Showing posts with label structural engineering. Show all posts
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October 01, 2018
Kier was the main contractor for this project. The contractor implemented BIMXtra, which was their own BIM management system to increase collaboration and integration within the team. The BIM software is a CDE, which consolidates information and data via a cloud based subscription service. Kier provided BIMXtra software free of charge in this project, which eliminated the need for expensive servers and systems. In BIMXtra, all information of the project was used for clash detection and coordination. Information from the BIM model was available directly to all subcontractors and fabricators for costing in an organized manner, which reduced double pricing or missing tasks.

Structural design changes were also done by BIM modeling. Structural engineers found out that the steel beams that were supporting the edge protection were not enough. They had to amend the design and construction process by fabricating the edge protection to be pre-installed to the steel beams before erection. Luckily, this was planned out during the design stage to avoid abortive works during construction.
Source: http://constructingexcellence.org.uk/

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September 30, 2018
The Shanghai Tower is the tallest building in China and second tallest in the world. In order to achieve its height, the geometry of the tower had to be optimized to resist lateral loads.

The cross sectional geometry is a spiral equilateral triangle with each floor rotated slightly from below, creating a 120 degree angle between the topmost floor and the lowest level.

The spiral geometry actually increases lateral resistance by 24%, as opposed to tapered box geometry. This saved $58 million in materials and construction procedures. There are over 7000 different geometries and 20000 dual-skinned curtain wall panels that make up the structural geometry.

Due to its complexity, it is quite difficult to utilize AutoCAD and other traditional 2D modeling methods to present and to analyze the spiral geometry. However, with Autodesk Revit Structure, the design team can design the geometry in 3D. Structural engineers can use this BIM model to analyze how the wind and seismic loads affect the spiral geometry. They were able to optimize both building materials and the structural framing system, by producing and choosing from more than 20 options of structural geometries.

Eventually, the design team chose the structural spiral geometry frame with super columns, outrigger trusses, and an inner concrete core due to its combined strong lateral resistance.

Revit and Naviswork were used for BIM clash detection. During the design of the tower’s basement, the team found seven design clashes using those BIM software. During construction, there were no more clashes and they focused their BIM efforts in combining Revit models and fabrication models in Navisworks for coordination.
Source: http://du.gensler.com/

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September 28, 2018
BIM is a trending digital phenomenon that is built for the multidisciplinary coordination of all project parties and the systematic integration of databases and information throughout the design, construction, and operational maintenance of a structure. By integrating elements of different disciplines and utilizing the various software and hardware technologies that BIM has to offer, various information can be centralized for the basis of enhancing decision making, problem solving, and productivity.

Recent developments in BIM help us structural engineers in the constructability and feasibility of our designs and construction methods. We will explore the use of BIM for our integrated structural designs and the BIM approaches in solving complex structural and construction challenges.
Source: http://enews.scia.net/

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September 26, 2018
The following aspects should be considered when designing for cost. These aspects should be consider when you want to decrease the cost of your structural design. These tips will definitely make your client very happy!
Foundations heavier superstructure increases foundation size, and therefore overall cost. Thus, the foundation size, the amount of reinforcement, the extent of foundation all depends on the superstructure and the loads that are acted onto it.

Superstructure
The cost of a heavier superstructure will not be greater than 0.04% of total structure cost.

Cladding
The cost of cladding is proportional to the area of the façade or the perimeter of the structure. The cost of cladding is about 25% of total structure cost.

Partitions
The labor to include sealing and fire resistance is cheaper for flat slabs, due to traditional methods. The use of flat soffits decreases 4% of overall structure cost and construction schedule.

Services
The soffit of a flat slab provides space for services. The lack of downstand beams provide large headroom space, which decreases cost of structure.

