DESIGN PRINCIPLES OF TRUSSED BEAMS

A beam which is stiffened by a system of braces constituting atruss of which the beam is a chord is called as Trussed Beam. Trussed beams are used for mainly for the Industrial buildings where free space requirement are essential for more working areas. The span of truss varies from 10’-0” to 300’-0” depending on the type of requirement and the available spaces.

A truss is a beam which is bent to the shape of the bending moment diagram in opposite direction.

The shape shown is better to take care of the Bending moment.

H= rise of the truss which is 1/8 for AC sheet.

As suggested earlier the top main chord (rafter) is divided into main divisions which in turn subdivided to suite the roof covering sheets.

Nowadays it became obsolete to use the rivets but customary to make use of welding as well as high tension bolts. There are four types of bolts available. Bolt G98 means bolt is having a strength of 9 Mpa 8 is % of strength used for calculation.

The structural design procedure consists of six principal steps.

Selection of type and layout of structure.

Determination of loads on the structure.

Determination of internal forces and moments in the structural components.

Selection of material and proportioning of members and connections for safety and economy.

Checking the performance of the structure under service conditions, and

Finial review.

Fabrication: Ease of fabrication and erection has an important influence on the economy of the design.

In general, small and medium trusses of symmetrical design are lifted at the ridge during erection. In order to prevent buckling of the bottom chord, it is necessary to proportion it to carry the compressive stresses developed during hoisting. An empirical relation is given by b/L =1/125. where b is the width of the bottom chord at its centre and L the span length.

For example a 50 m span truss shall have the top chord and bottom chord width =span/125.i.e. 50×1000/125=400mm. (?8times span- in mm).

This is to avoid bending of truss on either side during erection.

The shape of a truss :

TRUSSED BEAMS:

FORMULAE FOR TRUSSED BEAMS-REF: Building engineers hand book.

Trussed beams are used for long spans and are built up of wooden beams and struts of steel rods. But the wooden beams may be replaced by steel sections.

Es=2.1×10^6Kg/sq.cm(steel)

Design formulae.

Sl.no	Description	Single strut	Double strut
Uniformly distributed load –in Kg
1.	Tension in rod	0.312Wh/r	Wh/3r
2.	Compression in strut	0.625W	W/3
3.	Compression in beam	0.312WL/2r	WL/9r
Concentrated load over strut,Kg
1.	Tension in rod	Ph/2r	Ph/r
2.	Compression in strut	P	P
3.	Compression in beam	PL/4r	PL/3r

Lecture notes by Dr.L.S.Jeyagopal, a leading structural consultant.

Related Tags:
built up lumber beam for retrofit, truss frame design principles, truss frame design principles, general design principles of beams, trussed beam, general design principles of beams, built up lumber beam for retrofit, "trussed beam" steel, beams used in portal steel building, beams used in portal steel building, the constructor beams notes, design of beams,

Read more »

Posted in: design,Entries,Projects,Top 10

Precast Method of Bridge Construction

Precast Method of Bridge Construction

(i) Precast Beams:

Precast beam decks are generally used for short span bridges ranging between 5m to 50m – these may be railway or motorway bridges. Standard inverted tee beams or M-beams are chosen and positioned by crane.

Where precast beams are considered for a motorway bridge construction, the bridge cross-section for a typical carriageway will generally consist of four beams. Erection time of such bridge should have a rate of construction of four beams per day. A cast-in-situ slab top deck is normally used with an expected rate of construction of one span a week.

(ii) Precast Decks:

Precast deck construction is often used for the construction of long viaducts. It is a time saving method which is beneficial for long bridges where construction time for the final completion stage is tight.

A long viaduct can have a complete precast deck speedily placed with this method. The decks are positioned using either a large crane or purpose made gantry. A rate of construction of two spans per day is considered normal where a gantry system is in use, if this pace is maintained a one kilometer deck can be placed in three weeks.

However, if this method of construction is chosen it is imperative that the engineer has clearly organized the deck construction schedule. The speed of this method depends on the timely delivery of prefabricated decks, the engineer and deck contractor must set out a rate of construction which allows the supplier to produce a sufficient decks to time while the deck contractor must be ready to place and store decks on receipt of delivery.

(iii) Precast segmental decks:

Precast segmental deck construction is used for long bridges where the deck depth is difficult for cast in situ construction. Box girder deck segments are generally used where the segment can be 2m or less deep, between 2.5m and 4m long carrying a deck upto 15m wide are generally used.

Where in-situ post-tensioning is favoured the segments can be prestressed either internally or externally. Internal tendons must be protected from moisture attack.

The repetitive nature of this method allows for a variety of modern placement techniques to be used, though balanced or free cantilever about a pier is a preferred choice. With this method a crane or self launching gantry system can place upto six segments per day.

The rate of construction for internally prestressed segments is considered to be a span per week. If externally prestressed tendons are used it should be feasible to complete three spans per week.

