Three Pinned Portal Frame
This report found that plot 400 has many dead loads, live / imposed loads and wind loads in action on the buildings portal frame.
The building has a simple 3 pinned portal frame / arch structure which have been designed to resist all loads that are applicable. The building also has structural actions and different modes of collapse which act upon the supporting columns, rafter beams and supporting main beams of the portal frame.
The diagrams that have been produced illustrate how the different structural sections deform and how they would fail in the event of a mod of collapse, for example torsion, shear, fatigue or even compression.
The building has also been sketched and CAD drawings provided along with photographs of both the inside and outside structure which can be seen in appendix 4 of the report.
This report has also listed the client’s functional and spatial requirements and the applicable governing legislation which controls the design and use of buildings in England and Wales.
Contents Page. Page No.
1.0 Introduction 3
2.0 Brief 4
3.0 Structure Appraisal
3.1 Site Location 5
3.2 Building Function 6
3.3 Building Regulations 7
3.4 Building Drawings 8
3.5 Loadings 9 – 16
3.6 Structural Actions 17 – 24
3.7 Structural Stability 25 – 26
3.8 Deformation and Mode of Collapse 27 – 31
4.0 Conclusion 32 – 33
5.0 Bibliography 34
6.0 References 35
7.0 Appendix
7.1 Appendix 1 – Site Location Plan 37
7.2 Appendix 2 – Schedule of Drawings 38
7.3 Appendix 3 – Schedule of Photographs 39
7.4 Appendix 4 – Photographs 40
Introduction.
Buildings today are built to accommodate many different processes and purposes within the built environment.
When a building is at the feasibility stage, many key items must be considered to ensure that the building which is delivered to the client meets all the functional requirements and is built in accordance with all the relevant legislation which governs the construction industry and the built environment.
Some of the key design parameters would be:-
Spatial requirements
Materials used
Underlying use of building
Method of Construction
Finishes
Location
Size
Maintenance Considerations
Access and Use
Planning restrictions
This report will identify a building and investigate the design and comment on how the design meets the functional requirements for which it has been constructed.
Additional details such as drawings, photographs and sketches will also be provided to support this report.
Brief
This report has been produced to appraise a given structure.
The structure that has been chosen is Plot 400 at Ascot Drive, Derby in the East Midlands.
The building will be identified and suitable location plans will be provided for the site.
The buildings purpose will be explained in relation to its functional requirements and sketches and drawings will be provided to detail the building.
These drawings will be provided by hand and also by auto computer aided design technology.
Having described the building and its function, this report will then identify several different loads that the buildings portal frame must withstand with descriptions on how this portal frame is designed to resist such loads.
Many different structural actions are in force within the buildings main portal frame and load bearing elements, mainly:-
Tension
Compression
Shear
Bending
Shell action
Plate action
Joints
Arches
Suspension beams
Truss rafters
Having described these structural actions the report will then discuss how Plot 400 is designed and constructed to resist any overturning against dead loads, wind loads and fire conditions.
Finally sketches will be produced to show how the main load bearing elements of Plot 400 could deform under the action of several different loads, should the structure not be designed correctly to resist these loads and different modes of collapse or other possible failures will be described.
Conclusions will be made on the reports findings and the report will include appended drawings and a photograph schedule.
3.0 Structural Appraisal
3.1 Site Location
The proposed building is located in Derby within the East Midlands.
The full site address is:-
Plot 400
Ascot Business Park
Longbridge Lane
Derby
Derbyshire
DE24
Site location plans can be seen in appendix 1 which shows Plot 400 in relation to plots, 100, 250, and 550 off Longbridge Lane.
Appendix 1 also shows views from satellites and road location maps.
< www.multimaps.com>
Photographs of Plot 400 can be seen in appendix 4.
3.2 Building Function
The building at Derby, Plot 400 has been commissioned by the client Pattonair Limited to store, distribute and package parts for the aviation industry.
The building is to also allow space for ancillary processes such as ordering, accounts, HR, legal and every day processes such as welfare facilities.
The 3 storey office accommodation which is located to the front elevations of the building will cater for the following processes:-
Accounting (payments, orders etc.)
Orders
Management (staff, products)
Human resources
Legal responsibilities
General communications and data transfer
Information Technology
Welfare facilities (eating, washing toilets etc.)
Parking (staff and visitors)
Meetings
Protection of occupants from the elements (wind, rain, snow, cold and sun/heat).
The warehouse is a single storey building with a 3 storey office block attached to the front elevation (North Elevation).
