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How can floor vibrations be assessed?

Improvements in vibration performance after construction are likely to be difficult to achieve and very costly. The assessment of vibrations should therefore be carried out as part of the serviceability checks on the floor during the design process.

The vibration performance of the floor can be assessed using manual methods, a new simplified web-based tool or finite element methods. Where a BIM model of the building is being created by the design team, the model should contain all the necessary information required to carry out the analysis.

Manual methods

Simplified assessment can be carried out by hand methods of analysis, although such calculations are generally conservative and in some cases to a great extent. Various methods are available, one of which is set out in SCI publication P354.

To avoid the possibility that walking activities could cause resonance or near-resonant excitation of the fundamental mode of vibration of the floor, neither the floor structure as a whole nor any single element within it should have a fundamental frequency of less than 3 Hz.

The assessment procedure involves the following steps:

• calculate the natural frequency of the floor system;

• determine the modal mass, i.e. the mass participating in the vibration;

• calculate the critical rms acceleration and the response factor;

• compare the response factor with the acceptance criteria for continuous vibration.

If the response factor is not acceptable, try a more comprehensive method of analysis such as the new simplified web-based tool or finite element modelling.

Simplified web-based tool

A new Floor Response Calculator is available on www.steelconstruction.info that allows designers to make an immediate assessment of the dynamic response of a floor solution. The results from this tool provide an improved prediction of the dynamic response compared to the ‘manual method’ in SCI P354. The tool may be used to examine complete floor plans or part floor plans, comparing alternative beam arrangements.

The tool reports the results of approximately 19,000 arrangements of floor grid, loading and bay size, which have been investigated using finite element analysis. The designer must select between a variable action of 2.5 kN/m2 and 5 kN/m2, being typical imposed loads on floors. 0.8 kN/m2 is added to allow for partitions.

The designer must also select the arrangement of secondary and primary beams, with typical spans, which depend on the arrangement of the beams. Secondary beams may be placed at mid-span or third points. The pre-set damping ratio of 3% is recommended for furnished floors in normal use.

When a decking profile is selected, an appropriate range of slab depths are then available to be selected. Generally, thicker slabs will produce a lower response factor. When selecting the slab depth, solutions which result in a response factor higher than 8 (the limit for a typical office) are highlighted.

The primary and secondary beams are selected automatically as the lightest sections which satisfy strength and deflection requirements; these cannot be changed by the user.

The selection of the lightest sections is made to produce the most conservative dynamic response, as stiffer beams will reduce the response. A visual plot of the response is also provided for both the steady state and transient response.

Hovering over the plot shows the response factor. Generally the higher response will be in an end bay, where there is no continuity. The fundamental frequency of the floor is presented on the output screen. If the actual design differs from the pre-set solutions in the tool, users should note the following:

• Using stiffer beams will reduce the response

• Using thicker slabs, and stiffer beams, will reduce the response

• The gauge of the decking has no significant impact on the response factor

• Voids that break the continuity of beam lines will lead to higher response factors

Finite element analysis

The most accurate and detailed assessments of floor vibrations are made using finite element (FE) analysis. Simple methods can be applied with reasonable accuracy for orthogonal grids but where a floor plate is not orthogonal (e.g. curved in plan), simple methods are inadequate.

In FE analysis the floor slab, beams, columns, core walls and perimeter cladding are modelled with finite elements with appropriate restraints applied to the elements in the model.

A model of the whole building is often already available for Building Information Modelling (BIM) and an individual floor can be extracted and modified to provide a model that is suitable for vibration analysis.A modal analysis is carried out first to determine the natural frequencies, mode shapes and modal masses.

Steady state and transient responses are then calculated for each mode of vibration and each harmonic of the forcing function (the walking activity). The modal responses are then added up for all the mode shapes and harmonics considered, and a predicted rms acceleration calculated for each point on the floor.

The final step is to divide the acceleration by the base value to determine the response factor. The results can be plotted in a contour plot.

Mitigation

If the floor response is found to be unacceptable during the design assessment, the designer has some freedom to make adjustments to the structural arrangement such that the vibration response is reduced to acceptable levels.

Possible measures include increasing the mass, stiffness and damping of the floor, and relocating or reducing the length of corridors

What are floor vibrations and why are they important?

The term ‘vibrations’ when applied to floors refers to the oscillatory motion experienced by the building and its occupants during the course of normal day-to-day activities.

This motion is normally vertical (up and down), but horizontal vibrations are also possible. In either case, the consequences of vibrations range from being a nuisance to the building users to causing damage to the fixtures and fittings or even (in very extreme cases) to the building structure.

