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What are Truss Structures?

A truss is a special type of structure renowned for its high strength- to- weight and stiffness- to- weight ratios.

This structural form has been employed for centuries by designers in a myriad of applications ranging from bridges and race car frames to the International Space Station.Trusses are easy to recognize: lots of straight slender struts joined end- to- end to form a lattice of triangles, such as the bridge in Fig.1.

Fig.1 Truss bridge in Interlaken, Switzerland

In large structures, the joints are often created by riveting the strut ends to a gusset plate as shown in Fig.2.

A structure will behave like a truss only in those regions where the structure is fully triangulated; locations where the struts form other polygonal shapes (e.g., a rectangle) may be subject to a loss of stiffness and strength.

Fig.2 Joint formed by riveting a gusset plate to converging members

The special properties of a truss can be explained in terms of the loads being applied to the individual struts. Consider the three general types of end loadings shown in Fig.3 tension, compression, and bending.

If you were holding the ends of a long thin steel rod in your hands and wanted to break it or at least visibly deform it, bending would be the way to go. Thus, if we could eliminate bending of the struts as a potential failure mode, the overall strength and stiffness of the truss would be enhanced.

Fig.3 Different end loading possibilities. The dashed represents the deformed shape produced by the applied forces

This is precisely the effect of the truss geometry on the structure, as the stiff triangular lattice serves to keep any bending induced in the struts to a minimum.

What is Warm Mix Asphalt (WMA) ?

The increase of scheduled commercial flights at busy civil airports have made it imperative that airfield pavement rehabilitation and asphalt overlay be performed without disrupting airport operations.

For this purpose, the off-peak period (nighttime) construction has become one practical solution for airport authorities. Using this approach, the airfield facilities are closed at night for a few hours when the flight volume is at the lowest, and then quickly opened to air traffic in the next morning.

During this closed period, aircraft will use other runway facilities, if parallel runways are available, or airport operation will be postponed. Time is the essence of the construction during the off-peaktime.

The typical unoccupied time of airfield pavement rehabilitation is as short as 6–8 h per night. It is a period from 23:00 to 6:00 that was specified for runway overlay in Fukuoka airport. The similar night time construction period can also be found in these following airport projects: San Diego International airport in1980 (8 h), Frankfurt airport, Germany, in 2005 (8 h)and Hong Kong airport in 2006 (8 h).

However, with the increase of 24-hour airport operation, the period for night time construction has become limited. The decrease was observed in the largest Australian airports, where the available night time construction was generally reduced from eight hours in 2005 to five hours in 2015.

Rapid construction is expected to reduce the disruption due to the airport closure and allow more time for contractors to produce the maximum volume of asphalt each night to achieve satisfactorily constructed pavement.

One of the approaches for rapid night time construction is to shorten the cooling time of freshly paved asphalt overlay. In this case, with its advantage of lower production and compaction temperature, warm mix asphalt (WMA) gives an advantage of a lower cooling time of asphalt; thus, the pavement can be quickly opened to traffic.

In the situation where the closure of the runway is substantially critical, the use of WMA is expected to shorten the runway closure time each night. In addition, in the case that the closure hours are fixed for each night, the use of WMA would enable more volume of asphalt to be laid each night, increase the target length of pavement to be done each night, thus, shortening the overall project time, compared to HMA.

The use of WMA technology for airport pavements has been few until now. The technology has more popularly been adopted for road pavement projects than airfield pavements. However, extensive research has been carried out in the last few years on the use of WMA for airside applications.

Recent evidence suggests the suitability of using WMA for airfield pavement. Although considerable researches have been done, there has been no detailed investigation into the advantages of the use of WMA on shortening the construction time of pavement.

From sketch BIM to design BIM

As a promising technology for building lifecycle management, Building Information Modelling (BIM) has been extensively studied in the last decade.

A BIM is a shared knowledge resource for information about a facility forming a reliable basis for decisions during its life-cycle; defined as existing from earliest conception to demolition. BIM enables collaboration among varieties of stakeholders through data interoperability among different BIM applications.

Currently, BIM applications

involve every stage of a construction project, including conception, design, construction, and operation management.

BIM has been extensively applied in the design in construction sector. The design phase usually has two sub-stages: sketch design (short for sketch) and detailed design (short for design). Both sketch and design stages generate three-dimensional (3D) BIM models.

Due to different purposes, the sketch and design stages use different BIM tools. The most widely-used sketch BIM tools include SketchUp, Rhino, and Form-Z. The design BIM models are usually designed using software tools like Revit, ArchiCAD, Bentley Architecture, to name a few.

The different purposes and different tools result in the different organizations of sketch BIM models and design BIM models. Sketch BIM models can be considered as a project-level organization of BIM data, because all the data are aggregated into one or a few human/computer-unknown building elements.

Additionally, sketch BIM models usually have little property data. That is, a sketch BIM model usually focuses on 3D design from an overall view and ignores the definition of elements and their semantic data.

Contrarily, design BIM models are organized in the element-level, where each element has well-defined semantic and geometric data. Subsequently, a BIM application can obtain both semantic and geometric

data of a building element directly from a design BIM model.