The warehouse also includes a 3 storey mezzanine structure supported off the existing warehouse slab as shown in appendix 4 items 11, 22, 16, 24.
The main functions of the warehouse space are:-
Storage (mezzanine level)
Production
Packaging
Manual handling and labelling
Distribution
Protection of goods from the elements (wind, rain, snow, cold and sun/heat)
Protection for occupants and employees from the elements
Accommodation of occupants for processes
The building must therefore be designed to provide all these functions so that it will meet the client’s functional requirements for them to run a business.
3.3 Building Regulations and Planning Permissions
Now that the designers are aware of what functional requirements the building must provide, the design team must also ensure that the building meets with all the current legislation, these are mainly, for a building of this size and use:-
The Town and Country Planning act 1990
The Building Act 1984
The Building Regulations 2000
The Health and Safety at Work Act 1974
The Construction (Design and Management) Regulation 2007
The Regulatory Reform (Fire Safety) Order 2005
The Workplace (Health and Safety and Welfare) Regulations 1992
The Disability Discrimination Act 2005
These regulations apply to the building in both construction and occupation, the last 3 regulations only apply upon occupation of Plot 400.
3.4 Building Drawings
Having taken photographs and inspected the building, sketches will now be produced of the building.
3.5 Loadings
Buildings in England and Wales must be designed in accordance with Approved Document A of the Building Regulations 2000.
This document sets out minimum standards for all types of buildings and specifies that buildings must:-
“be constructed so that the combined dead, imposed and wind loads are sustained and transmitted by it to the ground –
Safely : and
Without causing such deflection or deformation of any part of the building, or such movement of the ground, as will impair the stability of any part of another building”
ODPM. (2004)
Buildings generally are designed for primary and secondary loads.
Primary loads are loads which act on a building in the first instance and can also be called principal loads, these loads are
Dead loads
Imposed loads
Wind loads
Dead loads
Dead loads are loads which are applied to a structure and are generally permanent and stationary in action.
The largest factor of dead loads is usually the self weight of the structural member itself for example:-
Steel column
Steel beam/rafter/perlin etc
Pre cast floor plank/beam/stair
Cladding (roof and walls)
Windows
Services (lights, sprinklers, HVAC)
Imposed loads
Imposed loads are also called “live loads” and these constitute loads which are movable and generally act on a structure when it is occupied.
Examples of live loads are:-
People
Furniture
Cars
Computers
Products (retail and storage)
Animals
Imposed loads can be further sub divided into imposed floor loads and imposed roof loads which are given in British Standard BS 6399 Part 1 : Code of practice for dead and imposed loads 1996.
Wind loads
Wind loads are as they sound, loads which are applied to a building in England and Wales due to negative and positive pressures both within and outside the building in question.
Wind loads can sometimes course actions to a building which may not have been considered and this load is the largest loading failure on the majority of buildings in England and Wales.
Wind loads of course vary depending on:-
Geographical location
Degree of exposure
Building height and size/shape
Time of exposure
Wind direction
Positive or negative pressures created
SEWARD, D. (2003)
Having discussed primary loads, it is also essential that secondary loads are also considered in the design of every building.
Secondary loads are caused mainly by the following factors:-
Temperature change within the element or surrounding elements such as walls, floors, plaster, blockwork, brickwork, finishes etc.
Shrinkage of materials/members such as plaster, timber, concrete etc.
Settlement of supports such as supporting columns, foundations, load bearing walls/floors and supporting beams.
SEWARD, D. (2003)
These loads are actions which effect the buildings supporting structure and the actions can be separated into 3 different categories.
The dead loads in a building are classified as ” permanent” actions.
The imposed loadings for floors and roofs (snow loads) are classified as “variable” actions.
The most recent action to now be included within the British Standard and approved document A of the building regulations is “disproportionate collapse” which is an “accidental” action.
The recent regulation states that:-
“The building shall be constructed so that in the event of an accident the building will not suffer collapse to an extent disproportionate to the cause”.
ODPM. (2004)
Within the guidance there are 3 main classes as shown in the table below:-
(Table taken from Approved Document A of the Building Regulations 2004, ODPM.)
Plot 400 at Derby is classified as a 2A Building as the warehouse is single storey and the offices do not exceed 4 storeys.
The way in which Plot 400 has been designed to resist disproportionate collapse is via effective horizontal ties of all suspended floors to walls, mainly the first and second floor pre- cast planks to the supporting steel beams. This was achieved with welded shear studs concreted into the pre- cast units at the external perimeter.