The severity of the consequences will depend on the source of the motion, its duration and the design and layout of the building.Floor vibrations are generally caused by dynamic loads applied either directly to the floor by people or machinery. The most common source of vibration that can cause nuisance in building applications is human activity, usually walking.

Although small in magnitude, walking-induced vibrations can cause a nuisance to people working or living in the building, especially to the use of sensitive equipment or to those engaged in motion-sensitive activities, e.g. surgery.

Naturally, the problem is more acute for more vigorous types of human activity such as dancing and jumping and therefore designers of buildings featuring a gymnasium or dance studio should take extra care to limit the vibrations in the rest of the building.Machinery-induced vibrations are best dealt with at source through the provision of isolating mounts or motion arresting pads.

Machines installed in factories tend to produce the most severe vibrations due to their size and the nature of their operation. However, floor vibration is rarely a problem in most factories, since it is accepted by the workforce as part of the industrial environment.

Once constructed, it is very difficult to modify an existing floor to reduce its susceptibility to vibration, as only major changes to the mass, stiffness or damping of the floor system will produce any perceptible reduction in vibration amplitudes.

It is important therefore that the levels of acceptable vibration be established at the concept design stage, paying particular attention to the anticipated usage of the floors. The client must be involved in this decision, as the specified acceptance criteria may have a significant impact on the design of the floor and the cost of construction.

Why Should Building Products Manufacturer Create BIM Content?

As the construction industry continues to evolve with the constantly growing technology, there are areas within the industry that needs to upgrade them. With that being said, it is building product manufacturers that particularly need to explore the potential of BIM for the growth of their business and gain that competitive edge.

BIM as grown significantly over the past half a decade and is still developing. It entered the industry with design tool for architects and today supports everyone involved in the construction project. Amongst all this changes, building product manufacturers need to reassess their usage of Revit Product data for Revit family creation and make the most of it. Incorporating BIM in the construction supply chain necessarily means every professional on a particular construction project should align themselves.

Why is BIM content creation is more than important for building product manufacturers?

Recently governments of several nations like UK and Singapore have mandated BIM Level 2 for every public construction project. This means that manufacturers of products such as steel stairs, MEP components, facades, furniture, shopfitting, sheet metal enclosures, ducts etc. need to align with the needs of sheet metal contractors. And these sheet metal contractors need to facilitate the general contractors and sub-contractors with the BIM content of the products that they supply for projects.

Thus, an ideal way to get the original and as-is BIM content of any product is from the original manufacturer of the product. BIM Content or Revit Families serve as an excellent project deliverables when supplied to the respective contractor along with the physical predicts. BIM-ready product data templates will ensure a seamless communication channel across the disciplines, cross-teams and during the development of LOD 500, 3D BIM models in Autodesk Revit.

Some of the apparent reasons to use BIM content for building product manufactures and product design engineers are as enlisted:

Lack of association of building designs with the manufactured products
Lacking control over design data ownership and out of date design data availability
Configured and customized products having higher design complexity
High monetary investments for to acquire skills for BIM content development

Well-structured digital information

To overcome these roadblocks, the recent focus of manufacturers has been shifted to BIM objects and data templates. These digitized models of frequently used standard components like MEP fixtures, cables, trays, switches, heaters, pumps, valves etc. helps in quick model development and coherent communication between designer, manufacturer, and the installation foremen.

Another advantage of BIM objects is that the manufactures and design engineers say that with the existing standard data, they can now quickly generate other similar objects with little customization. It has happened because of the standard data collected from manufacturers and the ones available online to establish a standardized approach.

Amongst the monetary benefits, it has been surveyed that of the total construction costs of UK about, 40% share is by the building products. This developed a natural attraction to BIM objects for building product design engineers, manufacturers as well as the project managers.

Opportunities to create Revit families/BIM objects

Creating Revit models from scratch for every object is possible but it needs rigorous training with Revit and AutoCAD both. One may have to pre-build each geometry configuration and export it for the manufacturing decisions making and finally maintain it.

Another option is getting BIM data directly from the manufacturer’s website with specifications, pictures, geometrical dimensions etc. By adding an update to native BIM models as product line evolves, helps the BIM expert to gain more specific insights about spatial occupation.

Such an approach aids the contractors to plan and schedule the site activities as per the PERT methods and lean construction techniques to achieve the ultimate aim of efficiency and economy.

