Steel Frame Construction Advantages And DisadvantagesIn contrast to conventional steel, mild steel is high quality steel which has mild and thin properties, but has conventional steel equivalent functions. This mild steel is a type of cold formed steel.
Although thin, mild steel has a high degree of tensile strength which is around 550 MPa, while ordinary steel is around 300 MPa. This tensile and stress strength to compensate for the thin shape.
Steel Frame Construction Advantages
- Because of its light weight compared to wood, the load that must be borne by the structure below is lower (so the structure is more economical)
- Mild steel is non-combustible.
- Mild steel cannot be eaten by termites
- Installation of lightweight steel roof truss is relatively faster when compared to wood frame.
- Mild steel has almost no expansion value and shrinkage, so it doesn't change because of heat and cold.
Steel Frame Construction Disadvantages
- The lightweight steel roof frame cannot be exposed like a wooden frame, the net-shaped frame system is less attractive without a ceiling cover.
- Because this structure is like a net, if there is one part of the structure that is miscalculated, it will drag the other part, meaning that if one part does not meet the security requirements, then the failure can occur as a whole (usually the structural calculation is done by a structural engineer from the applicator )
- Lightweight steel roof frames are not as flexible as wood which can be cut and formed various profiles.
Steel frame shapeThe steel frame has various shapes. The following reviews are limited to rectangular frames. The frame element can be bordered by a hinge or rigid angle. Hinges can be given both at the support and at the top of the column.
- 1. A single span steel frame is a rigid system, if four of the frame vertices are no more than three hinged and at least one is rigid. In this case, a rigid steel frame assembly must absorb all bending moments. This frame is called a hinged three frame.
- 2. If the frame has two rigid assemblies, the moment felt by the individual frame unit is almost half. Such a framework is called two hinges. If the hinges in the column support, only horizontal and vertical forces are transferred to the foundation of the wind load.
- 3. The most rigid single span frames do not have hinges. The foundation in this case also requires bending moments.
- 4. Other spans can be attached to a single span frame, which may only have rigid assembled hinges.
- 5. If in adjacent fields several or all nodes are rigid, the multi-span frame is obtained where the bending moments at the vertices of the horizontal load decrease accordingly.
- 6 and 7. Single or multi-span span frames are mounted on each other, forming a multilevel frame.
- 8. Medium support from bolt frame on post hinges.
- 9. On the attic floor, it is possible to use a slanted bar that matches the slope of the roof.
Optimal number of frame elementsOften the question arises whether all or only individual columns are involved in the framework as a frame rack. A different perspective is shown in a simple example.
Four-level four pillar steel frameAll transverse discs are a four-level framework of four pillars. The longitudinal disk is designed in the form of a four-column six-story frame. Horizontal strength is distributed to all columns. This solution is best suited for tall buildings.
Two-level four-level steel frameIn the transverse direction only two disks of a multilevel frame are formed and only the middle column is rigidly connected to the crossbar. In the longitudinal direction, only one range is made in the form of a two-tier four-level steel frame. Because the concentration of force in some internal columns is rigid, all other columns must be calculated as structures that only contain vertical loads. Thanks to this, overall, material savings are achieved. The same applies to the foundation. The foundation under the main pillar must be bigger than the others.
Multi-floor multi-span steel frameThe university building, which consists of standard elements, has the same rigidity in both directions because of the multi-storey multi-span steel frame. Internal partitioning does not violate the building's free layout. Transversal-level frames are formed from lines and channel columns placed in pairs, in the longitudinal direction, the same crossbars are placed between columns. The construction of the ceiling, which consists of sheet blocks with a distance of 1.8 m and a plate, depends on walking.
Three-level steel frameOffice buildings with square plans and 11 m square columns. Stiffness of the building is provided with two spaces in both directions in three ranges of multilevel steel frames. The building only has four internal columns. Because of frame construction, the plane between the inner columns is free of bonding or solid disks.
Steel frame garage buildingThe two parts of the garage building are offset by half the floor height, which are connected to each other using the Dumy ramps. To ensure the rigidity of the building in a transverse direction, the frame structure is designed. Ramps are rigidly bound to the center column of the frame.
The middle longitudinal column, rigidly connected with the guide for utilities forming multi-tiered multi-span frame by frame. So how many times does the static frame system have a significant margin, occasionally off work due to single bolt damage is not a problem.
This construction design is an example of the use of elements that are made economical for other purposes, to ensure building rigidity.
Provision of rigidity of high-rise buildingsIn high-rise buildings, the right choice of rigidity is very important, because an important part of the cost of building tall buildings falls on providing rigidity (up to 50% of steel consumption). The main horizontal load is determined by the influence of wind and seismic. Because the latter in many cases does not play a large role, only the influence of the wind is considered. The wind causes deformation and, in connection with the formation of vortices, building vibrations.
Wind velocityStatic wind pressure depends on wind speed, building height, building shape and surface smoothness. In most countries, based on calculations is the highest wind speed, measured over a period of 50 or 100 years at an altitude of h10 = 10 m above ground (estimated wind speed). In Germany, V10 = 30 m / s.
Vn wind speed increases with increasing altitude. The Vn / V10 ratio is shown in Figure. 1 for different conditions. DIN 1055 (sheet 4) only provides one stage of charge (a curve) in Germany and does not provide data for very high structures. In the US (according to Lipots), an increase in slower wind speeds with increasing altitude in the area built is accepted.
High speed wind pressure qb = V2 / 16.