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September 24, 2018

Reinforced Concrete vs Steel

Concrete is a mix of aggregate, cement, reinforcement (for reinforced concrete) and water. It is strong in compression and weak in tension. Insitu or precast or ready-mixed concrete (pumped or compacted) can be chosen for design, depending on the needs of the structure and the requests of the client. The type of reinforcement (mild, stainless, plan, etc) and the composition of the mix can also be designed to suit the structure. Formworks used are usually made out of steel, timber, or plastic that can help keep liquid concrete in shape until it forms into a solid state.

Steel has a good strength to weight ratio and has elastic behavior until yield strength occurs. It can be formed into many different sectional sizes and produce great connections with welds and other steel connection designs. Steel characteristics or grades are chosen based on serviceability requirements of the structure. Such characteristics include strength, weldability, and degree of carbon resistance.

Advantages
Reinforced Concrete
●     High strength and provide long spans (especially when using prestresed concrete)
●      Good durability, waterproofing properties, weather resistance, and chemical resistance, so not a lot of maintenance cost needed
●      Provides robustness for precast structures due to monolithic nature. Disproportional collapse can be prevented by careful detailing.
●      Flexible for late design changes
●      Typical, faster, and cheaper (i.e. services can be installed earlier in the construction schedule)
●      Provides resistance from lateral loads by reinforced concrete core walls
●      Suitable for swimming pools due to some concrete having waterproof properties
●      Readily available in local markets
●      Fire resistant properties for good thermal insulation
●      Can achieve a 60 year design life and can be recycled at end of life
●      Provides aesthetics in buildings by using precast or insitu concrete to create unusual shapes at a small cost (i.e. curved beams, circular columns, shells, concrete columns inside walls)
●      Provides good sound insulation and vibration resistance because of the heavy mass property of concrete. Great for educational and residential buildings in order to prevent sound and vibration passing and disruption through individual units and for hospitals and laboratories that contain sensitive equipment.
●      Reinforcement can be bent prior to site delivery to prevent delivery and inconvenience of handling long rebars
  Steel
●     ​Light weight property so foundation does not have to be big in size, thereby reducing cost
●      High strength and is not affected by time-dependent stresses (creep, shrinkage, etc)
●      Structure loading path is sufficient so it is not required for transfer structures
●      Simple to construct and assemble steel sections due to option of pre-fabrication
●      Common and flexible for pile foundations
●      Less columns needed (no core walls)  so more interior space in plan layouts
●      Suitable for long span structures
●      Less structural depth in steel sections so more clear headroom available
●      Steel trusses provide design of hanger columns and structural walls
​
Disadvantages
Reinforced Concrete
●     Transfer structures are required for load paths
●     Concrete elements are much heavier than steel so the structural design requires heavier and costly foundations

Steel
●     Less available and common in local markets so demand makes steel more expensive
●      Longer procurement times
●      Should not be used for water-tight structures
●      More maintenance cost because of corrosion
●      Requires more expensive fire resistance treatment and special corrosion protection measures
●      Not a lot of flexibility due to fixed layouts and fixed fabrication orders.
●      Changes in steel section orders will cause delay and cost
●      Method of bracing gives unaesethtic effect and gives less stiffness and robustness for lateral stability than core walls and RC beam-column frame. All beam to column connections would need to be moment (fixed) connections to make the structure more resistant to wind.
●      Require experienced and skilful labor for installation
●      When in a highway, site access is an issue for steel structural members

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September 23, 2018
Structures should be designed and constructed to resist loading combinations under these limit states.
ULS = ULS is used to design for safety of the building's users and occupants and for structural stability. ULS limit states include stability (overturning, sliding), deformation, rupture and fracture.

SLS = SLS is used to design for the structure's performance, structural appearance, and the comfort of the structure's users and occupants. The structure should be checked against deflection limits, durability, and vibration and noise effects so that the building finishes, ceilings, cladding systems, and the performance and comfort of the building will not be damaged.

We have listed the most common values used for weights and densities and material properties and types required for the use of scheme designs.

