How The Metal Bar Grating Load Tables Are Calculated

The procedures used to prepare data for metal bar grating load tables

***reference document “MBG534-12” METAL BAR GRATING ENGINEERING DESIGN MANUAL”

Bar grating load tables are critical engineering design tools that summarize the safe load-bearing capacity of grating panels. For engineers and designers, consulting accurate tables—including specialized versions for heavy duty grating load tables or specific materials like steel bar grating load tables—is essential for selecting safe and structurally compliant products. The data within these tables are derived from the rigorous application of established engineering formulas and material properties, as outlined in the calculation procedures below.

NOMENCLATURE

a = length of partially distributed uniform load or vehicular load, parallel with bearing bars, in.

b = thickness of rectangular bearing bar, in.

c = width of partially distributed uniform load or vehicular load, perpendicular to bearing bars, in.

d = depth of rectangular bearing bar, in.

A_c = distance center to center of main bars, riveted grating, in.

A_r = face to face distance between bearing bars in riveted grating, in.

A_w = center to center distance between bearing bars in welded and pressure locked gratings, in.

C = concentrated load at midspan, pfw

D_c = deflection under concentrated load, in.

D_u = deflection under uniform load, in.

E = modulus of elasticity, psi

F = allowable stress, psi

I = moment of inertia, in⁴

I_H20 = moment of inertia of grating under H20 loading, in⁴

I_b = I of bearing bar, in⁴

I_g = I of grating per foot of width, in⁴

I_n = moment of inertia of nosing, in⁴

K = number of bars per foot of grating width, 12"/A_w

L = clear span of grating, in. (simply supported)

M = bending moment, Ib-in

Mb = maximum M of bearing bar, Ib-in

Mg = maximum M of grating per foot of width, Ib-in

N = number of bearing bars in grating assumed to carry load

NbH20 = number of main bearing bars under load H20

NcH20 = number of connecting bearing bars under load H20

Pb = load per bar, Ib

Pu = total partially distributed uniform load, Ib

PuH20 = wheel load, H20, Ib

Pw = wheel load, lb

S = section modulus, in³

Sb = S of bearing bar, in³

Sg = S of grating per foot of width, in³

SH20b = section modulus at bottom of grating under H20 loading, in³

Sn = section modulus of nosing, in³

U = uniform load, psf

ABBREVIATIONS

in. = inch

ft = foot

Ib = pounds

Ib-in = pound-inches

pfw = pounds per foot of grating width

psf = pounds per square foot

psi = pounds per square inch

FORMULAS

1. Number of bearing bars per foot of width for welded grating

K = 12/AW

2. Section modulus of rectangular bearing bar

S_b = bd²/6 in³

3. Section modulus of grating per foot of width

S_g = Kbd²/6 in³ = KS_b in³

4. Section modulus required for given moment and allowable stress

S = M/F in³

5. Moment of inertia of rectangular bearing bar

I_b = bd³/12 in⁴ = S_b d/2 in⁴

6. Moment of inertia of grating per foot of width

I_g = Kbd³/12 in⁴ = Kl_b in⁴

7. Bending moment for given allowable stress and section modulus

M = SF Ib-in

The following formulas are for simply supported beams with maximum moments and deflections occurring at midspan.

8. Maximum bending moment under concentrated load

M = CL/4 Ib-in per foot of grating width

9. Concentrated load to produce maximum bending moment

C = 4M/L Ib per foot of grating width

10. Maximum bending moment under uniform load

M = UL²/(8 x 12) = UL²/96 Ib-in per foot of grating width

11. Uniform load to produce maximum bending moment

U = 96M/L² psf

12. Maximum bending moment due to partially distributed uniform load

M = P_u (2L - a)/8 Ib-in

13. Maximum deflection under concentrated load

D_c = CL³/48EI_g in⁴.

14. Moment of inertia for given deflection under concentrated load

Ig = CL3/48EDc in4

15. Maximum deflection under uniform load

D_u = 5UL4/(384 x 12El_g) = 5UL4/4608EI_g in.

16. Moment of inertia for given deflection under uniform load

Ig = 5UL⁴/4608EDu in⁴

17. Maximum deflection under partially distributed uniform load

D_u = P_u((a/2)³ + L³ - a² L/2)/48El_bN in.

GRATING SELECTION

Example

The concentrated midspan and uniform load bearing capabilities of W-19-4 (1-1/2 x 3/16) welded

A1011 CS Type B carbon steel grating and the corresponding midspan deflections will be calculated.

Allowable stress, F = 18,000 psi

Modulus of elasticity, E = 29,000,000 psi

Span, L = 54 in.

Bearing bar spacing, A_w = 1.1875 in.

Number of bearing bars per foot of width

K = 12/A_w = 12/1.1875 = 10.105

Section modulus of grating per foot of width

S_g = Kbd²/6 = 10.105 x 0.1875 (1.5)²/6 = 0.711 in³

Moment of inertia of grating per foot of width

I_g = Kbd³/12 = 10.105 x 0.1875 (1 .5)³/12 = 0.533 in⁴

Maximum bending moment for grating per foot of width

M_g = FS_g = 18,000 x 0.711 = 12,800 Ib-in

Concentrated Load

Load, C = 4M_g /L = 4 x 12,800/54 = 948 pfw

Defl, D_c = CL³/48El^g = 948 x (54)³/(48 x 29,000,000 x 0.533) = 0.201 in.

Uniform Load

Load, U = 96M_g /L² = 96 x 12,800/(54)² = 421 psf

Defl, D_u = 5UL⁴/4608El_g = 5 x 421 x (54)⁴/(4608 x 29,000,000 x 0.533) = 0.251 in.

Concentrated Mid Span Load per foot of width          Uniform Load per square foot