Source: www.hitechbimservices.com

History and Future of DWG

Created in the 1970s and launched by Autodesk in 1982, DWG has established itself as one of the most ubiquitous file format for CAD software on the planet. Everyone who uses CAD will be familiar with it – but how much do you really know about it? Here, we’re diving into the history of the DWG file format, from its origins to its current position as the world-leading CAD file format. We’ll also be taking a look into our crystal ball, to see what’s in store for the future of DWG…

What is DWG?

Before we dive into the story, let’s take a look at exactly what a DWG file is and does. The DWG file format allows users to store two and three-dimensional design data for use in CAD software. DWGs allow you to store vector entities, maps, geographic information, and even photos. Essentially, any information that you can enter into a CAD program can be stored in a DWG file.

The DWG file format is perhaps best known as the native format for AutoCAD, and is a proprietary file format owned by Autodesk, the creators of AutoCAD. As a proprietary format, it has been specifically designed to work with AutoCAD, rather than to function as an open standard across CAD software. Despite this, the format is supported across a range of CAD programs and DWGs can be viewed even without an AutoCAD licence. DWG also has applications across a range of industries, from architecture and engineering to virtual reality and game design – for a full rundown, visit our article exploring how different industries use DWG files.

The fast-growing game design sector uses DWG files day in, day out – this car is just one example

The Beginnings

The history of the DWG file format begins in the late 1970s. Programmer Mike Riddle found himself unsatisfied with the CAD programs available to him on the market – so instead of waiting around for someone to build something better, he built it himself. Starting in 1977, Riddle began work on a new CAD program, Interact CAD – and its native file format was to be (you guessed it) DWG, which stands for drawing. Initially released in 1979, Interact CAD was far from a runaway success: Riddle only managed to sell around 30 copies of the software.

Despite the rocky start, it was clear that the new software had potential, and in the early 1980s, Interact CAD was acquired by the newly-formed company Autodesk. Interact CAD was to form the architectural basis for a new CAD program, AutoCAD, which was launched in 1982 by Autodesk – at that time, a small company formed by John Walker and a handful of programmer friends, including Riddle. The DWG format was finally about to be introduced to the public on a much larger scale. While Interact CAD had reached only a handful of customers in its first few years of release, AutoCAD rose to become the most widely-used CAD program in the world just four years after its initial release.

One of the earliest forms of the DWG format shown in a 1982 edition of AutoCAD

DWG Dominance

Thanks to the global reach achieved by AutoCAD, the DWG file format quickly became the go-to standard for CAD designers. In fact, it was estimated that by 1998, there were more than two billion DWG files in existence – and considering that another 18 years have passed, the number has surely ballooned even further since.

Of course, DWG didn’t achieve its dominant position in the CAD world by standing still. Whilst the format exists to fulfil the same function as in 1982, it’s gone through a huge number of changes since then. The DWG format is subject to versioning. This means that every few years, major adjustments are made to the file format, as it adapts to technical advances and changes in software. New versions of AutoCAD will be able to support any DWG file, even those created for the first version of AutoCAD in 1982. Old versions of AutoCAD, however, won’t be able to open files saved to the newer versions of DWG.

All in all, there have been nineteen different versions of the DWG file format. Autodesk typically releases a new version of DWG every three years; the most recent version of the format, however, was released in 2013, and has been the native format for five consecutive editions of AutoCAD.

A DWG 2013 file open in the most recent version of AutoCAD – AutoCAD 2017

Legal Battles

Not everyone is happy with DWG’s status as a proprietary file format. In 1998, the OpenDWG Alliance was founded with the aim of making DWG an open standard for CAD software, much as the DXF format is. The organization was renamed as the Open Design Alliance in 2002. The group includes a number of competitors to Autodesk, and has aimed to reverse-engineer the DWG format, so that a method of reading and writing DWG files can be incorporated into other, non-licensed CAD programs.

Autodesk retaliated against this by introducing TrustedDWG technology, which verifies if a file was created and saved in Autodesk-licensed software. Autodesk made further efforts to defend their format by attempting to register “DWG” as a trademark with the U.S. Patent Office; this would have prevented other organizations from using the term. In support of their claim, they stated that “DWG” no longer referred solely to the file format, but also to a specific technology environment present within Autodesk software. However, the Patent Office issued a final refusal to register the trademark in 2011, with the refusal being affirmed once again in 2013.

