Introduction to High Rise BuildingsFull description
ml
Description complète
Full description
Full description
Week 4Descripción completa
privity of contractFull description
In the structural systems of high rise buildings there are a multitude of structural systems forms, such as twisted, tilted, tapered and free forms. The article describes the characteristics of the diagrid system and its applicability in the construc
Week 4
Notes for Cven3501 Water resources covering week two of content.Full description
AFAR
In the structural systems of high rise buildings there are a multitude of structural systems forms, such as twisted, tilted, tapered and free forms. The article describes the characteristics of the diagrid system and its applicability in the construc
Descripción completa
Full description
In the structural systems of high rise buildings there are a multitude of structural systems forms, such as twisted, tilted, tapered and free forms. The article describes the characteristics of the diagrid system and its applicability in the construc
mFull description
toefl
Full description
Signal & Systems Class Notes.Full description
Signal & Systems Class Notes.Description complète
control system formulaeFull description
Signals and Systems Notes
Lateral load resisting structural systems for high-rise buildings
Hira
esign of High-rise Structures - State-of-the-Art
LATERAL LOAD RESISTING STRUCTURAL SYSTEMS FOR HIGHRISE BUILDINGS - PART A Anil Hira
SUMMARY The significance of lateral loading increases with increasing height, in terms of serviceability, serviceability, strength and stability limit states. Although for normal normal buildings the structural system is generally governed by non-structural factors, for the taller and more slender buildings the structural form becomes increasingly important. This necessitates the structural engineer to choose the appropriate structural form at a very early stage of the design development and to have a greater interaction with the other disciplines. For the very tall buildings the structural engineer will work closely with the architect form the initial conceptual stage. Often the structural aspects will dictate the Architectural form of the building. This lecture will introduce the basic lateral load resisting structural systems and some basic guidelines on the structural efficiency of each system.
The action comprises of transferring the lateral shear (sum of the lateral forces imposed on the structure above the storey that is being considered) and the overturning moment (sum of the product of lateral forces above the storey being considered times the distance to load from the storey being considered).
In the previous lectures it has been noted that the vertical load effects essentially increase linearly with building height however the effects due to lateral loads increase more rapidly with increasing building height. It has been shown in simplistic terms assuming an idealistic uniform structural system up the height of the building, the following relationships hold:
The basic structural systems to transfer the lateral loads from one level to the next can be broken into four basic systems. They are:
The base overturning moment (M) 2
MαH
•
(1)
Deflection at the top of building (∆) 4
∆αH
(2)
By considering the above it is clear that the deflections and to a lesser extent strength increases rapidly with height suggesting the importance of overall stiffness. Providing adequate stiffness forms the primary objective in determining the structural form of high-rise buildings. 2.
Flexural cantilever – (see Figure 2) The lateral load is resisted by cantilever action, via flexural action (bending) of the vertical elements, which can be columns or wall elements. The relative deflection of the storey will be dependent on the EI value of the element. Conversely the load attracted by the vertical element will depend on the flexural stiffness. ie
k
EI
(3)
L
Assuming that the vertical elements between a floor are the same material the load resisted by each of the elements will be proportional to the “I” value.
BASIC STRUCTURAL SYSTEM TO RESIST LATERAL LOADS.
Before discussing LLRSS for buildings it is important to have a clear understanding of the basic structural systems to transfer lateral loads acting on a building to the foundation. The simple approach is to determine the ways of transferring the actions due to lateral loads from one storey to the storey below as illustrated in Figure 1.
In typical high-rise buildings, wall elements fall in this category where the lateral deflection is primarily due to flexural deformation.
•
Shear action – Similar to cantilever elements except the lateral load is resisted by cantilever action via shear deformation. The relative deflection of the storey is primarily dependent on the “EA” value of the element.
Squat reinforced concrete walls, where the length of the wall is relatively large compared with the height of the wall.
Figure 4 – Frame action
F
•
h
Braced action – (see Figure 5) The lateral load is transferred by direct axial tension and compression by triangulated member arrangement. A greater stiffness is associated with this system compared with the others in terms of mass of material to achieve the stiffness. It is by far the most efficient system as all the stiffness is provided by axial stiffness.
Although very efficient it has one big drawback as it imposes many constraints on the architectural planning and in many cases is not practical.