These loads are now known as “accidental” actions and have been introduced because of the incident that happened at the World Trade Centre, New York, America.
This incident caused an outcome which was disproportionate to the accident/attack and now all buildings have to be designed to resist such failures. Ronan point may also come under this classification.
Plot 400 has been designed to resist and withstand the following loads:-
Dead loads from structural elements
Dead loads from services
Dead loads from finishes
Dead loads from the external envelope
Imposed loads from people (UDL)
Imposed loads from products (UDL)
Imposed loads from furniture (UDL)
Imposed loads from computers (UDL)
Imposed loads from conveyors within the warehouse (UDL)
Wind loads to the Portal frame – uplift
Snow loads to the supporting roof structure
Wind loads – deflection
Wind loading – resistance to overturning
Hydrostatic Pressure (p) against all pad foundation bases
All these loads are applicable to the supporting structure of Plot 400.
The supporting structure is made up of the following elements:-
Pad Foundations which support columns
Columns which support rafters
Rafters which support the roof structure
Secondary rafters which support roof and wall cladding
These details can be seen in appendix 4 items 6, 9, 10, 17, 21, 25, 26, 28.
These elements are all designed to resist the previously mentioned loadings.
The way in which this is achieved is by applying the following specification.
The whole building is designed to transmit all dead, imposed, snow and hydrostatic loads safely to the ground.
Dead Loads
The dead loads are carried by all supporting beams, foundations and columns into the ground.
Each section is sized on the amount of dead, imposed and wind load (maximum) that could be applied to the section at any one time.
The section of each supporting element is governed by the following factors:-
Second movement of area (I)
Elastic modulus (Z)
Maximum bending capacity
Maximum shear capacity
Maximum compression capacity
Maximum torsion capacity
As each of these factors is increased, the size of section also increases
Live Loads
The Live loads generally affect the bending movement and deflection values of beams and floors/roofs etc.
Live loads are again designed against by increasing the section of material to resist the applied load.
Live loads can also be considered with the inclusion of bracing and gusset plates or stiffening plates etc.
These plates are some times included to reduce the live load affecting the size of section, in particular if the live load of a beam was particularly high (for example in a gymnasium or dance hall etc) the beams of that floor could be designed to resist the live load in the following manner:-
Larger section depth so that x – x is increased making the beam stiffer.
Smaller spacings of beams to create a stiffer floor.
Decrease the amount of allowable deflection, thus increasing the size of the beams possibly.
Stiffening plates to the beam sections to reduce the amount of I (second movement of area).
Wind Loads
All buildings must be designed to resist both vertical and horizontal loads.
If a building is designed to resist only vertical loads the possible deformation of the structure could take place.
This can be seen in appendix 2 diagram 11 which shows a portal frame building which has only been designed to resist vertical loads.
This structure has not been considered for possible horizontal loads and the failure of the elements is shown (The horizontal load could be a wind loading).
(Drawing adapted from SEWARD, D . (2003)
Buildings must also be designed for uplift. This is applied to a building when a prevailing wind acts directly onto the side elevation of a building.
This can be seen in appendix 2, diagram 9. This diagram shows how the wind load acts into the building causing positive pressures inside the building and causing uplift.
The way in which buildings are designed to resist uplift is as follows:-
The buildings overall mass may be increased to provide a sufficient dead load (weight to resist any vertical uplift forces.)
The buildings foundations can be oversized so that the buildings mass again is increased and the uplift force resisted.
The buildings shape and size can also be designed so that the wind load does not increase and uplift can not be applied, or is reduced to a minimum.
Such items as low buildings or buildings with sharp narrow elevations can reduce the amount of uplift.
SEWARD, D (2003).
The building may also encounter a sway case wind load. This wind load is generally created when the wind blows onto the side elevation of a portal frame building and can be seen in appendix 2 drawing 10.
This diagram shows how the wind causes external pressures to the portal frame.
Internal pressures are irrelevant in this condition.
The way that portal frames are designed to resist wind loads from such directions can be seen in appendix 2, diagram 7 and diagram 4 (wind bracing).
These diagrams show bracing within grid lines 19 – 20 and 1 – 2 (diagram 4) and bracing in bays A – B (diagram 7).
Bracing will also be provided in bays 1 – 2 and 19 – 20 vertically so that the roof bracing transmits all wind loads to the outer walls for which in turn, transmit the load safely to the ground.
When the wind blows perpendicular to the frame as in appendix 2, diagram 10, the stability is maintained by the rigid joints of the portal frame connections at both foundation/column and rafter/column locations.