As a downstream stage of the sketch, the design stage expects to reuse the data in sketch BIM models to improve the design efficiency. However, the different data organizations hinder the reusability of sketch BIM models in the design stage.

Because the design BIM tools can not directly obtain data of building elements from sketch BIM models. This triggers new demands to abstract reusable building elements from sketch BIM models to facilitate the BIM design.

Industrial Foundation Classes (IFC) is an open, vendor-neutral, international standard (ISO 16739–1:2018) of BIM. The mainstream BIM tools in both sketch and design stages support the IFC specification. Without loss of generality, a BIM model means an IFC file in this study. Although some studies investigated the geometric description of an IFC file and its applications, the recognition of reusable building elements from sketch BIM models remains unexplored.

The exploration of building element identification from a sketch BIM model will bridge the gap between sketch BIM models and design BIM models, smooth the reusability of data from the sketch stage to design stage and improve the design efficiency by avoiding the redesigning of building elements.

What is High Performance concrete – HPC?

1. Definition Of High Performance Concrete:

High-performance concrete may be defined as concrete with strength and durability significantly beyond those obtained by normal means. The required properties for concrete to be classiffied as high performance therefore depend on the properties of normal concrete achievable at a particular time and location.

At the present time, high-performance concrete in developed countries usually refers to concrete with 28-day compressive strength beyond 70±80 MPa, durability factor (defined as the percentage of original modulus retained after 300 freeze/thaw cycles) above 80%, and w/c below 0.35.

It is made with good quality aggregates, high cement content (450±550 kg mw-3), and a high dosage of both silica fume (5±15 wt.% of cement) and super plasticizer (5±15 l mw-3). Sometimes other pozzo-lanic materials are also used.

The high performance is achieved with the use of low w/c (0.20±0.35) as well as pozzolans to produce a dense microstructure that is high in strength and low in permeability.

Superplasticizer is added to keep the mix workable.With high cement content, the use of super-pasticizers and silica fume and the need for more stringent quality control the unit cost of high-performance concrete can exceed that of normal concrete by 30±100%.

2. When High Performance Concrete is Used:

Despite the higher material cost, the use of high-performance concrete is found to be economical for columns of tall buildings, as the amount of steel reinforcement can be reduced.

In bridges, the reduction in deck size and weight effectively increases the allowable unsupported span. For a continuous bridge, the number of piers can be reduced. In many infrastructure projects, high-performance concrete is chosen for its durability against various types of chemical (e.g., sulfate or chloride) and physical attack (e.g., abrasion).

High-performance concrete can also be produced with lightweight aggregates. However, the aggregate needs to be very carefully chosen to make sure it is sufficiently strong. As long as the light weight aggregateis strong enough, its use can indeed be advantageous.

By saturating their pores with water before mixing, these aggregates can act as internal reservoirs that supply water to ensure continued cement hydration and prevent auto geneous shrinkage due to self-desiccation. This aspect is of particular relevance to concrete with a very low w/c, in which the early development of high density and low permeability makes it difficult for water to penetrate uniformly forthe hydration process to continue.

Besides the production of high-performance concrete, superplasticizers are also commonly used in the production of high-workability concrete. With aslump value of 180±230 mm, high-workability concrete can be pumped rapidly over long distances, easily compacted in structures with highly congested re-inforcement, and can even be self-compacting (i.e.,requiring no external compaction effort).

With super-plasticizers, it is also possible to reduce the cement content while retaining the same workability. The possibility of thermal cracking in massive structures can therefore be reduced.

In the continual quest for improving concrete performance, it was soon realized that the size of aggregates is an important factor. By using very fine aggregates, superplasticizers, and a high dose of silica fume (about 20±30% of the cement content) concrete strength beyond 200 MPa can now be achieved by conventional techniques. One example is DSP–densified system with fine particles.

Using strongaggregates of small size (e.g., calcined bauxite withmaximum size of 4 mm), DSP with compressive strength over 250 MPa can be produced.

Reaction powder concrete (RPC) is another example. With the maximum particle size limited to 0.4 mm, the compressive strength reaches 170 MPa by 28 days under room temperature curing. Curing at 80±90∞C will further increase the strength to 230 MPa. If pressure is applied before and during setting, and curing is carriedout at 400∞C, strength as high as 680 MPa can be attained. With very high strength, both DSP and RP Care extremely brittle. Fiber reinforcement is therefore essential to prevent catastrophic failure at ultimateload.

Precast Prestressed Spun Concrete Piles

Precast prestressed spun concrete piles are closed-ended tubular sections of 400 mm to 600 mm diameter with maximum allowable axial loads up to about 3 000 kN.

Pile sections are normally 12 m long and are usually welded together using steel end plates. Pile sections up to 20 m can also be specially made.

Precast prestressed spun concrete piles require high-strength concrete and careful control during manufacture.

Casting is usually carried out in a factory where the curing conditions can be strictly regulated.

Special manufacturing processes such as compaction by spinning or autoclave curing can be adopted to produce high strength concrete up to about 75 MPa. Such piles may be handled more easily than precast reinforced concrete piles without damage.