Aerodynamic coefficientThe shape of the building is very important for wind loads w = cq, where c is the aerodynamic coefficient, which for tower buildings is 1.4-1.6. More accurate values for tall buildings can only provide aerodynamic tests. Always strive for symmetrical plans (Figures 2.1 and 2.2). Asymmetrical or upside-down plan, as shown in Figure. 2,3, it is not acceptable for high-rise buildings, because they cause, besides bending pressure, also torque. Giving their rigidity is the reason for a significant increase in value.
The rough or smooth surface of a building can give a value to the coefficient with a large influence determined by the study.
Horizontal building deformation
Deformation dimensionHorizontal deflection increases with building height and the less, the more reliable the stiffness system. Tolerance is not limited to instructions. According to American data, the theoretical deviation of high-rise buildings is in the range of 1/200 to 1/800 of their height. In general, buildings with deviations from 1/400 to 1/600 of their height can be considered as quite rigid.
Knowledge of expected deformations from structures important for calculation and installation of extensions that can be installed. External and internal walls that are not involved in the transfer of horizontal forces must be arranged so that they can withstand the expected deformation of the building without unwanted cracks or other damage.
Oscillation typeHigh structural vibration properties are subject to investigation. The frequency of natural oscillations must differ significantly from the frequency of wind oscillations. In order to accelerate obtained due to the influence of the wind, it is not pleasing to humans, it must be less than 0.5 m / s2.
On picture. 3 shows the relationship between oscillation characteristic time λ scale of building vibration (amplitude) AG and emerges from this maximum acceleration with% of gravitational acceleration g = 9.81 m / s2. Notation:
For illustrative explanations: 15% of the acceleration of g, which is 0.15g = 0.15 • 9.81 = 1.57 m / s2 according to, which is 10 seconds from a stationary position reaching a speed of 53 km h.
Constructive abilityThe rigidity of high-rise buildings is provided by the structural work described in this chapter: frame, bond, disk wall.
Often, all cross bars and columns are also firmly attached to the grating or downstairs, additional reinforced concrete walls are also added.
Structures that provide rigidity must pass through all floors to the ground. The requirements for large columns of free space on the ground floor can cause a significant increase in costs due to the introduction of special heavy elements for troop transmission from the top floor to the column columns of other columns.
Large diaphragm to provide building rigidityIf the needs of large buildings, such as wall firewalls, stair walls, elevator shaft walls or shaft equipment techniques, they are used to provide building rigidity. Because in construction using steel frames, building rigidity can be ascertained with the help of grating, massive walls are not economical.
The core rigidity with the placement of means of vertical transportationFrom reinforced concrete construction, the idea of a large "core" is taken, that is, from towers where elevator shafts, stairs and technical equipment shafts are located, and often sanitation units. This design is also suitable for structures with steel frames; but often other solutions are better, because for vertical communication in structures with steel frames, only holes in the floor are often needed.
Laying stairs and stair areas in this case is suspended from the beam. For fire protection vertical communication lines with steel frames, heavy reinforced concrete walls are not needed. There are lightweight and inexpensive construction materials. Nor does a sanitary nodule need to be placed in the core of massive violence. To pass through communication facilities, it is often necessary to have large holes in the wall shaft that use them as wind discs to be difficult. The stiffness of the wind in these cases is often better done with the help of large span vertical grating, but this is not the only solution.
Monolithic work methodMassive diaphragm wall or hard core can be made from monolithic reinforced concrete. At the core of high rigidity, you can use sliding formwork.
Provisions for constructionIn the construction schedule, the manufacture of reinforced concrete parts and the installation of steel frames must be carefully coordinated with each other. The hard disk wall must be made together with the installation of the frame. The stiffener core is recommended to fully produce first. This makes it possible to use a lifting mechanism to attach a steel frame.
ToleranceIt is necessary to consider the differences in tolerance taken during the construction of monolithic reinforced concrete and with steel frames. This must be borne in mind when designing connections between the core of carcass and concrete hardness, leaving enough gaps in this connection that are embedded in place during installation.
ConnectionSteel frame beams must be tightly connected with reinforced concrete walls that provide building rigidity. Beams convey vertical support reactions to reinforced concrete walls, and some horizontal strength.
Prefabricated reinforced concrete elementsThe organic construction process is obtained in cases when the walls are assembled from reinforced concrete elements. Efforts in the connection between the walls and steel structures are transmitted through a wrapped steel profile. Thanks to this, tight connections have occurred during the installation. However, this construction method only applies to buildings of limited height.
For objectivity solutions, we compare various points of view. Large walls or rigid cores are very useful:
- if an elevator or ladder can provide building rigidity and at the same time fire protection (when using heavy concrete, it is enough 14 cm for firewall walls and 10 cm for fireproof walls);
- if the lattice link specified in the frame is not rigid enough or if not at all;
- if the concrete tower lifts and stairs stand outside the building, and for flexible use of the floor area, a large step from the column is applied.
Network ties are recommended:
- if possible a rare arrangement of lightweight vertical ties;
- If the elevator and stairs do not fit together;
- If the stairs are not located on top of each other, but are installed on separate floors in different places;
- if the elevator and tower stairs are planned in light, glass frames outside the building;
- if the construction time of the core is large rigidity does not allow its use as a rigid connection;
- if the shaft wall has a large hole.
The rigidity of a 20-storey office building is provided by a triangular rigidity core, built in the form of formwork. At the core of hardness, the lifting mechanism is installed for mounting steel structures. After the completion of the main construction, the superstructure was built on two floors.
This twenty-two office building is reinforced with reinforced concrete core, which includes elevator shafts, stairs and technical equipment shafts. The hard core is made before installation in sliding work.
Two consoles for assembling steel structures connected to the core. To secure the beam in reinforced concrete, the steel plate is locked in steel, which during the movement of the sliding formwork is maintained by using anchular steel anchors.