Actions on Barriers and Vehicle Loading

Introduction

This post concerns the application of lateral loads to barriers and wheel axle loads from vehicles. The nature of this type of loads may vary from people simply leaning on barriers or vehicles colliding with them at high speed. Although Barrier and axle loading are defined as an imposed action, they are treated slightly different from other forms of imposed loading. While a blanket area may be used to define axle loads, we’re more concerned about the point loads from each wheel.

Principles

Table 2 of BS 6180: 2011 – Barriers in and about buildings – Code of Practice and Table 2 in PD 6688-1-1:2011 provide guidance on the application of loads to barriers in bulding. An extraxt of this table showing the commonly found loads has been included in this post (Table 1).

[supsystic-tables id=9]

When designing a structure, this loads must not be combined with other loads, as they are unique load cases. They do, however form part of the overall loads on the structure and should be grouped with imposed, dead loading where appropriate. We apply the line load at 1.1m above the floor finish level irrespective of the actual height of the barrier.

For barriers and other structures that could be subject to impact loads from vehicles that are travelling at more than 16 km/h (10 mph), please consult Eurocode 1-1-7 Clause 4.3 and Clause NA.2.11 of the UK National Annex to Eurocode 1-1-7 for further guidance

Vehicle Impact Loading

There are two variables that must be estimated when designing barriers that can be crashed into by moving vehicles. The first variable is the likely speed of the moving vehicle prior to the crashing into a barrier. Secondly, the mass of the moving vehicle must be estimated. These variables are needed in order to estimate the vehicle impact load.

Annex A of BS6810 and Clause B(2) of Annex B to BS EN1991-1-1 provides guidance on how the impact load may be estimated. This is based on the assumption that the load occurs over a length of 1.5m and at a height of 375mm above the floor finish level if located in a car park. The following equation can be used to estimate the Impact Load on the barrier.

F=\frac { 0.5m{ v }^{ 2 } }{ { \delta }_{ c }+{ \delta }_{ b } }

Where:

F = Impact Load
m=mass of the vehicle in kilogram (must not be greater than 2500kg)

v= velocity of the vehicle in m/s just before colliding with the barrier
δ_c = is the deformation in mm of the vehicle as it hits the barrier, which can be no less than 100mm
δ_b = is the deformation of the barrier as the vehicle hits the barrier

Supposing the vehicle strikes the barrier at an angle, Clause A.2 of BS6810 gives another expression taking into account the angle at which the car strikes the barrier. This is presumed to be the most common form of vehicular impact on barriers because barrriers are typically placed parallel to the direction of traffic. The expression for estimating F is therfore modified and defined as:

F=\frac { 0.5m(vsin\theta )^{ 2 } }{ csin\theta +b(cos\theta -1)+{ \delta }_{ c }+{ \delta }_{ b } }

Where:

Ѳ is the angle in º at which the vehicle hits the barrier
c is the dimension in mm from the centre of gravity of the vehicle to the front of it. see figure 1

b is the dimension in mm from the centre of gravity of the vehicle to the side that strikes the barrier. see figure 1
m,v, δ_b, δ_c are as defined in the preceding section

Figure 1: Defined variables for vehicles striking a barrier at an angle.

There are some specific figures in BS 6810 that can be used to obtain the impact load particularly figure A1 & A2. These figures are based on a plot of deformation against the ratio of F/m. Each vehicle has its own curve, thus do obtain the impact load for a vehicle, we simply multiply the ratio of F/m by the actual mass of the vehicle in kilograms.

Some specific recommendations are also made on barriers next to ramps and barriers next to a slope. Guidance on this can be obtained from the literatures listed in the further reading and reference section of this post.

Axle Load from Vehicles

BS EN 1991-1-1 Covers the loading from vehicles onto buildings, such as car parks or areas within a building that is trafficked by vehicles. Two categories are defined in the National Annex to BS EN 1991-1 which is replicated as Table 2:

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Application of the point load is defined in clause 6.3.3.2 of the code using figure one, this has been replicated as Figure 2on this post.

The area of contact of the vehicles is taken as 100×100 for lighter vehicles and 200×200 for heavier vehicles. This is classified as categories F & G loads respectively in BS EN 1991-1-1

Applying Partial Factors

As stated in the introductory part of this post, barrier and axle loads are imposed loads, therefore they must be treated as such. They are classified as a quasi-static variable action within the eurocodes, therefore, the same partial factors will apply to them as would any other imposed load.

Worked Example

A 1200mm high balustrade is to be installed along a walkway link bridge across an atrium in a shopping mall. Determine the ultimate lateral load on the balustrade and the resulting bending moment at the base of the balustrade.

Outside the shopping mall, there is a car park with a barrier around it perimeter. Determine the ultimate impact load of a car travelling at 16km/hr. at angle 20 degrees. The maximum deflection of the barrier is 100mm, similar to the deformation of the car as it hits the barrier. The assumed car dimensions are 2.2m wide x 5.2m long with a mass of 2.0 tonnes