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A Review of Two Codes For The Design of Pedestrian Bridges

Revision 0

Foreword

Bridge Engineering is international, but while the laws of equilibrium do not change from country to country, the practices to ensure that bridges satisfy those laws can vary widely, reflecting the position taken by the members of code writing committees.

In preparing this review, the objective has not been to provide a summary of design practices but to provide a basis for the preparation of a common document by signaling the areas of agreement and, more importantly, the areas of disagreement, at the level of the basic engineering, for future code writing bodies.

Alexandre de la Chevrotière, P. Eng.

Any statements expressed in this material are those of the individual author and do not necessarily represent the views of MAADI Group, which takes no responsibility for any statement made herein.

 Canada: CAN/CSA S6-06*
          Canadian Highway Bridge Design Code

 USA: AASHTO – 2009
          LRFD Guide Specification for the
              Design of Pedestrian Bridges

* Note: The CAN/CSA S6-06 code also implies the conception of pedestrian bridges.

PEDESTRIAN LOAD

CSA – Art. 3.8.9
p =5.0 – s⁄30 1.6≤ p ≤4.0 kPa (85 psf)
Where s is the total loaded length of walkway [m]
AASHTO LRFD PED. BRIDGES – Art. 3.1, 3.6
90 psf (4,24 kPa)
Load pattern to produce maximum load effects. No dynamic load allowance with this loading.

OTHER LIVE LOADS

CSA – Art. 3.8.10
Maintenance Access Loads
Ultimate limit states 1.6 kN over 1.00 x 0.50 m
AASHTO LRFD PED. BRIDGES – Art. 3.3
Equestrian Loads (if applicable)
1.00 Kip over square area 4.0”x4.0”

MAINTENANCE VEHICULE LOAD

CSA – Art. 3.8.11
When Vehicular Access Is Not Physically Prevented or specified by the client.
Clear deck width greater than 3.0 m
80.0 kN truck load
AASHTO LRFD PED. BRIDGES – Art. 3.3
When Vehicular Access Is Not Physically Prevented or specifed by client.
1) Clear deck width from 7 to 10 ft : 10 000 lb (H-5 Truck)

2) Clear deck width over 10 ft : 10 000 lb (H-10 Truck)

Vehicle impact allowance is not required

HORIZONTAL WIND LOAD

CSA – Art. 3.10
The wind pressure q [Pa] is for a return period of 50 years for bridge structures with a span shorter than 125 m.
Horizontal wind load per unit exposed frontal area of the structure: Fh= q ∙ Cg ∙ Ce ∙ Ch [Pa]
In the case of truss spans, this load shall be taken to act on the windward truss and an identical force shall be simultaneously applied to the leeward truss unless a recognized method is used to calculate the shielding effect of the windward truss.
AASHTO LRFD PED. BRIDGES – Art. 3.4
AASHTO SIGNS – Art. 3.8, 3.9
Pedestrian bridges shall be designed for wind loads as specified in AASHTO Signs, Articles 3.8 and 3.9.

VERTICAL WIND LOAD OVERTURNING

CSA – Art. 3.10
Vertical wind load per unit exposed plan area of the structure: Fv=q∙Cg ∙Ce ∙Cv [Pa]
The vertical load shall be taken to act either upwards or downwards. In addition to the application of Fv as a uniformly distributed load over the whole plan area, the effect of possible eccentricity in the application of the load shall be considered. For this purpose, the same total load shall be applied as an equivalent vertical line load at the windward quarter point of the transverse superstructure width.
AASHTO LRFD PED. BRIDGES – Art. 3.4
AASHTO SIGNS – Art. 3.8, 3.9 / AASHTO LRFD – Art. 3.8.2
A vertical upward wind force of 20psf times the width of the deck shall be considered to be a longitudinal line load. This lineal force shall be applied at the windward quarter-point of the deck width in conjunction with the horizontal wind loads.

LOAD FACTORS AND COMBINATIONS

CSA – Art. 3.5
Load Combination Ultimate limit states
ELU 1: 1.10∙D+1.70∙L
ELU 3: 1.10∙D+1.40∙L+0.50∙W
ELU 4: 1.10∙D+1.65∙W
ULS 4: 0.95∙D+1.65∙W (Overturning)
ELU 7: 1.10∙D+0.95∙W+1.3∙A
ELU 9: 1.35∙D
Serviceability limit states
SLS Combination 1: 1.00∙D+ 1.00∙L (Deflection)
L = live load (dynamic load allowance, when applicable)
D = dead load
W = wind load on structure
A = ice accretion load
AASHTO LRFD – Art. 3.4.1
Load Combination Limit State
STRENGTH I: 1.25(DC1 & DC2) + 1.75(PL) + 0(WS)
STRENGTH III: 1.25(DC1 & DC2) + 0(PL) + 1.40(WS)
SERVICE I: 1.00(DC1 & DC2) + 1.00(PL) + 0.30(WS)
DC = dead load of structural components
PL = pedestrian live load
WS = wind load on structure

PEDESTRIAN AND BICYCLE RAILINGS

CSA – Art. 12.4.4, 3.8.8.2
The load on railings:
wp = 1.20 kN/m applied laterally and vertically simultaneously.
Only one railing shall be loaded at a time when posts of post-and-railing barriers are being designed.

