Calculating Design Wind Loads to BS 6399-2 - HI-SPAN LTD

Thank you for selecting our construction CPD certified presentation on the British Standard 6399 part 2 wind code.

Agenda

During this presentation we aim to:

• Introduce you to our wind code calculator program, which is designed to take the strain out of the necessary calculations required by the code.
• Introduce you to Hi-span
• Look at climate change and how this affects us all.
• Explain why BS 6399-2 was introduced and why it was necessary to replace the existing wind code CP3.
• Look at the wind code calculator.

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Introduction

Hi-Span was established in 1962 and a part of the age young and Co group of companies whose core business is structural steelwork. For many years high span has been at the forefront in the design and development of cold rolled sections and they've been longstanding members of the cold rolled sections Association, as well as associate members of the British constructional steelwork Association. From our manufacturing base at Wyndham Norfolk Hi-Span can design, detail, fabricate and deliver the complete cold rolled package throughout the UK and abroad.

Hi-Span cold rolled products have been primarily designed for use within most types of buildings, as secondary supports for cladding materials, as well as a number of other applications. For example, mezzanine floor beams, high span offers a comprehensive range of Purlins, rails, C-Channels, Eaves, Beams and bespoke sections together with complementary Accessories. Significant recent capital investment in the very latest cutting edge cold rolling plant has enhanced high spans position as one of the UK's leading suppliers of cold rod products to the construction industry.

The Impact Wind Has On Community

A major factor when we consider the design of buildings is wind. Over recent years headlines such as these are becoming commonplace.

“high winds damaged city buildings.”

“nine dead as UK struck by storms.”

“Kensal rise London hit by a tornado.”

Our design code for the UK British standard 6399 Part 2 contains procedures for estimating wind loads on buildings and other structures for the majority of conventional building structures. Although the UK’s climate is highly variable, windstorms are a frequent feature due to its proximity to the Atlantic. The vast majority of windstorms pass over unnoticed but occasionally an event occurs that results in loss of life or catastrophic damage to property.

Windstorms Within the Community

As you can see from the graph the wind related damage over the 12 years has risen dramatically over the period 1990-2000 weather related insurance claims in the UK have ranged between 360 million and 2.1 billion per year. The majority of this damage is wind related, unlike other natural perils experienced in the UK over recent years notably flooding, windstorms have resulted in loss of life. Since 1987 some 160 people have died in windstorm events.

Climate Change

As most of us are aware climate change has been very much in the public eye over the last few years global temperatures are increasing, sea levels rising, and Arctic sea ice is thinning. There is evidence of more precipitation in large parts of the world. Here in the UK the picture is as follows; in April 2002 a new report called UK CIPS 02 was released, showing climate scenarios in the UK. These scenarios present four different possibilities of how our climate might change, they take into account the possible changes in technology and lifestyle over the next 100 years.

Temperature.

Annual average temperatures look set to rise by between 2 degrees centigrade and 3.5 degrees centigrade by the twenty 80s. The South and east of the UK will most likely see the largest rise in temperature in contrast to the north and West which will see the least. Most of the warming will be in summer and autumn.

In contrast rises in the winter in the North West of Scotland are predicted to be between one degree centigrade lower emissions and two degrees centigrade high emissions. Temperatures in the South East may rise by as much as 5 degrees centigrade on average by the twenty 80s according to the higher emission scenario. And over 4 degrees centigrade, with the medium high emissions.

Precipitation.

Precipitation in winter would increase in all areas of the country in every one of the scenarios the increase is predicted to range from between 10% and 20% depending on the area of the country for the low emissions. For the high emission scenario, the range increases to between 15% and 35%.

The summer will see less precipitation than we see now and will therefore be much drier. The low emission scenario predicts the country to become up to 35% drier, whereas the high emissions scenario forecasts 50% less rainfall than we experienced now by the 2080’s.

The largest changes are predicted for the southern and eastern part of England, the smallest changes are forecast to be in northwest Scotland. Less snow will fall throughout the UK, a decline of 60% in parts of Scotland and up to 90% elsewhere.

BS 6399-2 Loading for Buildings

The first design guidance for wind loading was published in 1944 and was called the CP4. Three major revisions were then published in 1952, 1970 and in 1972 with the guidance finally known as CP3 chapter five Part 2.

Unfortunately, these standards were badly worded and caused confusion within the industry. They were easily misinterpreted but were employed by designers to gain estimates of wind loads for typical building types. Although revised during its existence the methods used for analysing wind would not be as accurate as its replacement, the British standard 6399 dash 2 published in 1995.

