Lateral Bracing System

Lateral Bracing System:

Lateral Bracing System is a world-wide concept to increase the strength of a building structure. This design is found in buildings where the first storey contains a parking garage or an open commercial area for stores and the upper floors for house or offices or apartments.

This design creates a discontinuity of strength and stiffness. If all stories are approximately equal in strength, the entire building would bend in an earthquake. If the first storey is softer, or more flexible than the other stories, the bending would concentrate there. Because the first floor is also the most highly loaded, the problem is compounded, thus possibly causing column failure.

This also will put additional stress on the connection between the first and second stories and can cause the building to collapse. Building configuration can have significant effect on how a building performs in an earthquake. Generally the simpler the design and the more balanced the building and its structural and non-structural components, the better the building will perform during an earthquake.

Lateral Bracing is a Lateral Support of any structure to resist the upcoming Lateral Earth Pressure due to Earthquake. Lateral bracing is the term to refer to any pieces on a structure that help keep the top chord from bending horizontally.

Figure-01 indicates a lateral bracing system between two columns,

Figure-01: Lateral Bracing System between two columns

According to Johnson. Mark. A, Senior Vice President, Code Council’s of Business and Product Development,“Bracing is one of the most critical, yet most misunderstood, safety elements in one- and two-family dwellings and townhouses constructed under the IRC.”

Different Types of Lateral Bracing System:

In the world there are different types of Lateral Bracing are used in structure. The most commonly used Lateral Bracing are as follows,

a)      Knee Bracing

b)      Cross Bracing

c)      K-Bracing

d)      V-Bracing

a)      Knee Bracing:

A knee-brace system is a structural component that transfers wind pressures exerted on the sidewalls and roof of a farm building to its foundation. This, of course, induces shear and bending in the stud or pole. Knee braces are an effective wind-bracing system for either pole or stud-framed farm buildings. Figure 2.8 shows Knee Bracing system.

Figure-02 indicates a Knee Bracing System between two columns,

Drawing by Engr. Snehashish Bhattacharjee (Tushar), seasoft022.blogspot.com

Figure-02: Knee Bracing System between two columns

  

b)      Cross Bracing:

Cross bracing is a construction technique in which braces are crossed to form an X shape to support a frame. Cross braces are often metal, but they can also be made of wood. Cross bracing is used on many different types of items including furniture, ship frames, walls and flooring. Virtually any type of frame can be strengthened with a cross brace – even building frames.

Figure-03 indicates a Cross Bracing System between two columns,

Drawing by Engr. Snehashish Bhattacharjee (Tushar), seasoft022.blogspot.com

Figure-03: Cross Bracing System between two columns

c)      K-Bracing:

K-bracing is that type of bracing system where braces connect to the columns at mid-height.

Figure-04 indicates a K-Bracing System between two columns,

Drawing by Engr. Snehashish Bhattacharjee (Tushar), seasoft022.blogspot.com

Figure-04: K-Bracing System between two columns

d)      V-Bracing:

Inverted-V-braced frames are one type of Concentrically Braced Frame (CBF), in which the centerlines of members form a vertical truss system to resist lateral forces. As more emphasis has been placed on increasing ductility and energy dissipation capability of all types of structures in modern codes, design provision for a new type of braced frame, labeled the Special Concentrically Braced Frame (SCBF), have been developed (Goel1992, Bruneau et al. 1998).

Figure-05 indicates a V-Bracing System between two columns,

Drawing by Engr. Snehashish Bhattacharjee (Tushar), seasoft022.blogspot.com

Figure-05: V-Bracing System between two columns

Importance of Lateral Bracing System:

The load from gravity is easy to understand and constant on every house. Lateral loads are just as constant, but they vary in force and are not as recognized. The most common and universal lateral load is wind, with design velocities that vary from 85 to 150 miles per hour across the United States. The strongest wind loads, tornados, are not predictable and randomly occur in every part of the country. While it is not affordable or reasonable to design structures to withstand the strongest tornados, experience has taught us how structures can resist wind speeds of up to 150 mph and protect not only the inhabitants but also the integrity of the structure.

