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BS6399-2 WIND MODELLER

Monday 2 November 2009 15:44

BS6399-2 Wind Modeller Handbook page 2 CSCs Offices Worldwide

Monday 2 November 2009 15:44

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Monday 2 November 2009 13:53

page 4 Table of Contents

BS6399-2 Wind Modeller Handbook

Chapter 1 Introduction . . . . . . . . . . . . . . . 5

Chapter 2 Scope . . . . . . . . . . . . . . . . . 6

Chapter 3 Limitations . . . . . . . . . . . . . . . . 7

Chapter 4 Applying Walls and Roofs . . . . . . . . . . . . . 11Applying Walls . . . . . . . . . . . . . . . 11Applying Roofs . . . . . . . . . . . . . . . 11

Chapter 5 Running the Wind Wizard. . . . . . . . . . . . . 12

Chapter 6 Creating Wind Zones on the Building . . . . . . . . . . 13Basic Geometry . . . . . . . . . . . . . . . 13Wall Zones . . . . . . . . . . . . . . . . 14

Wall Type . . . . . . . . . . . . . . . . 14Windward Walls . . . . . . . . . . . . . . . 14Leeward Walls . . . . . . . . . . . . . . . 14Side Walls . . . . . . . . . . . . . . . . 15

Roof Zones . . . . . . . . . . . . . . . . 15Direction . . . . . . . . . . . . . . . . 15Automatic Zoning . . . . . . . . . . . . . . 16Non-Automatic Zoning . . . . . . . . . . . . . 17

User Modification of Zones . . . . . . . . . . . . . 17

Chapter 7 Load Decomposition . . . . . . . . . . . . . . 18Roofs . . . . . . . . . . . . . . . . . 18Walls . . . . . . . . . . . . . . . . . 18

Chapter 8 References . . . . . . . . . . . . . . . . 19

Chapter 1 : Introduction BS6399-2 Wind Modeller page 5BS6399-2 Wind Modeller

Chapter 1 Introduction

This handbook describes the BS 6399-2 Wind Modeller, a component of Fastrak Building Designer which allows you to load a model for wind in accordance with BS 6399-2 : 1997. The wind loading assessment is performed on the walls and roofs which are defined in your building model. The resulting wind loads are distributed back to the members for structural analysis and design.

You can use BS 6399-2 Wind Modeller: to determine site and effective wind speeds (standard or directional), to determine the zones of wind pressure on walls and roofs, to determine standard values of Cpe for each zone, to determine wind pressures on each zone, to determine wind loads and load cases for your structure.

Unless explicitly stated all calculations in BS 6399-2 Wind Modeller are in accordance with the relevant sections of BS 6399-2:1997 incorporating Amendment 1 and corrigendum No. 1. It is essential that you have a copy of this code with you while assessing wind on any structure.

In addition, you may find the following books useful to enhance your understanding of wind loading and the software:

Wind Loading - a practical guide to BS 6399-2(Ref. 2), Wind and Loads on buildings: Guide to Evaluating Design Wind Loads to

BS6399-2:1997(Ref. 3).

Unless explicitly noted otherwise, all clauses, figures and tables referred to in this document are from BS6399-2:1997.

BS 6399-2 Wind Modeller is a very powerful tool which has been developed to aid engineers in their assessment of wind loads on buildings. You will find that the determination of wind speeds, zones, pressures is rigorous but the final wind loads adopted are your responsibility.

Your attention is particularly drawn to BS6399-2:1997 Clause 1.1. For building shapes which are not covered by the Standard you will need to seek specialist advice.

BS6399-2 Wind Modeller page 6 Chapter 2 : ScopeChapter 2 Scope

BS 6399-2 Wind Modeller has been developed in order to provide you with a comprehensive design tool which can assess and apply wind loading to your Fastrak Building Designer model in advance of analysis and design.

The above model has to be clothed in walls and roofs. Wind is then applied intelligently to this building envelope within the scope below and the limitations laid out in the next section.

In the main, BS6399-2:1997 addresses rectilinear buildings. In order to develop a tool for engineers, we have extended this capability to address non-rectilinear buildings using the standard method. For more information, please refer to reference 2 (section 2.5.3.2.4, page 82 and 2.5.4.3.3 pages 89-90).

It is assumed that the wind loads are developed to assess the overall stability of the structure and for member design. The wind loads have not been specifically developed for the design of cladding and fixings.