In buildings where the frame is not a portal frame the design is some what different.
When buildings such as houses and apartment blocks or office blocks are built, which do not have a portal frames, shear walls are provided to maintain lateral support and resistance against wind loadings.
There are 5 main designs which can be provided to resist wind loads mainly:-
Shear walls parallel as in diagram 12, appendix 2.
Shear walls at the ends of a building shown in diagram 13, appendix 2.
Diagonal Bracing at the ends of a building as shown in diagram 14, appendix 2.
Central Core structure as the main support in a tall building as shown in diagram 15, appendix 2 and finally
Arched structures which have an inherent lateral strength and require little additional support. This can be seen in diagram 16, appendix 2.
All these options would provide support to resist wind loadings.
3.6 Structural Actions
Having described which loads are being applied to Plot 400, this report will now identify the different structural actions which may be taking place within the structural elements.
The main structural frame of Plot 400 consists of:-
Supporting pad foundations
Supporting columns
Supporting first and second floor beams to the office area
Supporting rafters to the roof
Supporting secondary rafters to the cladding (Z perlins)
The main structural actions that may be acting on the structural elements are as follows:-
Tension stress
Compression stress
Shear stress
Bending/Flexure (axial and bending) stress
Torsion
Deflection
Tension
Tension in objects is caused when a material/element is stretched outside its normal parameters.
For example the rafter beams supporting the roof of Plot 400, as shown in appendix 2, diagram 6 and diagram 17 shows how the top section of the supporting rafter is in tension and the underside of the beam is in compression.
Tension is a reaction force applied by the action force, in this case the self weight of the beam and roof cladding, acting on the support rafter.
Compression
Compression acts mainly in a single direction and small compression members are usually known as struts, larger sections are known as columns or traditionally as stanchions.
When a column is under compression it can variably fail in two areas, mainly under compressive strength/crushing and buckling.
SEWARD, D . (2003)
The elements within Plot 400 that are under compression are the supporting columns which support the rafters and supporting main beams of the first and second floors.
These columns can be seen in appendix 4 items 6, 10, 16, 17, 23, 25.
The supporting mezzanine legs of the mezzanine floor are also in compression as shown in appendix 4, item 2.
Shear
Shear stress is a force which acts parallel or tangential to the face of the connection, material or element.
The shear stress factor of a material is the ability or strength of the material to resist itself sliding (molecular structure).
If the material has a high shear stress factor its molecular structure will be strong, preventing the molecules from parting easily.
The items which are in shear stress in Plot 400 will be the column/rafter connections and the column/beam connections.
These connections will experience shear across the connecting bolts and fixing plates.
Bending
Bending, also known as flexure is the force which acts perpendicular to the axis of the element.
For example on a simply supported beam, the bending moment acts perpendicular against the x – x axis.
This bending moment then causes compression and tension on the beam as in diagram 17, appendix 2.
This bending moment also causes shear parallel to the lateral loading.
<http//:www.en.wikipedia.org>
Plastic bending can also occur in a material, this is when the stresses exceed the materials yield strength and all supporting elements in Plot 400 will have been designed so that the yield is not exceeded and therefore plastic bending should not occur.
The bending stresses applicable to the steel frame in Plot 400 can be seen in appendix 2, diagram 18.
This diagram shows the maximum bending moments applicable to the frame of Plot 400 which has been designed as a Ridgid Portal Frame.
This diagram also shows the point of contra flexure, the point at which there is no bending in the beams/rafters and columns.
CHUDLEY, R, GREENO, R (2004).
Torsion
Torsion is the torsional moment or torque, which like a bending moment has the units of force X distance (KNM).
There are two types of torsion, compatibility torsion and equilibrium torsion.
Compatibility torsion is caused when a steel column or member has to twist to accommodate the deflection of another member.
This happens when say 2 supporting beams have to deflect (twist) to accommodate the deflection of the centrally supported beam.
Equilibrium torsion is when a load is off centre to the central axis of the supporting member, the connections of this support must be of sufficient strength otherwise the element will fail.
The best way to eliminate torsion is to ensure that the supporting beam/element is placed directly under the supported load, for example placing an “I” beam so that the Y – Y axis is central to the load.
SEWARD, D . (2003)
The elements within Plot 400 are not under direct torsion.
The way that all supporting beams/rafters have been designed is with the “I” beam placed centrally under the supported loads.
Deflection
Deflection is the amount of movement or deflection that a structural member experiences when put under load.