Similar experiences with seismic activity have helped building codes develop methods that enable structures to survive earthquakes that used to cause damage beyond repair.

Lateral bracing serves to break the top chord into smaller sections, giving it more strength against any natural disaster.

A journal about the Importance of Bracing published on November 2002 written by Ian Giesler, of ICF Builders, involved with ICF's for many years with a lot of experience in design, engineering to construction.

According to Mr. Ian Giesler,

“A wall alignment bracing system is as much a standard tool for building with ICFs as a paintbrush is to a painter. If you think that you can build perfect walls without bracing systems, you're only kidding yourself. ICF walls may look plumb, square and level without bracing, but typically they aren't. In fact, many walls need even more than a wall alignment bracing system just to get them close to a tolerable level.”

Earthquake and Lateral Bracing

Earthquake:

Earthquake is one of the most devastating natural disasters. A major earthquake of 7.5 on the Richter-Scale has the potential to kill 88000 people and demolish 72000 buildings and cause damage to another 86000 pucca structures in Dhaka city alone. The devastation could even be worse in the port city of Chittagong because of its location along one of the fault lines. To counteract this type of damage and to provide enough safety for people due to earthquake, most engineers permit that the concept of lateral bracing assembly can be applied during construction of any structure. A braced assembly is defined by a pair of stiles, a top member and bottom member where diagonal braces and horizontal cross-braces create a triangular frame. Bracing is applied vertically between two columns. This vertical element can resist the all up coming lateral forces like earthquake so that it is called horizontal bracing system or, lateral bracing system. A lateral bracing has different shapes, it can be Cross bracing, Knee bracing, K bracing, V bracing. Three horizontal bracing systems can be used to resist earthquake forces. They are Braced frame systems, Moment-resistant systems, Shear wall systems.

According to seismic zoning map prepared by the Bangladesh University of Engineering and Technology (BUET),

43 percent areas of the country are rated as high risk, 43 percent medium risk and 16 per cent low risk. Fortunately, none of the tremors that had jolted this land for over more than century had been killer-type. It is though feared that the country runs the risk of being struck by major earthquakes anytime. This premonition has unfortunately failed to generate the necessary urge among the people and the successive governments to initiate damage-control measures.

Earthquake is a form of energy of wave motion, which originates in a limited region and then spreads out in all directions from the source of disturbance. It usually lasts for a few seconds to a minute. The effect of earthquake is the most important consideration in the construction of modern building. In Bangladesh, complete earthquake monitoring facilities are not available so that earthquake has a harmful effect more or less for any type of structure. The increase of the magnitude of the earthquake causes the damages of a structure. A modern building is a structure with more rigid at upper stories and flexible at the first storey. This design is found in that type of buildings where first storey contains garage or an open commercial area for stores and the upper floor contains office or residential apartments. This type of design creates a discontinuity of strength or stiffness. If the first storey of a building is softer then the other stories, it more damages occur due to earthquake because the first floor is generally highly loaded and thus damage causing column failure.

Lateral Bracing:

Lateral bracing is the term that can help to keep the top chord from bending horizontally. In this world, one of the primary concerns in any flexural design is the use of lateral bracing to control lateral-torsional buckling. The concept of lateral bracing is not a new idea. In many countries in this world, lateral bracing is used to provide more safety for a structure such as John Hancock Building (Chicago, Illinois), Alcoa Building (San Francisco, California), IBM Office Building (Pittsburgh, Pennsylvania), Transamerica Building (San Francisco, California), Eiffel Tower (Paris, France) etc. Now-a-days the application of lateral bracing is also increased in Bangladesh day-by-day. Crandell, J., and S. Herrenbruck (August 2006) states that “Some important rules need to follow to provide lateral bracing. Bracing angle must be at least 45 degrees and not more than 60 degrees from horizontal”.  Joseph A. Yura, Ph.D, Professor Emeritus states that “Design rules based on strength considerations only, such as a 2% rule, can result in inadequate bracing systems. Both strength and stiffness of the brace system must be checked”. In the fourth edition of Steel Structures: Design and Behavior by C.G. Salmon and J. E. Johnson, a five step procedure is given for the design of lateral bracing. "Steel Structures: Design and Behavior," shows seven types of definite lateral support. Such lateral support is attached to the compression flange and to the web-lateral support of the compression flange being most common. Guide to the IRC Wood Wall Bracing Provisions by American Planning association (APA) suggests that “To provide the bracing for a residential structure, use of the International Residential Code (IRC) is very important and necessary to resist the lateral loads that can result from wind and seismic events”.