The scope of BS 6399-2 Wind Modeller encompasses: Enveloping the building with walls and roofs is undertaken in Fastrak Building Designer in

the normal manner. There is only limited validation of the envelope defined (for example connected walls must have consistent normal directions). The onus is on you to model the building shape as completely and as accurately as you determine necessary.

Choice of method: BS6399-2:1997 - Standard Method - Standard effective wind speeds with standard

pressure coefficients, BS6399-2:1997 Hybrid Method - Directional effective wind speeds with standard

pressure coefficients. Basic Wind Speed or Dynamic pressure is determined using BREVe Active X Control(Ref. 4). Having defined walls and roofs (defaults are standard wall, flat or monopitch roof

depending on the slope), you are able to specify the type in more detail e.g. multi-bay, monopitch / duopitch etc.).

The main wind parameters, are calculated for you but conservatively, (for example Crosswind Breadth, B, is determined for the enclosing rectangle of the whole building). Wherever possible other attributes are determined conservatively, but you are able to override the values should you need to.

Given the above, zoning is semi-automatic, (not attempted for roofs with more than 4 sides which are defaulted to single conservative coefficient), with full graphical feedback. Provision is made for you to modify the zoning. For example you can define a manual zone layout, you can override the coefficients

Load decomposition is fully automatic where valid, (walls and roofs need to be fully supported in the direction of span).

BS 6399-2 Wind Modeller is a very flexible tool that can, should you wish, be used purely for wind assessment by setting up a model of consisting only of walls and roofs (no members) the software can determine the wind loading on the building envelope.

Chapter 3 : Limitations BS6399-2 Wind Modeller page 7Chapter 3 Limitations

Throughout the development of BS 6399-2 Wind Modeller extensive reference has been made to references 1-3 and we consider it advisable that you are fully familiar with these before using the software.

In addition, because wind loading is complex and its application to general structures even more so, it is essential that you read and fully appreciate the following limitations in the software:

Caution You should seek specialist advice for building shapes that are not covered by the Standard see Clause 1.1 of BS6399-2:1997.

Limitations

Geometric Limitations Exposed members are not considered, for example lattices, trusses Open sided buildings are not considered. Free standing walls and sign boards are not considered. Parapets and free-standing canopies are not considered. Barrel-vault roofs are not considered. Dominant Openings are not explicitly handled Clause 2.6.2. However, you can use Table

17 to calculate the necessary Cpi values and manually apply them to zone loads.

General items Horizontal Loads Clause 2.1.3.6 and Table 5a. No account is taken of the reduction on

overall loads to allow for non simultaneous actions. This is conservative. Frictional Drag - Clauses 2.1.3.8, 2.4.5 and 2.5.10. Should you consider these to be

significant, you will need to make a separate assessment of this load. Asymmetric loads Clause 2.1.3.7 no account is made for torsional effects. If you consider

these to be significant, you should create additional load cases in which to define these loads.

Division by Parts rule for buildings, Clause 2.2.3.2, is not considered. This is conservative. BS 6399-2 Wind Modeller does not check that the horizontal component of the factored

wind load is greater than the factored dead loads BS5950-1:2000 Clause 2.4.2.3. Dynamic Augmentation Factor, (Cr), including check for applicability of Clause 1.6.1 is

assumed to be undertaken by you outside the software. BS 6399-2 Wind Modeller ignores the difference between the loaded area of walls and roofs

defined at the centre-line rather than the sheeting dimension. Asymmetric Loads for Standard Method Clause 2.1.3.7. One interpretation of this clause

is that asymmetry is handled by using both the positive and negative coefficients in the tables for roofs reference 3, Chapter 8, page 53. Also SCI AD 273 advises engineers that

they do not need to consider the downward pressures for ordinary portals with slopes less

BS6399-2 Wind Modeller page 8 Chapter 3 : Limitationsthan 20 for SLS only.

In BS 6399-2 Wind Modeller, no automatic reduction is made for beneficial load. When you edit the Zone Load Data for a wind direction, having generated wind load cases, there is an option to allow for beneficial loads.

Wind loading on walls Automatic zoning applies to all walls subject to the other limitations described below:

Walls that are more than 15 from the vertical are outside the scope Clause 2.4.1.5. The inset storey clause 2.4.4.2 b) is not implemented. You can edit the zones manually

according to your engineering judgement to include zone E if you consider this necessary. Walls of internal wells are not automatically identified Clause 2.4.3.2a. You can

manually edit the zones to apply the roof coefficient to the walls.