The amount of allowable deflection is set out in the building regulations and is set to stop alarm and panic of building users.
Other reasons are to stop any damage being caused to finishes such as plaster and wall coverings etc.
Deflection is also limited to stop movement and possible cracks being caused which may cause water penetrations and structure damage, for example a roof deflecting more than the allowable limit may mean that tiles become separated and allow water to penetrate into the roof space.
The elements of Plot 400 will all be under deflection.
The rafters supporting the roof, the beams supporting the upper floors and the columns supporting the rafters will all be subjected to deflection.
The reason they do not fail, or should not fall is due to the fact that the allowable deflection will have been calculated and each section will deflect, but will not deflect outside its design limit.
Shell Roofs
A shell roof can be defined as a structural curved skin/material over a given plan shape.
The main design of a shell roof system is:-
The roof is primarily a structural skin/element which acts as a single element
The basic strength of the roof is determined by the shape
The amount of material used to cover a shell roof is generally less than a standard roof design.
CHUDLEY, R. GREENO, R. (2004)
Shell roofs can be constructed from concrete, steel and timber.
The shell action of the roof gives the roof its strength and the building at Derby does not have any shell actions or a shell roof.
Joints
Plot 400, Ascot Drive, Derby has many joints within the structure.
These joints connect columns to pad bases, columns to rafters, rafters to rafters and beams to columns.
The main connections within the structure at Plot 400 are:-
Welded Connections
Bolted Connections (both standard and HSFG)
Welded Connections
Welded connections are generally created using electric are welding.
The main two welds are a butt weld and a fillet weld.
The connections in Plot 400 which are welded are all plates to each end of the rafters and main beams.
Bolted Connections
Bolted connections are provided to support a given load.
There are 2 main types of bolted connections:-
ordinary bolted connection
This connection depends on the strength on contact between the bolt shank and the sides of the hole in the plates. This is commonly known as a dowel-pin action.
HSFG
High Strength Friction Grip bolted connections rely on tension between the two surfaces being bolted.
This connection provides a very high strength ridged connection and is ideal for fixing elements which are subjected to a load reversal such as wind bracing.
SEWARD, D. (2003)
STROUD FOSTER, J, HARINGTON, R. (2000)
This report will now identify the structural elements of Plot 400 which work as a:-
Suspension beam
Truss
Arched structure
The following structural elements in Plot 400 are acting as a suspension beam:-
Supporting universal beams which are providing support to the pre-cast floor units at First and second floor levels.
These can be seen in appendix 2, diagram 5.
The following structural elements in Plot 400 are acting as a truss:-
ii) The truss rafter beams which are supporting the roof element.
These can be seen in appendix 2, diagram 6.
The building at Derby does not contain any plate structures or shell structures.
However, these items will be described on how they act.
Arched Structures
Arched structures are a natural shape which inherently has a good level of strength and support.
The arch was utilised some 2000 years ago by the Roman Empire as large open spans could be achieved.
The support reaction of an arch has a vertical as well as a horizontal component.
An arch depends entirely on the ability of its supports to resist the horizontal components without excessive movement.
The most common cause of arch failures is due to foundation failure.
SEWARD, D. (2003).
Examples of arched structures are:-
the old Roman aqueducts
the Gothic Arch buildings
Bridges
Portal frame buildings (Plot 400)
The common portal frame building today consists mainly of a 3 or 2 pin structure.
The building at Derby consists of a 3 pin structure, the central rafters are pinned and both columns are pinned at foundation base level.
This type of arch is statically determinate and fairly simple to analyse. Examples for the bending moments applicable to Plot 400 can be seen in appendix 2, diagram 18.
The portal frame/arched structure has 3 maximum load cases that must be applied mainly:-
maximum vertical load
maximum sway
maximum uplift (overturning)
Once all these cases have been designed into the portal frame, the structure will be sufficient to accommodate all wind, dead, live/imposed and overturning loads.
The building at Derby does not contain any shell structure this has already been considered in the previous section.
The last remaining item is a plate structure.
This is a structure which acts together as a single element in a horizontal plane.
The building at Derby does not contain any plat structures; the closest item within the building which may constitute a plate structure is the composite, hollow rib deck floor system.
This is a system which comprises of a concrete, in-situ floor which is reinforced to give lateral strength, this composite deck is then connected to the supporting main beams and in some cases positively connected to resist any disproportionate collapse risk.
Structural Stability
The building at Derby has been designed as a simple portal frame structure which has to provide many functions as detailed in section 3.2 of this report.