The update has less scope for misunderstanding and misapplication and more background information on wind speeds and pressure coefficients. Nevertheless, like all codes British standard 6399-2 can only cover basic shapes and when it moves into more intricate arrangements.

The clauses do get more complex or perhaps don't exist.

How Does it Compare with CP3?

Reported perceptions are the design loads are significantly higher with the new code, this is untrue. A major reason for this is the misapplication of CP3 in two main areas. Firstly, by applying terrain categories incorrectly, secondly not using the topography rules of CP3 if misapplied this would give incorrect design pressures. The second calibration showed that misapplied terrain categories in CP3 British standard 6399 would appear to give design pressures up to 25% greater.

British standard 6399-2 gives two methods for calculating wind loads, standard and directional. The simpler method contains conservatism that depends on site location and terrain, to get the best of the code you need a clear understanding of the detail and the underlying behaviour of wind around structures.

Compare BS 6399-2 with CP3 Chapter 5 Part 2

Many arguments have been raised against British standard 6399-2 due to its increased wind loads compared to CP3 chapter five Part 2. In order to show that this is not necessarily the case we can investigate two identical buildings and evaluate the wind loads using both codes and methods.

Initially we can see that the dynamic pressure queue calculated on both buildings is similar leading us to believe that the pressures and sanctions applied will also be similar.

Compare BS 6399-2 with CP3 Chapter 5 Part 3

However, as you can see the loadings calculated are not hugely different between the codes in some instances CP3 values are larger than British standard 6399 loads. The most noticeable difference between the codes with regard to secondary steelwork design is that local coefficient effects must now be taken into account, when they could previously have been ignored using CP3.

Our Objective

Our objective with this CPD presentation is to establish the necessity for accurate wind code analysis. We will then take a step by step guide through the wind code calculator during which we will relate the values calculated to the various sections of the code. When loads have been calculated we will look at how best to use them to achieve economies in your design. This free issue design software is available on our website at www.hi-span.com. 

Guide to Wind Loadings

When planning a new development designers need to understand the local wind environment. They need to know what impact it will have on the new structure and whether the structure will cause problems for neighbouring buildings and the surrounding area. Estimates of wind loads on structures are carried out using a wide range of information sources, including in house data from previous studies on similar structures, published data various national codes and general expertise.

For the majority of conventional building structures, the use of wind loading information in codes is perfectly adequate. However, for structures with an unusual geometry or with nonstandard structural properties it is often not possible to obtain accurate wind loading data from codes to achieve reliable design.

Designed wind speeds for a site are either measured data or basic wind speed information in design codes. The variation with direction and height is determined using established techniques based on roughness and topography of the site. 

The Wind Load Calculator

Hi-Span have a free Wind Load calculator within our free issue design suite, which simplifies the many necessary calculations required by the code into a windows based program.

Using Ordnance Survey references as shown, wind speeds in 12 directions around the site are calculated; it can also calculate altitude, distance to see or in town and obstacles spacings and Heights all of which can be edited by the user should they deem necessary. The final printable results are laid out in clear easy to follow diagrams, for the fall relative faces of the structure giving the user a complete wind code analysis.

Once the software has been installed you can access the wind code calculator by clicking here this is step one of a four step program. First you must locate the site this can be in the use of software such as autoroute or by using the various Internet programs such as multi map.  The site location can be provided as either an Ordnance Survey reference or as latitude and longitude coordinates. As an alternative you can directly select the location of your site simply locate your grid reference which in turn allows you to select your site location to the nearest 100 metres. On return to the opening screen you can see the grid reference chosen is displayed here.

Next you must choose the roof type which best suits the building you're going to analyse. The roof type shown coverall the standard roof types provided in British standard 6399-2. It is then necessary to calculate the dynamic augmentation factor CR. This value is dependent upon the height above ground level and the building type factor KB. CR is calculated automatically and corresponds to figure three of the code, this value determines where the British standard 6399-2 is suitable to carry out wind loading calculations on your building. If the geometry is outside the scope of this standard the code offers references for further information on analysis of dynamic structures.

Once calculated the dynamic augmentation factor can be used to increase overall loads when carrying out calculations for non-simultaneous action between faces as stated in clause 2136. 

The following three options will affect how the breve engine calculates the directional effective wind speeds developing the first stage of our hybrid interpretation of the wind code.

• Option one show Wizards gives the user a higher degree of control over which parameters the engine uses when calculating wind speeds. With this option selected the breve site wizard is activated when the user attempts to continue to Step 2. This option should only be used when the user has a good understanding of the site and the surrounding area, or if they wish to adjust the program to provide more conservative values.
• Option two allows the user to ignore the effects of topography. This means that topography is considered insignificant when calculating the altitude factor essay. When this option is left unselected the program takes into account its own values for topography based upon the Ordnance Survey reference previously input. This can also be edited in the next stage of the program.
• The third of the breve’s options allows you to take into account the displacement effects of surrounding buildings to the site, as with all of the breve’s options values are automatically input based on the Ordnance Survey reference. Again, this can be edited in the next stage.