Future of DWG

As previously mentioned, Autodesk failed to release a new version of DWG for the 2016 and 2017 versions of AutoCAD, as would normally be expected. The head of the Open Design Alliance, Neil Peterson, speculated this could be due to a lack of new features. A former head of the ODA, Arnold van der Weide, even suggested that Autodesk could be planning to do away with the DWG format altogether. Could Autodesk’s move towards cloud-based software kill DWG entirely?

Probably not. With DWG maintaining its status as the dominant CAD format, it’s unlikely to be going away any time soon. Whilst there may have been few recent developments in the desktop version of AutoCAD, DWG is still the native format for Autodesk’s new cloud-based software AutoCAD 360. Not only that, but the ubiquity of DWG means many would still use the format regardless of any abandonment by Autodesk. ODA head Neil Peterson suggests that DWG would still be safe even if it were scrapped:Even if Autodesk did away with it, our 1,250 members and millions of their customers would keep right on using DWG.Neil Peterson, President of the Open Design Alliance

Conclusion

So, DWG is not dead, but it’s definitely evolving. DWG is no longer a desktop-bound format, and while mobile apps are still currently seen as complementary to the ‘main’, desktop editions of software, all that is set to change. Smartphones and tablets are likely to be key platforms for tomorrow’s CAD designers. Far from becoming irrelevant, DWG is set to be a feature on more screens than ever before.

Top 10 Fastest Trains in the World

Traveling Europe by train is already faster than by plane right now, and Japan is testing a “Supreme” version of its popular high-speed trains, set for a 2020 debut ahead of the next Winter Olympics. You can’t ride that one just yet, but there are more than a few bullet trains available to speed up your travels. Here are the world’s fastest high-speed trains in commercial service, ranked by speed:

1. Shanghai Maglev: 267 mph

The world’s fastest train isn’t the newest, the shiniest, or even the one with the most expensive tickets. Charging $8 per person, per ride, the Maglev runs the nearly 19 miles from Shanghai’s Pudong International Airport to the Longyang metro station on the outskirts of Shanghai. That’s right—the train, which takes just over 7 minutes to complete the journey using magnetic levitation (maglev) technology, doesn’t go to the city center. As such, the bulk of the passengers since its 2004 debut have been travelers on their way to and from the airport, cameras out and ready to snap a photo of the speed indicators when the train hits 431 km/hr (267 mph).

2. Fuxing Hao CR400AF/BF: 249 mph

China wins again, also serving as home to the world’s fastest non-maglev train currently in service. The name “Fuxing Hao” translates to mean “rejuvenation,” and each of the two trains have been branded with nicknames: CR400AF is “Dolphin Blue,” and the CR400BF is “Golden Phoenix.” The “CR” stands for China Railway. Both take just under five hours to zip up to 556 passengers each between Beijing South and Shanghai Hongqiao Station, easily halving the nearly 10-hour time it takes to ride the conventional, parallel rail line between these two megalopolises. The “Rejuvenation” also beats China’s next fastest train, the “Harmony” CRH380A; it has dazzled since 2010, with speeds of up to 236 mph on routes connecting Shanghai with Nanjing and Hangzhou, and Wuhan with Guangzhou.

3. Shinkansen H5 and E5: 224 mph

Japan is celebrating the 54th anniversary of high-speed train travel this year, since it was way back in 1964 that the Hikari high-speed train launched service between Tokyo and Osaka, cutting travel time between the country’s two largest cities from nearly seven hours to a mere four by rail. The H5 and E5 series Shinkansen, respectively running the Tohoku and Hokkaido services, are two of the newer bullet trains on Japan’s tracks, and so far the fastest in regular commercial service in the country.

4. The Italo and Frecciarossa: 220 mph

Italy’s dueling train operators, NTV and Trenitalia, each flaunt a high-speed train that tie as Europe’s fastest, capable of shuttling passengers from Milan to Florence or Rome in under three hours, with a new route to Perugia debuting this year. The Frecciarossa, or “red arrow,” was unveiled during Expo 2015, held in Milan, and the train is remarkable as much for its speed as for its construction; its components are nearly 100 percent renewable and sustainable.

5. Renfe AVE: 217 mph

Spain’s fastest train is the Velaro E by Siemens, and it is used for long-distance services to major Spanish cities and beyond: traveling from Barcelona to Paris can now be accomplished on high-speed rail in six hours.

The DeutscheBahn ICE reaches speeds of 205 mph.Courtesy Deutsche Ban

6. Haramain Western Railway: 217 mph

The Mecca-Medina high-speed link stretches the 281 miles between Saudi Arabia’s most holy cities and has been in partial operation since December 2017, with full completion set for early summer 2018. Traveling the length of the route takes two and a half hours, compared to five hours by car. Speed isn’t the entire justification for the construction of this railway, however; the Haramain is expected to carry three million passengers a year, including many Hajj and Umrah pilgrims, relieving traffic congestion.