This section will now discuss how the building has been designed to resist any overturning.
The building at derby, Plot 400 has 2 cases of overturning.
The fist case of overturning is due to wind loads, these wind loads cause large negative and positive pressures within and outside the building envelope causing an “uplift” force as on a wing of an aeroplane.
The second area of overturning that must be designed into the building is into the Foundation bases which are situated in a Fire boundary Condition. These bases area also known as Constrada bases and can be seen in appendix 2, diagram 2 and 20, on grid line A bases 1 – 20.
All these bases are oversized to resist any overturning of the supporting column in a fire condition.
This has to be provided to satisfy the requirement in the building Regulations 2000, each base is provided with a minimum volume to ensure that should there be a fire, and the roof section collapses pulling the truss rafters inwards, the portal frame columns do not fail.
Constrada bases are not required if the building has a sufficient sprinkler system installed as the sprinklers tend to control the size of a fire, but do not put the fire out.
The stability of the Portal frame structure is mainly provided by the self weight of the building and the foundations provided at each column location.
The basis of a Portal frame structure (arched structure) is that there are only the vertical loads on the structure and that for horizontal equilibrium, the horizontal reactions must be equal and opposite, thus resisting any overturning motion on the portal frame.
This can be applied to Plot 400 and can be seen in appendix 2, diagram 19.
This diagram shows that the vertical forces create exact equal and opposite forces, the equilibrium of the horizontal forces create stability and resist any overturning motion.
The buildings self weight also plays a part in keeping the structure in the ground and not overturning, the amount of force required to overturn the building would have to be larger in magnitude than the buildings total dead load.
Also discussed was the possible overturning condition to foundation bases in a fire situation
The way that foundations are designed to resist this can be seen in appendix 2, diagram 20.
This diagram shows how the large volume of concrete foundation overturning (torsion/rotation) of the column.
The forces of the overturning motion are equal and opposite to the forces provided by the large surface area of the Constrada base.
The self weight of the structure along with the frictional resistance of the column foundation, provided by the large surface area, satisfies the resistance requirement in this condition.
The Steel Construction Institute (2002).
3.8 Deformation and mode of collapse
Having identified the different loads applicable to the portal frame of Plot 400 mainly:-
Dead loads
Imposed (live) loads
Wind loads
Snow loads
Hydrostatic loads
This report will now represent via sketches how each structural element would deform under the action of each load.
These diagrams represent how each element would deform within its design limit, this would not be a failure as all structural elements deform when subjected to a load. The diagrams all show exaggerated deformation.
Dead load
Diagram 21 appendix 2 shows how the rafter truss supporting the roof structure would deform; in this case deflection would be the action of deformation.
Diagram 22, appendix 2 shows how a main beam supporting the first and second floor levels would deflect under dead loads.
Diagram 23, appendix 2 shows how a supporting column may deflect, in this case buckle when subjected to a dead load from the roof, cladding and services etc such as lighting, sprinklers and heating/ventilation systems.
Imposed loads
The imposed loads of the building mainly apply to the main beams supporting the upper floors.
The imposed loads within Plot 400 are:-
People
Furniture
Computers
Snow loads
Wind loads
Diagram 24 shows how the main beam of the offices may deform under deflection when live loads are applied.
Wind loads
The building will deflect under wind loads; however the building is sufficiently designed so that each gable end transfers the wind loadings to the outer walls and safely to the ground.
Diagram 25 shows how the steel frame would deflect under wind loadings.
Snow loads
Snow loads are applied to the supporting rafter beams, columns and in turn foundation bases.
Diagram 26a shows how the rafter would deform under snow loadings.
Hydrostatic loads
The hydrostatic loads would be applicable to the pad foundations, in particular if the ground became water logged and saturated.
Diagram 27a shows the hydrostatic forces applicable to a pad foundation.
Having detailed how the structure would deform under the previously mentioned loads, the typical modes of collapse will now be detailed within this report.
The typical modes of collapse within Plot 400 Derby are as follows:-
Fatigue
Shear
Deflection
Torsion (warping)
Creep
Fracture (ductile and Brittle)
Buckling (Euler buckling formula)
Compression
Fatigue
Fatigue failure is caused by fluctuations in stress.
As a material goes through stress cycles the element is put under stress which may go outside the materials yield point.
If the yield is exceeded and a constant stress cycle is endured, small cracks form within the material and eventually cause failure.
In Plot 400, this would not usually be the typical failure, this type of failure is usually related to elements which experience vibrations such as aeroplane wings and car chassis.