Weights & Densities
Water = 10 kN/m^3
Soil = 20 kN/m^3
Mass concrete = 20 kN/m^3
Lightweight concrete = 18 kN/m^3
Stone cladding = 1.2 kPa
Facade (curtain walls) = 1.0 kPa
Services attached to ceiling = 0.5 kPa
Partitions = 4.5 kPa
Car parking (mostly for kerbs) = 0.5 kPa
Partial Safety Factors
γm = 1.50 for concrete
γm = 1.15 for steel
​Concrete
Grade 40
Strength fcu = 40 N/mm^2
Density = 24.5 kN/m^3

Reinforcement
Type II high yield deformed bar
Grade 460
Strength fy = 460 N/mm^2

Links
Mild Steel
Grade 250
Strength fy = 250 N/mm^2

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September 21, 2018
Possible Constraints:
1.    The constraint of the site can affect placement or availability of site offices and storage locations.
2.    The proximity of adjacent buildings to the proposed structure might affect structural (especially the foundations) and temporary works design.
3.    Constraints in site access can affect allowable equipment size.

Specific site boundary?
Yes-
The structure should not extend beyond site boundary.
Exterior face of all bored piles should be at least 1000mm away from the site boundary.
Not mentioned.-
The building boundary is the site boundary. No structures should be constructed beyond the boundary.

Development Type
The proposed structure is in the City Centre-
Working hours for construction should be reduced.
Less wind pressure should be used with structural design.
Avoid pile driving.
Noise and pollution should be controlled.
Shoring required for basement construction due to adjacent structures.
No open cut method due to lack of space.
Self-compacting concrete should be used to prevent vibration

The proposed structure is in a Remote Site-
Large wind pressure should be used with structural design.
Have enough space for open excavation.
Some materials, labour, plant, and equipment are not available and might not access the site.
Less restrictions on the access of the site and the use of large machines, such as bored pile machines.
Less restrictions on site operations.
​Limited, Restricted Access to the proposed structure-
Should not use open cut excavation due to it taking a lot of space for other usage and construction works.
Should not use structural steel due to material delivery issues.
Site conditions, space,  and traffic flow should be controlled.
Prefer pre-fabricated construction.
Can use temporary on-site concrete batching plant
Possible for access and use of large machines



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September 20, 2018
Existing underground structure under proposed building?
Yes-
Ground slabs on top of the existing underground structure should be suspended and should not be on grade.
Bored piles with sleeves and/or pile caps should be used. This will not create any undue stress or surcharge load on the existing foundation.
Diaphragm walls are preferred to be used in order to reduce vibration
Basement walls should be set back at least 500mm during excavation in order to reduce stress onto existing foundation in order to avoid imposing additional loading on the existing structures.
Soil Data
Sandstone-
Assume sandstone soil has hard bedrock with a bearing capacity of 3000kPa
Pad footing- Consideration of differential settlement if the foundation is on stiff clay. Footings cannot be used for fine sand soil.
When there is differential settlement, the designer should make the structure more flexible, so no damage will be done onto the structure due to settlement.
Weak soil stratum under layers of strong soil stratum-
Assume 30 degrees of load spreading.
Organic-
Soil disposal should not be frequent and soils should only be disposed at approved dump sites.
Clay-
There will be heaving caused by excavation and swelling, which is caused by changes in water table levels. The effects can be reduced by using an approximately 500mm compressible layer between the basement structure and the clay layer.
  Ground Water Table?
GWT above or near proposed building structure (i.e. Ground Floor)-
Should not use ground bearing slabs
Provide waterproofing
Prefer to use bored piles
Provide dewatering for excavation
Should not excavate more than the water table since groundwater and dewatering will be problematic
Provide drained cavity floor
Reduce allowable bearing pressure for shallow footing design
Consider uplift in stability calculations and that uplift pressure is usually greater and a crticial case When there is high water table, a recharge well can be constructed adjacent to the structure for dewatering purposes. Piezometers and settlement markers should be installed to measure the changes and the effects of the water table levels.