The minimum height (H):
Pedestrian : 1.05 m
Bicycle : 1.37 m
OPENINGS IN BARRIERS:
1) Pedestrians
Openings in pedestrian barriers shall not exceed 150 mm in the least direction or shall be covered with chain link mesh. Openings in chain link mesh shall not be larger than 50 × 50 mm. The wires making up the mesh shall have a minimum diameter of 3.5 mm.
2) Bikes
Openings in bicycle barriers for the lower 1050 mm of barrier shall not exceed 150 mm in the least direction or shall be covered with chain link mesh. Openings in chain link mesh shall not be larger than 50 × 50 mm. The wires making up the mesh shall have a minimum diameter of 3.5 mm.
AASHTO LRFD – Art. 3.4.1 – Art. 13.8, 13.9
The load on railings:
w = 0.73 N/mm applied laterally and vertically simultaneously.
In addition, each longitudinal element will be designed for a concentrated load of 890 N, which shall act simultaneously with the above loads at any point and in any direction at the top of the longitudinal element.
The posts of pedestrian railings shall be designed for a concentrated design live load applied transversely at the center of gravity of the upper longitudinal element or, for railings with a total height greater than 1500 mm, at a point 1500 mm above the top surface of the decking.
The value of the concentrated design live load for posts, PLL, in N, shall be taken as:
PLL =890+0.73L where, L = post spacing (mm)
The minimum height (H):
Pedestrian : 1070 mm
Bicycle : 1070 mm
(Guide for the Development of Bicycle Facilities, 3rd Edition, requires 1100 mm)
OPENINGS IN BARRIERS:
1) Pedestrian
A pedestrian rail may be composed of horizontaland/or vertical elements. The clear opening between elements shall be such that a 150-mm diameter sphere shall not pass through.
When both horizontal and vertical elements are used, the 150-mm clear opening shall apply to the lower 685 mm of the railing, and the spacing in the upper portion shall be such that a 200-mm diameter sphere shall not pass through.
A safety toe rail or curb should be provided. Rails should project beyond the face of posts and/or pickets.
2) Bicycle
The height of a bicycle railing shall not be less than 1070 mm, measured from the top of the riding surface.
The height of the upper and lower zones of a bicycle railing shall be at least 685 mm. The upper and lower zones shall have rail spacing satisfying the respective provisions of pedestrian railing.

FATIGUE LOAD

CSA – N/A
Not applicable otherwise specified by the Engineer
AASHTO LRFD PED. BRIDGES – Art. 3.5
The fatigue loading used for fatigue and fracture limit state (Fatigue 1) shall be as specified in Section 11 of AASHTO Signs. The Natural Wind Gust specified in Article 11.7.3 and the Truck-Induced Gust specified in Article 11.7.4 of that specified need only be considered, as appropriate.

MAXIMUM DEFLECTION

CSA – Art. 3.4.4
Art. 3.4.4 et Figure 3.1
Deflection limits for highway bridge superstructure vibration.
ASHTO LRFD PED. BRIDGES – Art. 5
AASHTO LRFD – Art. 3.4.1
Deflections should be investigated at the service limit state using load combination Service 1.
For spans other than cantilever arms, the deflection of the bridge due to the unfactored pedestrian live loading shall not exceed 1/360 of the span length. Deflection in cantilever arms due to pedestrian live loading shall not exceed 1/220 of the cantilever length. Horizontal deflections under unfactored wind loading shall not exceed 1/360 of the span length.

VIBRATIONS

CSA – Art. 3.4.4
Art. 3.4.4 et Figure 3.1
Deflection limits for highway bridge superstructure vibration.
ASHTO LRFD PED. BRIDGES – Art. 6
Deflections should be investigated at the service limit state using load combination Service 1.
The fundamental frequency in a vertical mode of the pedestrian bridge without live load shall be greater than 3.0 hertz (Hz) to avoid the first harmonic. In the lateral direction, the fundamental frequency of the pedestrian bridge shall be greater than 1.3 Hz. If the fundamental frequency cannot satisfy these limitations or if the second harmonic is a concern, an evaluation of the dynamic performance shall be made. Other considerations must be taken into account in Art. 6.

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Last modified: June, 2015

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