7. DeutscheBahn ICE: 205 mph

The distinctively futuristic white and silver of the Inter-City Express, or ICE, combined with its sharp red cheatline, makes an impressive sight speeding through scenic German countryside, especially on its newest route connecting Berlin and Munich. Similar to Spain’s Renfe AVE train, Germany’s fastest train is another Siemens design, the Velaro, and was built to fit through the Channel Tunnel. That’s a serious asset for DeutscheBahn’s long-term plans to operate these trains from Frankfurt to London.

8. Korail KTX: 205 mph

South Korea’s high-speed rail network is far from the newest (the KTX debuted in 2004), but it does hold its rank among the fastest. The latest route, opened just in time for the 2018 Winter Olympics, connects Incheon International Airport in the west to the coastal town of Gangneung in the east, stopping in Seoul along the way. The KTX cuts the transport time to reach the ski slopes of PyeongChang from six hours by conventional train to under two hours.

9. Eurostar e320 and TGV: 200 mph

Both the TGV and Eurostar e320 trains are tied for next on the list, but the latter underwent a redesign in 2015. Named for its top speed of 320 km/hr (200 mph), the e320 series is the first tip-to-tail redesign of a Eurostar train in the company’s 22-year history. The speedier trains—20 km/hr faster than the earlier, e300 series—are capable of trimming another 15 minutes off the already zippy Eurostar trips of around two hours between Brussels, Paris, and London (and Amsterdam, later this year). Since Eurostar delivers its passengers right to the center of each city and fares are available with Rail Europe from $70 one-way, it’s a wonder anyone still flies between the cities.

10. Thalys: 186 mph

Connecting Amsterdam, Brussels, Paris, and Cologne with multiple daily services, the Thalys is one of Europe’s most important train lines for both leisure and business travelers; in fact, its ridership is almost an even split between the two categories. In December 2015 the German route was extended as far as Dortmund, though the Brussels-to-Paris run remains critical, making up more than half the business.

The Benefits of Understanding your Track Geometry Measurements

The Goal

The overall goal of track maintenance is to deliver the track, to support the timetable. This is achieved by Rail and Transit owner-operators through four objectives. First, to deliver safety to all passengers and staff, safety for passengers who depend on the rail system. Second, to deliver a reliable rail system, to ensure that the required services are available and that all assets are fit for purpose. Third, to deliver economic prosperity for a rail organization, through optimal and sustainable maintenance activities; and lastly, to deliver a comfortable ride for the consumer, reducing noise and improving ride quality. To deliver these goals, a rail organization must understand the criticality of all their assets, and the condition, along with the quality of the track they own and operate on.

Linear Measurements

Track Geometry measurements are a key component to understanding if the track is fit for purpose and is in a state of good repair. Periodic track condition measurements are required to evaluate the track quality and maintain an effective railway track system. Today most railroads already collect track geometry measurements from recording vehicles; however, often rail operators are not utilizing the data to its full advantage and sometimes data sets are held in siloed systems; making it almost impossible to visualize different condition data at the same time. Track Geometry contains a wealth of information that can support a range of maintenance and renewal decision support.

Core Track Geometry Measurements and Calculations

Gauge; the distance between the running edge of the left and right rail. There are several gauges used globally, the standard gauge is 1,435 mm (4 ft 8 1⁄2 in) and is typically used in North America and most of Europe. As the linear asset degrades, the distance between the rails will increase. This deterioration can cause a train to derail.

Curvature; one way to survey a track alignment is to measure the offsets from a chord to the running edge of the rail at the centres of successive overlapping chords, laid out along the outer rail of the surveyed track, this offset is called a versine.

Superelevation; is a difference in height between the left and right rails. It is generally applied in curves, with the low rail being on the inside edge of the curve. It is applied to offset the lateral forces that are felt when a vehicle traverses a curve.

Vertical Track Variation; a challenge for some individuals to understand is that the underlying geometry is not important; for example, a hill or a valley was already designed into the system. However, if there is a bump or a dip in the track on a hill, that is the information that is needed for extraction. The hill is viewed as a zero, we want to pay attention to the oscillations or variations in the track geometry data. As a general rule, if it takes less than two seconds to go through the variation at line speed, then we consider the variation; and if it takes more than two seconds, then we don’t consider the variation.