No GWT-
Uplift can be ignored

GWT 5m below-
Uplift can be ignored

Differences in ground level?
Yes-
Retaining wall should be used for the steps to accommodate for foundation level changes in order to increase resistance to surcharge and water loads

Ground level with slopes?
Yes-Retaining walls should be constructed along the building perimeter in order to resist surcharge, soil, and water loads.
Consideration for uneven uplift forces should be used when some sections of basements are at or near the water table.

Open cut with 30◦ degrees
Yes-
Open cut can be used when there are no adjacent structures.
Open cut should be used because its is easy and inexpensive since no temporary support are required.

Note: When geological conditions are unknown, more GIs must be implemented and a trial pile should be constructed to find the site’s bearing capacity for initial design.


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September 19, 2018
Progressive collapse
Story Number < 4 stories
Progressive collapse is not critical. However, the structure should be designed with minimum vertical and horizontal ties and that all connections for beams and columns should be cast monolithically. This is so the RC structure will have adequate reinforcement within to resist and avoid progressive collapse.
For buildings with more than 4 stories, there should be adequate horizontal and vertical ties and an alternative load path should also be designed. All connections should also be cast monolithically.
Cladding
Brickwork or brick cladding -
No need to set back columns since there is no glazing
Perimeter fully glazed -
Columns and slabs should be set back 200mm to cater for cladding or glazing installations and connections.
No external structural walls or columns should be allowed.
Steel truss at roof level
Yes -
Roller supports should be used to allow structural movement caused by thermal expansion
  Torsional effect
Yes-
​Torsional effect is created by the asymmetric arrangement of core wall. Its effect on the structure should be checked during detail design.
  Structural Element to be Exposed
Yes-
In order to provide good appearance, fair face concrete or circular columns will be used and that no false ceiling for slab soffit should be exposed. Fair face concrete or circular columns should be constructed by using special material with high standards of construction care. There should be no external cladding along the external perimeter walls.
No-
There is no need to set back edge columns
Plan Layout?
Open Plan Layout-
Only columns at 5m c/c should be used.
No structural walls should be used.
Column Spacing where  _m center to center-
All internal columns, perimeter columns, core walls should be spaced  __m center to center
Movement Joint?
MJ for structures with different height
Movement joints should be used to separate the structure into different independent sections in order to prevent differential settlement and stress caused by movement.
Movement joints reduce thermal effect and cracking effect caused by shrinkage.
Movement joints are composed of polythene sheet with bituminous paint, water stop, and joint sealant and are usually 25mm-50mm wide.

MJ for buildings longer than 60m
Movement joints should be used to separate the structure into different independent sections in order to prevent differential settlement and stress caused by movement.
Movement joints reduce thermal effect and cracking effect caused by shrinkage.
Movement joints are composed of polythene sheet with bituminous paint, water stop, and joint sealant and are usually 25mm-50mm wide.
Structures with irregular shapes
Movement joints should be used to separate the structure into different independent sections in order to prevent differential settlement and stress caused by movement.
Movement joints reduce thermal effect and cracking effect caused by shrinkage.
Movement joints are composed of polythene sheet with bituminous paint, water stop, and joint sealant and are usually 25mm-50mm wide.
Preferred no MJs should be used
Shrinkage strips should be used to reduce thermal effects and shrinkage cracking.
For carparks, shrinkage strips should be provided at the middle of the carkpark to reduce shrinkage on large plates of concrete.
Shrinkage strips should be used for basement slabs and swimming pools since movement joints are not allowed in these situations due to shrinkage strips are more effective for substructures in wtaer.
MJ for significant change in foundation load and height of structure
Movement joints should be used to separate the structure into different independent sections in order to prevent differential settlement caused by different types of loads and uses above and stress caused by movement.
Movement joints reduce thermal effect and cracking effect caused by shrinkage.
Movement joints are composed of polythene sheet with bituminous paint, water stop, and joint sealant and are usually 25mm-50mm wide.
Lift Core or Service Core?
Yes-
Lift pit should be considered by assuming its level to be 1.5m - 2m deep below.
Lift machine room should assumed to have a 4m height on roof level.  The lift cores should be used to resist lateral loading.
Lighting?
High degree of natural light-
Perimeter should be glazed. Location of perimeter walls or columns should be considered for light paths.
​Artificial lighting used-
The structure can have perimeter walls or structural walls at the exterior enclosure.
Roof used as plant area with wall enclosed?
Yes-
The plant room should have lightweight waterproof as its exterior enclosure.
Core Wall Dimensions
Yes-
Core dimensions is to be the external setting out dimension
Parapet Wall at Roof
Yes-
Parapet wall should be designed as 1400mm height
Heaving Effect Negligible
Yes-
This is because the basement or ground floor slab is on top of sand and gravel
Earthquake Load
Yes-
Earthquake loads are not significant in Hong Kong
Elegant structure?
Yes-
Structure should be light in weight, have small member sizes, and have good appearance
Swimming pool?
Yes-
Waterproofing details should be shown, as well as construction approach.
Requires a Fast Construction Programme?
Yes-
Construction methods should be standardized and repetitive.
Flat slabs should be used in order to speed up construction.
Standardized formworks should be used to speed up construction.