Step two of the wind code calculator displays the Ordnance Survey reference input in step one along with the altitude and basic wind speed of the chosen site. The site editor option allows you to edit the various options you selected in step one, giving the user more control over what parameters the program uses to evaluate wind loadings.

In order for the program to calculate zone dimensions for the different coefficients that the code requires basic geometry must be input, as part of the geometry the diagonal dimension A is required for the roof sides and gable elevation of the structure. This value represents the largest diagonal dimension for which load bearing takes place. Steel Construction Institute guidance recommends using the span of the purlins on, butted designs and a maximum of two spans for continuous design. This value is used to calculate size effect factors for the external pressures defined.

Parapets could also be added to the structure some simple geometry is required in order for the program to evaluate coefficients in accordance with clause 28 free standing walls, parapets, and sign boards. It is worth noting that the reduction factor is not accounted for within this program if the user sees it necessary to reduce the parapets wind loads further, please refer to table 21A of the code. If buildings are situated in close proximity of one another a check must be made to assess whether funnelling occurs. The funnelling pop up Jakes parameters laid out in the code for distances between buildings. If funnelling is present the wind flow is accelerated increasing the negative coefficient. These increased coefficients are taken from Table 5 and are used increase wind suction. The revised loadings can be reduced through interpolation further advice can be sought from clause 2414.

Step three allows you to input the orientation of the building this can greatly reduce wind loads to certain elevations of your structure depending upon its relative position to North. If the orientation is changed unknown the direction factor SD will be taken as one point zero equivalent to the wind continuously approaching the structure from 240 degrees from North shown in Table 3.

The options for internal pressure coefficients are taken directly from the code these values represent tables 16 CPI for enclosed buildings. In accordance with steel construction Institute guidance of positive internal pressure of plus 0.20 can only occur in an enclosed building when two opposite walls are equally permeable, the other faces are impermeable, and the wind direction is normal to the impermeable face. In all other cases the internal pressure is negative.

Dominant openings affect the internal pressure coefficients when their area is equal to or greater than twice the sum of the openings in the other faces. Table 17 has been replicated to offer you the two options available in the code. The values two and three relate to the ratio of dominant opening area to the sum of the remaining openings.

It is possible to consider a dominant door to be shut during a storm as stated in clause 2613. When doing this you are advised to consider the door open as a service ability limit. Considering A plus 0.20 internal pressure is a conservative approach to this. Negative values will have to be assessed depending upon which soon the dominant opening lies.

The final step of this programme brings all the available results from the input data. The screen displays a plan on the roof of the building with accompanying elevations for the wall loads. You can view how the wind loads change when blowing on different faces of the building simply by clicking the appropriate face number. The display then represents the wind load values for wind blowing on the selected face, or wind loads, and zone geometry can be simply printed using the report button, which prints the face you currently have selected.

Further results are also available this print out will provide the user with all the various coefficients and size effect factors used to calculate the pressures and suctions shown on the diagram. The BreVe reports list all the information that has been assumed or edited by the user in relation to the specific site chosen; these values include distance from c to site topography considered, obstructions considered and the effective gust dynamic pressures for 30 degree intervals around the site.

Now that loads have been calculated they need to be interpreted in order to use them for designing section sizes. Typically, there are large uplifts of the eaves and the apex. This may increase the section size but with the purlins had reduced centres in these zones, there is no need to increase the sanction. To analyse your purlins for a typical wind, load the user must take account of the end bay condition with the wind against the gable face. Small zones of high uplift occur followed by reduced loads along the span. These zones of high uplift rarely extend to the full length of a bay so in order to design your purlins to the correct wind load you must use the uniformly distributed load calculator.

By imputing the various loading and their associated distances in relation to the span the program will calculate an equivalent UDL would produce directly in the highest band software. This value takes into account the moment produced by the varying loads and produces a UDL accordingly. The same approach is used for designing side Gable rails normally there are large sections to the corners of a building zone A but again they do not regularly span a whole bay. The same technique is applied to reduce the load to an equivalent UDL therefore avoiding over designing the section sizes. This approach is system specific and subtle differences in the calculations occur for butted, sleeved, and heavy end bay systems.

Thank you for taking the time to listen and understand our CPD presentation on our wind code calculator software. This should hopefully allow you to comfortably assess wind loads using the software in only a matter of minutes.