Lateral Bracing System in Bangladesh:

Bangladesh is a country of seismic zone. In Bangladesh, many residential, commercial, industrial structures which are constructed without considering the rules and regulations of Bangladesh National Building Code (BNBC). There are different ways to ensure the enough safety of a structure. Use of the lateral bracing in a structure is an economic solution which can ensure more safety of a structure. This system may also be used to increase the strength of the structure. In case of building different types of lateral bracing can be used such as, Cross Bracing, Knee Bracing, K-Bracing, V-Bracing. Lateral bracing system depends upon the upcoming wind force and seismic load. This system is more effective when the magnitude of wind force and seismic load is very high on a building structure. Actually this lateral bracing system is not innovative concept; it is a worldwide system which is used to ensure more safety of a structure against wind force and seismic load. In case of low rise buildings where ground floor consists of garage, shop, open place; lateral bracing system is more applicable and economical and in case of high rise building lateral bracing system is needed at the every storey as well as top floor also.

Road Pavement

Pavement: The Road Pavement or, Pavement is the portion of the road located directly above the sub-grade and beneath any wearing surface. In urban areas it is often bordered by kerb and channel and in rural areas by road shoulders. It is typically constructed from compacted from imported material such as crushed rock.

In other words,

That portion of a Road designed for the support of, and to form the running surface for, vehicular traffic is called Road Pavement. Or,

The portion of the road, excluding shoulders, placed above the design sub-grade level for the support of, and to form a running surface for vehicular traffic is known to us as Road Pavement.

Figure-01 indicates a cross section of Road Pavement,

 

Figure-01: Typical Cross Section of a Road Pavement

There are Two (02) main types of Road Pavement, These are,

            a)      Flexible Pavement,

            b)      Rigid Pavement.

       a) Flexible Pavement: A Flexible Pavement is that type of pavement with a structure that deflects, or flexes, under loading. A Flexible Pavement Structure is typically composed of several layers of material. Each layer receives the loads from the above layer, spreads them out, then passes on these loads to the next layer below.

Figure-02 indicates a cross section of Flexible Pavement,

 

Figure-02: Typical Cross Section of a Flexible Pavement

      

       b) Rigid Pavement: Rigid Pavement is another type of Road Pavement Structure which deflects very little under loading due to the high Modulus of Elasticity of their surface course. A Rigid Pavement is typically composed of a P.C.C surface course built on top of either the sub-grade or an underlying base course, because of its relative rigidity, the pavement structure distributes loads over a wide area with only one, or at most two structural layers.

Figure-03 indicates a cross section of Rigid Pavement,

 

Figure-03: Typical Cross Section of a Rigid Pavement

Difference between Flexible Pavement and Rigid Pavement:

Topics	Flexible Pavement	Rigid Pavement
Life Time	20 Years	40 Years
Initial Cost	Less	High
Effect on Environment	Most hazardous effect	Less hazardous effect
Maintenance Cost	High	Low
Reinforcement	No	Yes
Cracking	Does not crack by over-loading	Crack because of over-loading
Labor	Skilled labor not essential	Skilled labor essential
Material	Not available	Avaiable

Advantages & Disadvantages:

Advantages & Disadvantages of Flexible Pavement:

Advantages:

---Design is Empirical,

---Life time is 10 to 20 years,

---Initial cost is low.

Disadvantages:

---Hazardous effect on environment,

---Maintenance cost is high,

---Expensive than Rigid Pavement,

---Manufacturing materials are not available.

Advantages & Disadvantages of Rigid Pavement:

Advantages:

---Long life time about 40 years,

---Less hazardous effect on environment,

---Low maintenance cost,

---Economical than Flexible Pavement,

---Materials are available.

Disadvantages:

---High initial cost,

---Does not fit into stage construction.