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September 18, 2018
Active soil pressures are generally used for soil load calculations. Active pressures are applied loads induced by the soil onto the contained environment. Passive pressures are forces induced by the soil's resistance to applied loads. Passive pressures are generally not conservative for calculations. This is because there will be a worst case scenarios if the soil mass is removed or when an empty space develops between the wall and the soil due to hydration. There will be no passive resistance due to lack of forces induced by soil resistance and there will only be passive resistance until the wall moves towards and is in direct contact with the soil for the soil to resist the wall mass. Constants used to determine soil pressures include the angle of repose and soil/wall friction.
Soil load calculations are often calculated for soil pressure induced onto retaining walls and basement walls. Here are some typical loads used:
Uniform surchage load = 10 kPa
Soil load = 20 kPa using dead load factors
Hydrostatic water pressure should also be designed for, assuming it acts onto ⅓ height of the wall. However, full hydrostatic head will be used for soils with high water table.
Propped retaining wall     Ko = 0.7
Overconsolidated clays   Ko = 1.5
Overconsolidated sands  Ko = 1.0
​​Other pressures
Pressures that need to be considered when constructing retaining walls, basement walls or any other substructures include the following.
1.    Earth pressure
2.    Water pressure (accounting for extreme flood conditions, buoyancy where groundwater is taken the full basement depth)
3.    Surcharge from adjacent structures

Fluid Loads

Fluid loads should be calculated by using the static pressure of liquid and its effect on its containment (pressure = density x height of fluid in the contained environment).
Common uses include water tanks and basement walls.

Vehicle Loads

Vehicle loads should be incorporated into the structure’s design, when the structure consist of parking lots, ramps, highways, loading bays at industrial buildings, bridges, and any other structure that has vehicles moving on it. Not only do self-weight of the vehicles should be included in the design, vehicle impact loads should be included, as well as, the design of kerbs and rails and other protective and preventative measures for vehicle impact.
Normally, 2.5 kPa should be used for uniformly distributed loads for a car parking area and 5 kpa  should be used for uniformly distributed loads for a commercial vehicle area.

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September 17, 2018
Wind load values are based on the height of the structure, in which design wind pressures with respect to height about site or ground level can be obtained. Values are also dependent on the shape of the building, in which shape factors can be obtained. Wind forces are derived with the following equation obtained from the Code of Practice on Wind Effects in Hong Kong 2004. Wind load cases that govern critical load combinations include tall and slender structures and roof slabs with long spans.
​Source: Code of Practice on Wind Effects in Hong Kong 2004
​Source: Code of Practice on Wind Effects in Hong Kong 2004

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