SEWARD, D. (2003)
A fatigue failure can be found in diagram 26b, appendix 2.
This diagram shows a beam which has failed due to constant stress cycles which gradually went beyond the materials yield strength and through many cycles caused a crack.
Shear
Shear is caused through bending movement and shear stress is experienced parallel or tangential to the face of the material as opposed to normal stress which acts perpendicular to the face in question.
<http//:en.wikipedigl.org>
A shear stress fracture can be seen in appendix 2, diagram 27c.
This diagram shows a concrete section which has sheared at its weakest point.
Photograph taken from:-
<http//:images.google.co.uk>
Deflection
Deflection is mainly a problem in beams which span large distances, cantilever beams and elements which have a low elastic limit, for example materials such as timber and brittle materials such as un-reinforced concrete beams etc.
Diagrams 22, 24 and 26a show how beams mainly the rafter beams and supporting floor beams of Plot 400 could deflect under self weight /dead loads and imposed loads.
Torsion (warping)
Torsion is caused when a material is subjected to a rotation force, sometimes materials or structural elements have to accommodate an adjacent element deflecting, thus causing torsion.
There are no elements in Plot 400 that are put under pure torsion, compatibility torsion or equilibrium torsion as all supporting main beams have been designed to support all loads directly under the centre to eliminate any eccentrically loaded structural elements.
Diagram 27b shows how a beam would fail under torsion should this be applicable.
Creep
When a load is applied to a structural member it causes strain within the elements molecular structure, if this load is applied to the element for a long period of time the member will deform, this is known as creep.
Diagram 28 shows how a section of lead pipes have failed under creep over a number of years due to the self weight of the lead and the molecular strength.
Plot 400 would generally not have any elements which would be subject to creep as this would have been included within its design state/limit.
<http//:images.google.co.uk>
Fracture
Fractures are caused in structural elements when the elements design limit is exceeded.
Diagram 29 shows a ductile fracture of a HSFA bolt. This bolt failed due to the size of strain put onto the bolt.
The elements of Plot 400 which could fail under a ductile or even brittle fracture could be:-
Supporting main beam to first/second floor levels (massive overloading causing excessive deflection and eventually a shear/brittle fracture of the steel beam or connection plate).
A supporting column due to massive overloading or excessive wind/snow loadings could cause the column to buckle and eventually shear.
A supporting rafter beam due to excessive dead/live load from snow and wind loads causing deflection and eventually a shear failure.
Buckling
Buckling happens to long slender structural elements such as the main supporting columns of Plot 400.
The amount of buckling is calculated by the “Euler Buckling Formula” which is calculated using the modulus of elasticity (E), second movement of area (I) and the length (L) between the two pins, in this case the column to foundation base connection and the rafter/column connection.
By assessment the longer and smaller the column the more prone to buckling the column will be.
Diagram 30, 31, and 32, appendix 2, show beams and columns which have buckled under excessive strain/forces being applied.
The elements within Plot 400 which could fail in this manner are the supporting columns to the portal frame and supporting main beams over the first/second floor office accommodation.
Compression
Compression occurs when loads are applied to members eccentrically or a bending moment can also cause localised crushing in compression.
A compression failure can be seen in appendix 2, diagram 33.
This diagram shows a concrete reinforced column which has failed due to huge compressive forces acting down the column.
The columns of Plot 400 and supporting main beams are all under compression.
Should the main beams or columns fail they would either buckle (columns) or twist (torsion on main beams).
This can be seen in appendix 2, diagram 32 which shows a main beam under compression, for example if the office loadings to Plot 400 had not been calculated correctly and under estimated, the beam would deflect under excessive compression as shown.
An example of a major collapse and structural failure can be seen in appendix 2, diagram 35 and 36.
These diagrams show the Tocoma Narrows Bridge failure.
It was November 7, 1940 at approximately 11.00 am when high winds were experienced by the bridge and due to wind induced vibrations and torsional mode oscillations, the bridge collapsed.
The centre of the bridge was stationary at the time this happened but the 2 edges were moving in opposite directions (resonance).
<http//:enm.bris.ac.uk/anm/tocoma>
4.0 Conclusion
This report found that Plot 400 contains a 3 pinned portal frame which has been designed to resist the following loads:-
Dead loads
Imposed loads (live loads)
Wind loads
Snow loads
Hydrostatic loads
The report was commissioned to carry out a structural appraisal of Plot 400, Derby to determine the loads which have been designed for and what possible failures could occur within the structure.