Lateral Track Variation; horizontal track geometry is generally filtered in the same way as vertical track geometry. We are looking for features that can be traversed in less than two seconds at line speed. The two seconds is derived from ISO 2632 – Human Comfort.

Track Twist; the difference in cross-level between two points or the rate of change of superelevation and measured over the impact on the bogie; this should be calculated based on the smallest wheelbase used by the owner-operator. A worst-case scenario is that the front wheel would drop onto a twist causing the rear wheel to climb, resulting in a train derailment.

Core Channels from a Track Geometry System

Location; beyond recording what the track geometry is, the system needs to record where it is as well. This is often one of the main issues with geometry data, as the same feature can be recorded at slightly different locations on different recording runs. Distance measured along the track can be derived from a tachometer fitted to one of the axles. This is a reasonably effective mechanism, but errors can be introduced as the wheel wears and if the recording vehicle runs around curves at different speeds. This can be corrected by GPS where available, or by detecting known features along the network and marking them against the recording. Lastly, the location should be reported against the linear referencing system (LRS) for the track, not against the distance traveled by the recording vehicle.

Speed; the FRA defines the maximum allowable posted timetable operating speed using the Vmax formula. We can take our curve data and plot the max speeds on our track charts. Additionally, we can calculate the equilibrium speed, which is the minimum speed that should be traveled through a curve. If locomotive traverses through the curve above Vmax speed, this causes extra wear on the outside rail; and if the locomotive traverses through the curve below the equilibrium speed, then this causes extra wear on the inside rail. This can be tracked and shown in our track charts with real locomotive speed data, to ensure operators are traversing through curves at the appropriate speed.

By Robert Henderson – Rail and Transit Consultant at Bentley Systems

What are stone columns ?

Stone columns are constructed using down-hole vibratory probe methods similar to those used in vibro-compaction. The main difference is that instead of using coarsegrained soil to simply fill the void created by the vibro-compaction operation, stone or other clean, coarse grained materials are placed, and compacted, to form a narrow structural element (i.e. a column) which functions as one or more of the following:

1. enhance the average shear strength and bearing capacity of a weak soil mass,
2. transfer a surface load to deeper competent materials, or
3. provide easy drainage of temporarily high pore water pressures.

Stone columns are ideally suited for improving soft silts and clays and loose silty sands. Stone columns under suitable conditions will:

• increase a soil’s bearing capacity and shear resistance
• reduce settlements,
• increase the time-rate of consolidation,
• reduce liquefaction potential, and
• stabilize existing slopes affected by low shear strength soils .

Stone columns, in general, are most economically attractive for sites requiring column lengths less than 35 ft. deep and preferably about 20 ft. deep below the surface.

Unsuitable soil conditions for stone columns include soils having thick layers of very soft or sensitive clays and organic materials. If the thickness of the unsuitable soil layer is more than the diameter of the stone column, then stone columns may not be appropriate because the very soft soils will not provide adequate lateral support of the stone column. In addition, stone column construction can be hampered by the presence of a thick, dense overburden, or soils with boulders, cobbles or other obstructions that may require pre-drilling prior to installation of the stone column.

Stone columns are constructed using either a vibro-replacement or vibro-displacement installation with the stone aggregate placed using either top or bottom feed methods.

Vibro-Replacement :
Vibro-replacement involves a wet installation method that replaces deep, narrow pockets of the in-situ soil with stone aggregate columns. In this method a high-pressure water jet, located at the tip of the probe, is used to excavate a narrow, open (uncased) hole. Once the hole is progressed to the design depth, the hole is flushed out several times by raising and dropping the probe to remove any loose silt and sand at the bottom of the hole. The vibro-probe is retracted and a limited amount of stone is placed into the hole from the top. The probe’s vibration mode is turned on and it is inserted into the hole to compact the lift of stone. The probe is retracted again and the process repeated until the stone column is formed to the ground surface. During the entire operation, water is continually pumped into the hole to prevent collapse and to keep the aggregate clean. This method is best suited for sites with soft to firm soils with undrained shear strengths of 200 to 1,000 psf and a shallow groundwater table, and where drill wash and spoil
containment and disposal can be practically handled.

Vibro-Displacement :
When a cleaner or lesser environmental impact operation is preferred, stone columns should be constructed using the vibro-displacement method. The operation is a dry installation method where the stone aggregate can be placed into the hole from the top or from aggregate ports at the bottom of the probe. Although the probe’s dead weight and vibration, in lieu of water jetting, is used to excavate the hole, air jetting and/or pre-augering may be used to prevent clogging of the aggregate ports or to assist in advancing or extracting the probe. This method is best suited for
sites where collapse of the hole during the column’s installation is unlikely.