Wind loading on roofs Automatic zoning only applies to all triangular and quadrilateral roof items that are not

concave, i.e. all of the internal angles < 180. The inset storey clauses 2.5.1.7 a) and b) are not implemented. In clause a) the software

sets Hr and H equal conservatively. You are obviously able to edit the zones manually according to your engineering judgement to include the further zones indicated in Figure 18 should you consider this necessary.

It should be noted that in Table 8 for curved and mansard eaves, the zones start from edge of horizontal roof and not from the edge of the feature.

Special care should be taken for winds blowing on duopitch with slopes that differ by more than 5. If the wind is blowing on the steeper slope (that is that the less steep slope is downwind of ridge), the downwind slope should be set to be a flat roof with mansard at eaves for this wind direction.

Mansard and multipitch roofs are not detected automatically. However, you can manually apply the relevant roof type, apex type and bay position parameters for each appropriate wind direction to match the requirements of Figure 22 and Figure 23.

Roof overhangs are not explicitly handled. To model an overhang, you should define a roof object that has a partial overhang. You can then define Cpi values manually to either have the same coefficient as the adjacent wall, (Clause 2.5.8.2 Small Overhangs), or as an open sided building (Clause 2.6.3).

Additional wind loads There may be situations when you perceive a need to manually define loads that can not be determined automatically. You can do this by defining additional wind load cases to contain these loads and then include these with the relevant system generated loads in design combinations in the normal way.

Chapter 3 : Limitations BS6399-2 Wind Modeller page 9Load decomposition onto shear wallsAll wall loads are decomposed into loads on columns. In a building that contains shear walls, the analytical model of the shear wall consists partly of a mid-pier vertical column at the centre of the shear wall, hence wind wall loads will be decomposed onto the mid-pier column.

Wall loads are only decomposed as nodal loads on this mid-pier vertical column. (For real columns and/or gable posts, wall loads can optionally be decomposed as either nodal loads or element loads.)

This decomposition on to the mid pier column could in certain cases result in an averaging of the wind pressure profile that removes the localised pressure increase at the corners of the building.

The example below illustrates the problem and provides an alternative model as a workaround:

Physical model of shear wall

Although not shown here, wind walls are also added to all four faces of the building.

Wind zones from BS6399 Part 3

The zones are generated on the wind wall faces.

Resulting stepped wind pressure on wind wall faces

Highest pressure occurs in Zone A, lesser pressure exists in other zones

BS6399-2 Wind Modeller page 10 Chapter 3 : LimitationsWind pressure decomposed on to the shear wall

Stepped pressure gradient is averaged over the face of the shear wall and then decomposed on to the mid-pier column at its centre. Hence only a single point load is applied at each floor level.

Alternative Model

Define two adjacent shear walls, making the first as wide as wind zone A. This results in a more accurate decomposition of the wind load, reflecting the stepped profile of the wind pressure.

Chapter 4 : Applying Walls and Roofs BS6399-2 Wind Modeller page 11Chapter 4 Applying Walls and Roofs

All the calculations for wind depend on the geometry and interconnectivity of the walls and roofs that envelope the building. You must therefore define the model, together with its walls and roofs before you can start to calculate the wind loading using BS 6399-2 Wind Modeller.

Whilst defining the models walls and roofs, it is essential that you define the largest planar surfaces possible for these if you want to get the best results from the software. If you ignore this advice, then the calculation of the reference height can be unconservative.

Applying Walls A single wall is determined to be a single planar surface. The outward face is vitally important for determining the wind direction relative to the wall, that is windward or leeward.

It is recommended that you use BS 6399-2 Wind Modellers Show/Alter State feature to check the face orientation quickly and correct any mistakes by clicking once on an item to reverse the direction. However, whenever automatic zoning is carried out, for example at the end of the Wind Wizard, the connected walls are checked to ensure that the normal directions are not inconsistent.

The following additional wall properties can be specified: Span Direction Funnelling Gap Parapet Load on Member

To set this information you should use the Structure window, and select the walls in the usual manner. You can then use the Properties pane to set the details you require. For details refer to What information can I change which is Wind direction independent? in the BS 6399-2 Wind Modeller Help.