Many different types of legislation govern the design and use of buildings in England and Wales and the main legislation which is applicable to Plot 400 was listed in section 3.3. of this report.
Plot 400, Ascot Drive, Derby has been designed to meet the client’s requirements for both spatial and operational purposes.
These purposes were mainly:-
Accounting
Ordering of materials
Management
Human Resources
Legal Responsibilities
Communications and data transfer
IT
Welfare facilities
Parking
Meetings
Weather resistance for occupants and products/materials
The building at Plot 400 achieves all these requirements and also meets the functional requirements of the Planning Regulations and Building Regulations etc.
Drawings have been produced of Plot 400 which are Auto CAD and sketches which were produced from the photographs which can be found in appendix 4 of the report.
These drawings show both plan layouts, elevations and sections of all parts of the building.
Having determined which loads were applicable to Plot 400, the structural actions were identified and any modes of failure which may be applicable to the supporting structure of the Plot 400.
It was found that the following structural actions were present within the supporting frame at Plot 400:-
Tension stress
Compressive stresses
Shear stress
Bending stress (flexure)
Torsion
Deflection due to bending etc
Within Plot 400 there is also some design features that have been included to resist the factor of overturning.
These design factors were mainly the increase in dead load of the building itself and secondly for the Fire Boundary Condition that exists to grid line A, the design of Constrada bases was included.
Both these factors ensure that the building will not overturn due to wind loadings (differences in pressures) in a normal condition and the portal frame columns will not overturn in a fire condition, thus decreasing the amount of protection to adjacent buildings when the rafter beams/roof section collapse inwards.
The building has many structural actions in momentum and diagrams have been produced to show how a structural element would deform when put under this action. These diagrams are all in appendix 2.
The modes of collapse were also identified these were mainly:-
Fatigue
Shear
Deflection
Torsion (warping)
Creep
Fracture (ductile and brittle)
Buckling (Euler Buckling Formula)
Compression
The way in which structural elements collapse under these modes has been highlighted within appendix 2.
These diagrams show how each mode of collapse would cause a structural element to fail.
5.0 Bibliography.
1) Understanding structures, analysis, materials, design, 3rd Edition, Derek Seward.
2) Advanced Construction Technology, 4th Edition, Roy Chudley and Roger Greeno.
3) Building Construction Handbook, 5th Edition, Roy Chudley and Roger Greeno.
4) Mitchell’s Structure and Fabric, Part 2, 6th Edition, Jack Stroud Foster and Raymond Harington.
5) Single storey steel framed buildings in fire boundary conditions, 1st edition, The Steel Construction Institute.
6) Approved Document A, 2004 Edition, ODPM.
6.0 References.
British Standard Institution (1988) Loading for buildings – part 3: code of practice for imposed roof loads. BS 6399, London:BSI (report)
British Standard Institution (1996) Loadings for buildings – part 1: code of practice for dead and imposed loads. BS 6399, London: BSI (report)
British Standard Institution (1997) Loading for buildings – part 2: code of practice for wind loads. BS 6399, London:BSI (report)
British University, home page, (no date) Tacoma narrows bridge disaster [online]. [cited 14th October 2007]. <http//:enm.bris.ac.uk/anm/Tacoma>
CHUDLEY, R., GREENO, R. (2004) Building construction handbook. 5th ed., Great Britain: Elsevier Butterworth Heinemann.
CHUDLEY, R., GREENO, R. (2006) Advanced construction technology. 4th ed., England: Pearson Prentice Hall.
Google Images, home page (no date) Shear Fractures [online]. [cited 03rd December 2007]. <http//:images.google.co.uk>
Multimap, home page (no date) Location finder [online]. [cited 14th October 2007]. http//:uk8.multimap.com
ODPM. (2004) Approved Document A., London: The stationary office
Seward, D. (2003) Understanding structures, analysis, materials, design. 3rd Ed., China:Palgrave Macmillian.
STROUD FOSTER, J., HARINGTON, R. (2000) Mitchell’s structure and fabric part 2. 6th ed., England: Pearson Longman.
The Steel Construction Institute (2002) Single storey steel framed buildings in fire boundary conditions. Department of Trade and Industry. 1st Ed., Berkshire: The Steel Construction Institute.
University of Washington Libraries, home page (no date) Special programs [online]. [cited 14th October 2007]. <http//:www.lib.washington.edu/special>
Wikipedia, home page (no date) Mechanical resonance [online]. [cited 14th October 2007]. <http//:en.wikipedia.org>
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