What is vibro-compaction?

Vibro-compaction is a ground improvement method that uses a specialized vibrating probe for in-situ subsurface compaction of loose sandy or gravelly soils at depths beyond which surface compaction efforts are effective.

The vibrating probe densifies loose granular, cohesionless soils by using mechanical vibrations and, in some applications, water saturation to minimize the effective stresses between the soil grains which then allows the oil grains to rearrange under the action of gravity into a denser state.

Vibro compaction to densify loose, silty sands for an interim spent fuel cask storage pad in Braceville, Illinois.

Generally, vibro-compaction can be used to achieve the following enhanced soil performance or
properties:
• Increased soil bearing capacity
• Reduced foundation settlements
• Increased resistance to liquefaction
• Compaction to stabilize pile foundations driven through loose granular materials
• Densification for abutments, piers and approach embankment foundations
• Increased shear strength
• Reduced permeability
• Filling of voids in treated areas

Two rigs completing vibro compaction for liquefaction mitigation and settlement at a casino.

The vibrator is hung from a crane cable or, in some instances; it is mounted to leads in a similar fashion as foundation drilling equipment. The vibrator penetrates under its self weight (or crowd of the machine if mounted in leads) and, at times, with assistance from the action of water jets. The goal is that the vibration and water imparted to the soils ransforms the loose soils to a more dense state.

The Vibro Compaction Process

Advantages, Disadvantages and Limitations

1. Advantages

As an alternative to deep foundations, vibro-compaction is usually more economical and often results in significant time savings. Loads can be spread from the footing elevation, thus minimizing problems from lower, weak layers. Densifying the soils with vibro-compaction can considerably reduce the risk of seismically induced liquefaction. Vibro-compaction can also be cost-effective alternative to removal and replacement of poor load-bearing soils. The use of vibro-compaction allows the maximum improvement of granular soils to depths of up to 165 feet. The vibro-ompaction system is effective both above and below the natural water level.

2. Disadvantages and Limitations

Vibro-compaction is effective only in granular, cohesionless soils. The realignment of the sand grains and, therefore, proper densification generally cannot be achieved when the granular soil contains more than 12 to 15 percent silt or more than 2 percent clay. The maximum depth of treatment is typically limited to 165 feet, but there are very few construction projects that will require densification to a greater depth.

Like all ground improvement techniques, a thorough soils investigation program is required. Yet, a more detailed soils analysis may be required for vibro-compaction than for a deep foundation design because the vibro-compaction process utilizes the permeability and properties of the in-situ soil to the full depth of treatment to achieve the end result. A comprehensive understanding of the total soil profile is therefore necessary which typically requires continuous sampling or in-situ testing.

Equipment access over the site must also be considered. Since the operation requires use of a large crane, a relatively flat work bench with a width of at least 25 ft must be possible near all areas to be treated.

Wet vibro-compaction requires the use of water to jet the vibrator into the ground. The effluent from the jetting process requires at least temporary containment to allow any fine soil particles to settle out and be disposed. Further, this method of ground improvement may not be acceptable if the existing subsurface environment, either soil or water is contaminated. If contamination is present, use of water jetting may cause its dispersion and therefore other ground improvement methods should be considered.

What type of pavement is used for airports runway?

The materials used for airports is generally the same as what is used for roadways, however, the depths, or thicknesses are different, and the tolerances are much tighter at an airport. The material for runways usually needs to meet a much tighter spec.

A typical section for an airport can use asphalt or concrete. Below is a generic look at the structural section for either asphalt or concrete from an FAA Advisory Circular on Aiport Pavement Design and Evaluation.

You will notice that the materials in the middle are thicker and then taper to thinner. This is because the loads on the runway are primarily from the 2 landing wheels, which will be in the middle of the runway. The effective tire width is pictured below.

The surface must be smooth and well bonded, and resistant to the shear stresses of the airplane wheel loads. The non-skid surface must not cause undue wear on the airplane tires . The surface must be free of loose particles that could damage the airplane or people. In order to meet this requirement, there must be good control of the mix. This usually requires a central mixing plant be used for the hot mix asphalt.

The base course is integral to flexible pavement design such as asphalt. The loading in flexible pavements transfers downward and outward. For this reason, the base, subbase, if used, and subgrade contribute to the strength of the pavement section. For concrete pavement, the concrete provides the strength to the structural section.