Applying Roofs A single roof is determined to be a single planar surface. The orientation of a roof is automatically determined when placed based upon the slope vector the line of maximum roof slope.

Initially the roof type is set to Default. This is interpreted as Flat if the roof slope < 5 degrees, otherwise it is interpreted as Monopitch. You must use BS 6399-2 Wind Modellers Show/Alter State feature to adjust the roof type as necessary for all other situations (i.e. For Duopitch, Hip Main, Hip Gable or Mansard).

To set the span direction you should use the Structure window, and select the roofs in the usual manner. You can then use the Properties pane to set the span direction you require. This is defined as an angle, where 0 is parallel to the X axis and 90 is parallel to the Y axis.

BS6399-2 Wind Modeller page 12 Chapter 5 : Running the Wind WizardChapter 5 Running the Wind Wizard

Once the walls and roofs are in place, you use the Wind Wizard to define sufficient site information to calculate the effective wind speeds and dynamic pressures for the required wind directions and heights around the building, (that is the Reference Height (Hr) for each wall or roof). The BREVe ActiveX control is used for this purpose, 2009 CSC (UK) Ltd; BRE Ltd; Ordnance Survey.

BREVe is an aid to the use of BS6399-2:1997. BREVe automates the wind speed parts of the Standard and Directional methods of BS6399-2. The information held within BREVe is based upon the Ordnance Survey data of Great Britain.

It should be noted that BS6399-2:1997 recommends that the Standard Method requires assessment of orthogonal load cases for wind directions normal to the faces of the building. The wizard permits you to create wind load for any wind direction and thus it is up to you to create those loads for the directions most appropriate to your structure.

Chapter 6 : Creating Wind Zones on the Building BS6399-2 Wind Modeller page 13Chapter 6 Creating Wind Zones on the Building

At the end of the Wind Wizard, the system creates default zones for all the walls and roof items for each of the defined wind directions.

Whenever this process occurs, any error and/or warning messages are written to the Output window. Where appropriate, double clicking on a message highlights the item, (although if the problem is direction specific, then you may have to switch to the relevant view).

Basic Geometry The basic building geometry is assessed as follows:

Reference Height (Hr) is taken as the difference between highest point on wall or roof and ground level.

Wall height (H) is taken as the difference between highest and lowest points on the wall.

Roof height (H) is taken as the difference between highest point on wall or roof and ground level. This definition does not handle the upper roof of inset storey but is conservative and only affects the scaling dimension, b see Clause 2.5.1.7.

BS6399-2 Wind Modeller page 14 Chapter 6 : Creating Wind Zones on the Building The Building Breadth, B is calculated from the smallest enclosing rectangle around the whole building (considered over all roof and walls only) for the given direction. You can override the calculated value in case the Fastrak Building Designer model does not include the whole building.

Wall Zones

Wall Type We assess each wall to determine if it is a windward, leeward or side wall. We classify the type of wall dependent on :

Windward, Leeward, Other walls are classed as Side.

Windward Walls Windward walls have a single zone and we use Table 5 with interpolation for D/H.

Leeward Walls

60 120Leeward walls have a single zone and we use Table 5.

Chapter 6 : Creating Wind Zones on the Building BS6399-2 Wind Modeller page 15Side Walls Side walls are assessed for recesses (narrow or wide), irregular flushed faces, downwind re-entrant corners. In all cases, side walls have the relevant number of zones. BS 6399-2 Wind Modeller uses Table 5.

Roof Zones BS 6399-2 Wind Modeller automatically generates roof zones, where possible, for each wind direction. In essence each roof item is assessed in its own right based on its properties. The interconnectivity of touching roof items is not generally considered.

Direction Internally the roof slope vector (line of maximum slope) is determined from the normal vector, with its direction always giving a positive slope angle, i.e. the roof slope vector must always point up the slope.

We calculate the angle between the wind direction and projection of roof slope vector onto horizontal plane ( in range -180 to +180).

BS6399-2 Wind Modeller page 16 Chapter 6 : Creating Wind Zones on the BuildingAutomatic Zoning Automatic zoning normally only applies to all triangular roof items and quadrilateral roof items that are not concave, that is that all of the internal angles < 180. However, additionally, it only applies to Hip Gable roofs if they are triangular, and Hip Main roofs if they are quadrilateral. Further, Downwind Slope Hip Gables must not have 2 upwind corners.

Dimensions All zone dimensions are specified in plan.