V M
M = Fh V=F Figure 3 – Frame action
•
Frame action – (see Figure 4). This system consists of columns and beams connected rigidly. The lateral load resistance is provided by the flexural resistance is provided by the flexural resistance of the columns, beams and the joints. Buy considering the actions in Figure 4, lateral stiffness will also be dependent on the axial stiffness of the vertical elements.
Such a system has the advantage in highrise construction due to maximum flexibility in architectural planning, as the frames can be located along the perimeter or internally with minimal interference with space.
Figure 5 – Braced action
The basic structural systems given above to transfer lateral load from one level to the level below can be extended vertically and horizontally with the objective to transfer the lateral loads down to the ground.
A typical frame subjected to lateral loads is illustrated in Figure 6. The size of members in a moment resisting frame, subjected to lateral loads, is often controlled by stiffness rather than strength to control to control the deflection, especially with frames of increasing height.
b) c)
Frame racking due to beam flexure (5060%) Frame racking due to column flexure (1520%)
The maximum height for an efficient rigid frame system is approximately 30 stories after which the required beam stiffness and column stiffness to limit the deflection due to shear racking starts becoming excessive.
Figure 6 – Frames subjected to lateral loads.
The lateral stiffness of the frame is made up of two components
• •
Cantilever moment
Figure 7 – Cantilever moment and shear racking.
Shear racking 4.
The components can be explained by considering a prismatic cantilever beam where the deflection is made up of two components: bending deflection and shear deflection. For typical cantilevers with span to depth ratio is greater than 10, bending deflections is the predominant component and shear deflections are small and often ignored. The deflection characteristic of a rigid frame on the other hand is just the opposite where the shear deflection of the frame (shear racking) can amount to as much as 80% of the total and the remaining 20% is attributed to flexural deflection. (Cantilever moment) The two components of the frame behaviour are illustrated in Figure 7. By noting the behaviour of the frame it can be seen that the total stiffness (lateral load deflection at the top of the building) is dependent on a)
Axial deformation of the column (15-20%)
BRACED FRAMES
A braced frame attempts to improve upon the efficiency of a rigid frame by virtually eliminating the bending of columns and beams. This is achieved by adding diagonals to the frame and hence triangulating the frame and therefore allowing the applied horizontal forces to be resisted by direct axial forces. The stiffness of the braced frames therefore is directly related to the axial stiffness of the diagonals and column. Similar to the rigid frames the stiffness of the braced frame can be broken into the two components.
•
Flexural deformation due to the axial extension and shortening of the columns and,
•
Shear deformation due to the axial deformation of the chord members.
The above deformations are illustrated in Figure 8
The bracing system is not just restricted to cross bracing. Other forms of bracing are illustrated in Figure 9. It can be seen by considering Figure 10 that some of the configurations will attract gravity loading into the diagonals elements whereas for some configurations the diagonals only attract lateral loads.
Figure 9 Bracing types a. single diagonal bracing b. X bracing c. Chevron bracing d. Single diagonal (alternate direction) e. Knee diagonal
Figure 10 – Gravity load paths in bracing frames. 5.
STRUCTURAL SYSTEMS FOR BUILDINGS
The structural system for buildings will comprise of one of the systems or more likely a combination of the systems discussed in this lecture. There are many factors that will dictate the likely system. Some of the factors include
• • • • • • • •
Height of the building Magnitude of the design wind loads Magnitude of the seismic load Architectural considerations Limitation on size of the members Ductility of the frame. Material availability Specialised labour availability.
Are just a few of the main parameters. The following sections will discuss some of the commonly adopted structural systems in highrise building design.
In tall building design it is important to recognise the importance of the interrelationships between the many disciplines that participate to develop that final solution. The crucial conceptual stage of the design process is often very short in duration. The distinctive features of tall building design at conceptual stage are: •
- 5.5 -
close communication required with other disciplines, in particular the clients and Architect.
structural system is based on minimum information and therefore significant engineering judgement is required. short time duration during concept stage often precludes the luxury of carrying out complex analysis etc. once the structural system is selected there is virtual impossibility of altering the system.
Regardless of all the factors that dictate the structural system it is fundamental that the select system satisfies all the structural criteria outlined earlier. The following sections will outline a guide on the common types of structural system adopted and its optimum structural efficiency in terms of building height. Each of the systems is essentially a combination of the basic lateral load systems outlined in the pr evious sections.