This type of piles is generally less permeable than reinforced concrete piles and may be expected to exhibit superior performance in a marine environment. However, they may not be suitable for ground with significant boulder contents. In such cases, preboring may be required to penetrate the underground obstructions.

Spalling, cracking and breaking can occur if careful control is not undertaken and good
driving practice is not followed

Precast Reinforced Concrete Piles

Precast reinforced concrete piles are not common nowadays.

These piles are commonly in square sections ranging from about 250 mm to about 450 mm with a maximum section length of up to about 20 m. Other pile sections may include hexagonal, circular, triangular and H shapes. Maximum allowable axial loads can be up to about 1 000 57kN.

The lengths of pile sections are often dictated by the practical considerations including
transportability, handling problems in sites of restricted area and facilities of the casting yard.
These piles can be lengthened by coupling together on site.

Splicing methods include welding of steel end plates or the use of epoxy mortar with dowels.

This type of pile is not suitable for driving into ground that contains a significant amount of boulders or corestones.

Large-displacement piles Advantages and Disadvantages

Large-displacement piles include all solid piles, including precast concrete piles, and steel or concrete tubes closed at the lower end by a driving shoe or a plug, i.e. cast-in-place piles.

Advantages of Displacement Piles

Material of preformed section can be inspected before driving.
Steel piles and driven cast-in-place concrete piles are adaptable to variable driving
Installation is generally unaffected by groundwater condition.
Soil disposal is not necessary.
Driving records may be correlated withinsitu tests or borehole data.
Displacement piles tend to compact granular soils thereby improving bearing capacity and stiffness.
Pile projection above ground level and the water level is useful for marine structures and obviates the need to cast insitu columns above the piles.
Driven cast-in-place piles are associated with low material cost.

Disadvantages of Displacement Piles

Pile section may be damaged during driving.
Founding soil cannot be inspected to confirm the ground conditions as interpreted from the ground investigation data.
Ground displacement may cause movement of, or damage to, adjacent piles, structures, slopes or utility installations.
Noise may prove unacceptable in a built-up environment.
Vibration may prove unacceptable due to presence of sensitive structures, utility installations or machinery nearby.
Piles cannot be easily driven in sites with restricted headroom.
Excess pore water pressure may develop during driving resulting in false set of the piles, or negative skin friction on piles upon dissipation of excess pore water pressure.
Length of precast concrete piles may be constrained by transportation or size of casting yard.
Heavy piling plant may require extensive site preparation to construct a suitable piling platform in sites with poor ground conditions.
Underground obstructions cannot be coped with easily.
For driven cast-in-place piles, the fresh concrete is exposed to various types of potential damage, such as necking, ground intrusions due to displaced soil and possible damage due to driving of adjacent piles.

Pile Classification – The Four Types Of Piles

Piles can be classified according to the type of material forming the piles, the mode of load transfer, the degree of ground displacement during pile installation and the method of installation.

Pile classification in accordance with material type (e.g. steel and concrete) has drawbacks because composite piles are available. A classification system based on the mode of load transfer will be difficult to set up because the proportion of shaft resistance and endbearing resistance that occurs in practice usually cannot be reliably predicted. In the installation of piles, either displacement or replacement of the ground will predominate.

A classification system based on the degree of ground displacement during pile installation, such as that recommended in BS 8004 (BSI, 1986) encompasses all types of piles and reflects the fundamental effect of pile construction on the ground which in turn will have a pronounced influence on pile performance.

Such a classification system is therefore considered to be the most appropriate.

Piles are classified into the following four types :

(a) Large-displacement piles, which include all solid piles, including precast concrete piles, and steel or concrete tubes closed at the lower end by a driving shoe or a plug, i.e. cast-in-place piles.

(b) Small-displacement piles, which include rolled steel sections such as H-piles and open-ended tubular piles.
However, these piles will effectively become largedisplacement piles if a soil plug forms.

(c) Replacement piles, which are formed by machine boring, grabbing or hand-digging. The excavation may need to be supported by bentonite slurry, or lined with a casing that is either left in place or extracted during concreting for re-use.

(d) Special piles, which are particular pile types or variants of existing pile types introduced from time to time to improve efficiency or overcome problems related to special ground conditions.

How to Estimate the Load Capacity of Piles? (Formulae)

The following article will guide you to learn about how to estimate the load capacity of piles with the help of static and dynamic formulae’s.

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BIM-Based Collaborative Building Process Management

The book reports on the great improvements in the information and knowledge management due to the digitalization of the building sector.

By summarizing several research projects addressing the implementation of BIM in different stages of the building process, and the definition of standards at Italian, European and international levels for managing information relying on the implementation of BIM-based processes, it showcases the efforts, especially within the Italian building sector, to build a standardized structure of information and develop tools for collecting, sharing and exchanging information between stakeholders involved in different stages of the building process, so as to enhance the storage, traceability, usability and re-usability of information management.

Further, it presents an enhanced use of information that relies on the adoption of the standardized structure of information, and proposes dedicated applications for automating the process of information fruition. Lastly, it features a digital platform for different stakeholders in the building sector, such as manufacturers, producers and construction companies.

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