Flat Roofs See Clause 2.5.1, Figure 16 and Table 8.

Monopitch Roofs See Clause 2.5.2.3, Figure 19 and Table 9.

Duopitch Roofs See Clause 2.5.2.4, Figure 20 and Table 10.

Hip Gable See Clause 2.5.3, Figure 21 and Table 11.

Hip Main See Clause 2.5.3, Figure 21 and Table 11.

Chapter 6 : Creating Wind Zones on the Building BS6399-2 Wind Modeller page 17Mansard Roofs If you manually set the connected roof types to Mansard, then the program will correctly identify the special cases in BS6399 Figures 17c, 22a and 22b, and use the correct tables and values. See BS 6399 Clauses 2.5.1.6.2 & 2.5.4

Multi-bay Roofs We allow you to interpret Clause 2.5.5 and Figure 23 as you think appropriate and manually define the roof types and sub-types accordingly. You also have the ability to manually set the multi-bay position for each roof item for each wind direction:

Not Multi-Bay - for this wind direction (conservative default), Upwind Bay first bay of many for this wind direction, Second Bay for this wind direction, Third or more Bay for this wind direction.

Where the reduction applies, the values of all coefficients are reduced according to Table 12.

Non-Automatic Zoning Where automatic zoning does not apply, the system creates a single zone covering the entire roof as follows:

Flat B, Monopitch B, Duopitch B for upwind, F for downwind, B for side, Hip Gable B for upwind, G for downwind, I for side, Hip Main B for upwind, F for downwind, I for side.

User Modification of Zones Initially the expectation is that only Expert users may want to make changes to the actual zone layouts or other data.

Whenever you edit the zones for a wall or roof item, please note that the zone layout will not be updated to reflect changes elsewhere in the model, you must make any necessary changes yourself.

BS6399-2 Wind Modeller page 18 Chapter 7 : Load DecompositionChapter 7 Load Decomposition

Roofs The direction of the one way decomposition of the wind zone loads to roof members is as specified by the span direction of the roof. All types of elements (except bracing and cold rolled members) are considered during the load decomposition.

Walls Wall load decomposition depends on the setting of the Load on Member wall attribute:

The default setting for this attribute is No and results in nodal loads on the supporting members. This setting is generally appropriate to avoid lateral loads on simple beams and distributed loads on simple columns.

Setting Load on Member to Yes allows the generation of UDL's on portal stanchions and gable posts without the need to model side rails.

Irrespective of the setting of this attribute, the initial decomposition of wind zone loads to wall members is similar to the roof decomposition. Again all types of elements are considered except bracing and cold rolled members.

If Load on Member is set to No a second decomposition stage is undertaken: Full/partial UDLs and VDLs on elements (lengths of beams/columns between nodes) are

distributed back to nodes as if the elements were simply supported at either end. The final nodal load is the sum of all incoming element loads.

Note This second stage is always performed if the members supporting the wall are simple columns or shear walls, irrespective of the Load on Member setting.

Chapter 8 : References BS6399-2 Wind Modeller page 19Chapter 8 References

1. British Standards Institution (2000). Loading for Buildings Part 2: Code of practice for wind loads. BS6399-2:1997 Incorporating Amendment No. 1.

2. Cook, N.J. (1999). Wind Loading - a practical guide to BS 6399-2 Wind Loads on buildings. Thomas Telford, London. ISBN: 0 7277 2755 9.

3. Bailey, C.G. (2003). Guide to Evaluating Design Wind Loads to BS6399-2:1997.SCI Publication P286.

4. BREVe software package version 3. Copyright 2009 CSC (UK) Ltd; BRE Ltd; Ordnance Survey.

BS6399-2 Wind ModellerContentsChapter 1 IntroductionChapter 2 ScopeChapter 3 LimitationsLimitationsGeometric LimitationsGeneral itemsWind loading on wallsWind loading on roofsAdditional wind loadsLoad decomposition onto shear walls

Chapter 4 Applying Walls and RoofsApplying WallsApplying Roofs

Chapter 5 Running the Wind WizardChapter 6 Creating Wind Zones on the BuildingBasic GeometryWall ZonesWall TypeWindward WallsLeeward WallsSide Walls

Roof ZonesDirectionAutomatic ZoningNon-Automatic Zoning

User Modification of Zones

Chapter 7 Load DecompositionRoofsWalls

Chapter 8 References