Read more »

Posted in: design,Projects,Steel Bridges

AIRCRAFT CONSTRUCTION

The airframe of a fixed-wing aircraft consists of the following five major units:

Fuselage

Wings

Stabilizers

Flight controls surfaces

Landing gear

A rotary-wing aircraft consists of the following four major units:

Fuselage

Landing gear

Main rotor assembly

Tail rotor assembly

The primary factors to consider in aircraft structures are strength, weight, and reliability. These factors determine the requirements to be met by any material used to construct or repair the aircraft. Airframes must be strong and light in weight. An aircraft built so heavy that it couldn’t support more than a few hundred pounds of additional weight would be useless. All materials used to construct an aircraft must be reliable. Reliability minimizes the possibility of dangerous and unexpected failures.

Many forces and structural stresses act on an aircraft when it is flying and when it is static. When it is static, the force of gravity produces weight, which is supported by the landing gear. The landing gear absorbs the forces imposed on the aircraft by takeoffs and landings.

During flight, any maneuver that causes acceleration or deceleration increases the forces and stresses on the wings and fuselage. Stresses on the wings, fuselage, and landing gear of aircraft are in tension, compression, shear, bending, and torsion. These stresses are absorbed by each component of the wing structure and transmitted to the fuselage structure. The empennage (tail section) absorbs the same stresses and transmits them to the fuselage. Stresses are analyzed and considered when an aircraft is designed.

Members of an aircraft are subjected to following stresses:

Tension

Compression

Shear

Bending

Varying stresses

All structural members of an aircraft are subject to one or more stresses. Sometimes a structural member has alternate stresses; for example, it is under compression one instant and under tension the next. The strength of aircraft materials must be great enough to withstand maximum force of varying stresses.

Fig: Engine torque creates tension stresses in aircraft fuselages.

Fig: Bending action occurring during carrier loading

AIRCRAFT CONSTRUCTION MATERIALS

An aircraft must be constructed of materials that are both light and strong. Early aircraft were made of wood. Lightweight metal alloys with strength greater than wood were developed and used on later aircraft. Materials currently used in aircraft construction are classified as either metallic materials or non-metallic materials.

METALLIC MATERIALS

The most common metals used in aircraft construction are aluminium, magnesium, titanium, steel, and their alloys.

Aluminium

Aluminium alloys are widely used in modern aircraft construction. Aluminium alloys are valuable because they have a high strength-to-weight ratio. Aluminium alloys are corrosion resistant and comparatively easy to fabricate. The outstanding characteristic of aluminium is its lightweight.

Magnesium

Magnesium is the world’s lightest structural metal. It is a silvery-white material that weighs two-thirds as much as aluminium. Magnesium is used to make helicopters. Magnesium’s low resistance to corrosion has limited its use in conventional aircraft.

Titanium

Titanium is a lightweight, strong, corrosion resistant metal. Recent developments make titanium ideal for applications where aluminium alloys are too weak and stainless steel is too heavy. Additionally, titanium is unaffected by long exposure to seawater and marine atmosphere.

Alloys

An alloy is composed of two or more metals. The metal present in the alloy in the largest amount is called the base metal. All other metals added to the base metal are called alloying elements. Adding the alloying elements may result in a change in the properties of the base metal. For example, pure aluminium is relatively soft and weak. However, adding small amounts or copper, manganese, and magnesium will increase aluminium’s strength many times. Heat treatment can increase or decrease an alloy’s strength and hardness. Alloys are important to the aircraft industry. They provide materials with properties that pure metals do not possess.

Steel Alloys

Alloy steels used in aircraft construction have great strength, more so than other fields of engineering would require. These materials must withstand the forces that occur on today’s modern aircraft. These steels contain small percentages of carbon, nickel, chromium, vanadium, and molybdenum. High-tensile steels will stand stress of 50 to 150 tons per square inch without failing. Such steels are made into tubes, rods, and wires. Another type of steel used extensively is stainless steel. Stainless steel resists corrosion and is particularly valuable for use in or near water.

NON-METALLIC MATERIALS

In addition to metals, various types of plastic materials are found in aircraft construction. Some of these plastics include transparent plastic, reinforced plastic, composite, and carbon-fiber materials.

Transparent Plastic

Transparent plastic is used in canopies, windshields, and other transparent enclosures.

Reinforced Plastic

Reinforced plastic is used in the construction of radomes, wingtips, stabilizer tips, antenna covers, and flight controls. Reinforced plastic has a high strength-to-weight ratio and is resistant to mildew and rot.

Composite and Carbon Fiber Materials

High-performance aircraft require an extra high strength-to-weight ratio material. Fabrication of composite materials satisfies this special requirement. Composite materials are constructed by using several layers of bonding materials (graphite epoxy or boron epoxy). These materials are mechanically fastened to conventional substructures. Another type of composite construction consists of thin graphite epoxy skins bonded to an aluminium honeycomb core. Carbon fiber is extremely strong, thin fiber made by heating synthetic fibers, such as rayon, until charred, and then layering in cross sections.

FIXED-WING AIRCRAFT

Fig: Principle Structural units of a fixed wing aircraft

Related Tags:

aviation fail, light aircraft wing design, aircraft airframe, aircraft construction, aviation fail, aircraft design, airport structural design, aircraft construction, aircraft construction with picture, aircraft design, fuselage stresses, STRUCTURAL ANALYSIS,

Read more »

Posted in: AIRCRAFT CONSTRUCTION,design,Projects,Top 10