s

For flat plate A_s is flat area of plate.

For domes and spheres, As = p r². r is radius of domes or spheres.

V_i can be multiply by 0.8 for vertical plate, 0.6 for horizontal plate.

4. Ice density is 56 lb/ft³ minimum.

5. Normal ice thickness, t, due to freezing rain at a height of 33 ft and the concurrent wind speed is determined from Figure 10-2, 10-3, and 10-4 on ASCE 7-02.

 6. Adjustment factor for height is

f_z = (z/33)^0.1 for 0 to 900 ft., fz = 1.4 for 900 ft and higher.

7. Important factor is listed in Table 10-1.

8. Topographic Factor,

K_zt = (1+K₁+K₂+K₃)²  where K₁, K₂, K₃ are determined from Figure 6-4 of ASCE 7-02.

9. Design ice thickness is  determined as

t_d = 2 t I_i f_z (K_zt)^0.35

 Section 10.5 Wind on ice-covered structures.

Important factors and equations:

1. The projected area exposed to wind shall be increased by t_d to all boundry.
2. Wind load on iced structure shall be based on concurrent wind speed, V_c from Figures 10-2,10-3, and 10-4 instead of 3 second gust from wind storm in Section 6.
3. Procedure for calculating wind load is the same as in Section 6.

4. Parameters for calculating wind on ice-covered Chimney, Tanks, and  similar structures are the same as those in Table 6-19 except that DÖq_z shall be used for all round cross section.
5. Calculation of wind on ice-covered solid freestanding walls and solid signs are the same as those in Table 6-10.

Velociaty pressure, q_z = 0,00256 K_z K_zt K_d V_c² I_i.

K_d = 0.85 for using load combination, Kd = 1 for wind along.

K_z = 2.01 (z/z_g)^2/a

where z is height above ground, z shall not be less than 15 ft. except that z shall not be

less than 30 ft for exposure B for low rise building and for component and cladding.

a and z_g are taken as follows:

6. Parameters for calculating wind on ice-covered open signs and lattice frameworks are the same as those in Table 6-21 except that DÖq_z shall be used for all round cross section.  The solidity ratio shall be based on the projected area including ice.
7. Parameters for calculating wind on ice-covered trussed towers are the same as those in Table 6-22.  The solidity ratio shall be based on the projected area including ice.

Design Procedure:

1. Determine normal ice thickness and concurrent wind speed from Figure 10-2,10-3, 10-4, or a site-specific study.
2. Determine topographic factor, K_zt.
3. Determine importance factor, I_i.
4. Determine height factor, f_z.
5. Determine design ice thickness, t_d.
6. Determine weight of ice.
7. Determine wind parameter K_z and K_d.
8. Determine velocity pressure, q_z = 0,00256 K_z K_zt K_d V_c² I_i.
9. Determine force coefficient, Cf.
10. Determine gust effect factor G.
11. Determine wind pressure, p = q_z G C_f.
12. Determine wind force, F = p A, where A is projection area including ice cover.

  Design Examples:

  Example 1: A 10 ft diameter sign logo on the side of 10 story building

Location: New York City, New York

Design data:

Height above ground: 120 ft

Diameter of logo sign: d = 10 ft

Requirement: determine weight of ice on the logo and concurrent wind pressure.

Solution:

Design procedure (ASCE 2-02 section 10.7) :

1. Detarmine normal ice thickness from Figure 10-2: t = 1 in

Determine concurrent wind speed: V_c = 50 mph

2. Determine topographic factor from section 10.4.5 (Figure 6.4)

K₁ = 0, K₂ = 1, K₃ = 1

Kzt = (1+K₁ K₂ K₃)² = 1

3. Determine importance factor (Table 1.1 & 10.1), I_i = 1

4. Determine height factor from section 10.4.3,

Z = 120 ft, fz = (Z/33)0.1 = 1.14

5. Determine ice thickness at roof from section 10.4.6

t_d = 2.0 t I_i f_z K_zt ^0.35 = 2.28 in

6. Determine weight of ice from section 10.4.1

     Area of ice of logo, A_f = p d²/4 = 78.5 ft²

     Weight of ice, W_si = Af td x 56 pcf = 834 lb

7. Determine velocity pressure

            Kd = 0.85, I = 1

    Exposure B, exposure coefficient

            a = 9.5, Z_g = 900 ft, Z = 120 ft

            K_z = 2.01 (Z/Z_g) ^2/a = 1.32

8. Determine velocity pressure

q_z = 0.00256 K_z K_zt K_d (V_c)² I = 7.2 psf

9. Determine wind force coefficient (Table 6.21)

            C_f = 1.4

10. Gust effect factor (Section. 6.5.8)

            G = 0.85

11. Determine design wind pressure with ice load

            p = q_z G C_f = 8.5 psf

12. Concurrent wind force on logo

            A = p (d + 2 t_d)²/4 = 84.6 ft².

            F = p A = 720.3 lbs