This article aims to demystify the calculations used in wind speed calculations and give professionals involved in the handling of A-frames and Stillages a useful primer in the factors involved in wind loading.
Disclaimer: each loading scenario, site and environmental conditions require a separate assessment so this article is for informative purposes only. If you need any further advice, please contact us directly on (01663) 719 300 or send an email to email@example.com discuss further.
In simple terms, there are two factors that determine whether a stillage will overturn: firstly, the position of the centre of mass relative to the base; secondly, the tipping force applied to the frame.
1 — Centre of Mass
When loaded, any given A-frame will have a centre of mass. The taller the A-Frame, the higher the centre of mass. As common sense dictates, the higher the centre of mass, the more unstable the A-Frame.
The frame itself will have its own centre of mass (CoM) but this will of course be altered by loading the frames. The glass company should always try to load stillages with glass so that the load is balanced. JS Burgess designs bespoke A-Frames and can use CAD Data to determine the CoM but this is largely academic as the handling characteristics of the frame are changed the second it is loaded. This is why you will often see structural engineer’s reports that calculate site-specific tipping conditions.
The A-Frame or stillage will only tip when the CoM acts outside the base of the frame. This is shown in the sketch above where both frames are shown to be on the verge of tipping.
With a tall, thin-based A-Frame, the tipping angle is relatively small. Having a shorter wide-based A-Frame means that the tipping angle is much larger and therefore the stillage can be considered to be more stable.
As we will see shortly, the economics of transport means that it isn’t always possible just to specify a wide base on the A-Frame and so the glass handler may have to deploy other means of preventing the frame from tipping.
2 – Tipping Forces required to overturn an A-Frame
As shown above, the glass handling A-Frame or stillage will tip as soon as the vertical line of the CoM falls outside the corner of the base in contact with the ground; but what can cause the frame to tip?
The site manager has complete control over the first two items, they can ensure the stillage is stacked on stable, level ground and they can introduce preventative measures to protect the frame from unwanted impact or improper handling. The final point, wind loading, is less predictable and needs the input of an independent structural engineer.
You may have found this article because you have been passed a structural engineer’s report (or been asked to commission one!) to assess the wind loading factors on your site. These reports at first glance seem complex but they generally share the same features.
Not all sites require these reports but if they do, they have to be produced by a third party structural engineer experienced in the use of EN1991-1-4:2005 Wind Actions on structures.
These reports at first glance appear very complicated but typically comprise of the following sections:
a. Static analysis of the loaded A-Frame – the structural engineer will determine the position of the CoM of the loaded frame which will require the glass handler to provide a loading plan. From this the tipping angle can be determined.
b. Wind Pressure Calculations – EN1991-1-4:2005 Wind Actions on structures has various tables that give constants specific to different sites, topographies and seasonal variations. These pressure coefficients are used to determine the peak force for a given wind speed.
c. Tipping Analysis – the final part of the report comprises of a moment balancing equation which balances the gravitational moment of the CoM of the frame against the counter-moment produced by the peak wind force. This will determine whether the stillage is likely to overturn for a given wind speed.
Preventing an A-Frame from tipping, in theory at least, is very simple. Keep the CoM low and the base wide!
Practicalities however mean that having a very wide base is not always cost efficient.
Flat bed lorries are around 2400mm wide meaning that if an A-Frame base is 1200mm wide or less, then two can be stacked across the width of the wagon. Going above 1200mm doubles transport costs so simply specifying a wide base is not always economically feasible
A potential solution is to specify extension bars which are bolted on to the base of the frame. This will have the effect of widening the base without affecting the transport volume. It does of course increase costs and introduce demountable components that may be lost or damanged.
Keeping the CoM low is possible through careful load planning but sometimes the situation will be “what it will be”. This is particularly the case when a single large panel is placed on either side. In the case where multiple units are being loaded, care must be taken to balance the load on each side of the A-Frame.
The final consideration is very much under the site manager’s control. Once a suitable flat stable site has been chosen, stillages can be stacked at 90degrees to the prevailing wind direction so that the wind is more likely to blow through the ‘A’ of the stillage rather than act on the “sail” that is the glass panel. The A-Frames can be block stacked to provide extra stability and it may also be possible to tether the frames to the ground.
With most of the weight of a frame in the base anyway, A-Frames are naturally very stable structures, but with increasing health and safety demands it may be necessary to commission a wind loading report, especially if the frames are to be used at height.
JS Burgess are very experienced in working with glass & curtain wall manufacturers and can help with every aspect of the design and build of A-Frames and curtain wall module stillages. If you need further advice, please contact us on (01663) 719 300 or send us an email firstname.lastname@example.org to discuss. We’d be delighted to help.
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