Design Info
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
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
What materials should be used for structures?
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
Steel
Disadvantages
Reinforced Concrete
Steel
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
Cost
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!
FoundationsA 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.
FoundationsA 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.