The base course must be of sufficient quality that it won’t fail, or allow failure in the subgrade. It must be able to withstand the forces from the airplane wheel loading without consolidating which would cause the surface course to deform. The base course uses very select material with very hard and durable aggregate. The requirements for the base course are very strict.

What is Concrete Slab Moisture

WHAT is the Problem?

Concrete slab moisture can cause problems with the adhesion of floor-covering material, such as tile, sheet

flooring, or carpet and bond-related failures of non-breathable floor coatings. Many adhesives used for installation of floor coverings are more water-sensitive than in the past, due to restrictions on the use of volatile organic compounds (VOCs).

To warranty their products, manufacturers require that the moisture emission from the hardened concrete slab be less than some threshold value prior to installing floor coverings or coatings. Fast-track construction schedules exacerbate the problem when floor-surfacing material is installed before the concrete slab has dried to an acceptable level.

WHAT are the Sources of Concrete Slab Moisture ?

a. Ground water sources and when the floor slab is in contact with saturated ground, or if drainage is poor. Moisture moves to the slab surface by capillary action or wicking. Factors affecting this include depth of the water table and fineness of soil below the slab. Fine grained soil promotes moisture movements from considerable depths compared to coarser subgrade material.

b. Water vapor from damp soil will diffuse and condense on a concrete slab surface that is cooler and at a lower relative humidity due to a vapor pressure gradient.

c . Wetting of the fill course/blotter layer, if any, between the vapor retarder and the slab prior to placing the slab will trap moisture with the only possible escape route being through the slab. A blotter layer is not recommended for interior slabs on grade (CIP 29).

d. Residual moisture in the slab from the original concrete mixing water will move towards the surface. It may take anywhere from six weeks to one year or longer for a concrete slab to dry to an acceptable level under normal conditions.

Factors that affect the drying rate include the original water content of the concrete, type of curing, and the

relative humidity and temperature of the ambient air during the drying period. This is the only source of moisture in elevated slabs. Any wetting of the slab after final curing will elevate moisture levels within the slab and lengthen the drying period.

HOW do You Avoid Problems?

Avoiding problems associated with high moisture content in concrete can be accomplished by the following means:

• Protect against ingress of water under hydrostatic pressure by ensuring that proper drainage away from the slab is part of the design.

• Use a 6 to 8 inch [150 to 200 mm] layer of coarse gravel or crushed stone as a capillary break in locations with fine-grained soil subgrades.

• Use a vapor retarder membrane under the slab to prevent water from entering the slab. Ensure that the vapor retarder is installed correctly and not damaged during construction. Current recommendation of ACI Committee 302 is to place the concrete directly on a vapor retarder for interior slabs on grade (CIP 29).

• Use a concrete mixture with a moderately low water-cementitious material (w/cm) ratio (about 0.50). This reduces the amount of residual moisture in the slab, will require a shorter drying period, and result in a lower permeability to vapor transmission. Water reducing admixtures can be used to obtain adequate workability and maintain a low water content. The water tightness of concrete can be improved by using fly ash or slag in the concrete mixture.

• Curing is an important step in achieving excellent hardened concrete properties. However, moist curing will increase drying time. As a compromise, curing the concrete under plastic sheeting for 3 days is recommended and moist curing times greater than 7 days must be avoided. Avoid using curing compounds on floors where coverings or coatings will be installed.

• Allow sufficient time for the moisture in the slab to dry naturally while the floor is under a roof and protected from the elements. Avoid maintenance and cleaning operations that will wet the concrete floor. Use heat and dehumidifiers to accelerate drying. Since moisture transmission is affected by temperature and humidity, maintain the actual service conditions for a long enough period prior to installing the floor covering.

• Test the slab moisture condition prior to installing the floor covering. When concrete slab moisture cannot be controlled, consider using decorative concrete, less moisture-sensitive floor coverings, breathable floor coatings, or install moisture vapor suppression systems (topical coatings).

HOW is Concrete Slab Moisture Measured?

Various qualitative and quantitative methods of measuring concrete slab moisture are described in ASTM E 1907.

Test the moisture condition of the slab in the same temperature and humidity conditions as it will be in service.

In general, test at three random sample locations for areas up to 1000 sq. ft. [100 m2] and perform one additional test for each additional 1,000 sq ft. Ensure that the surface is dry and clean. Record the relative humidity and temperature at the time of testing. Some of the common tests are:

Polyethylene Sheet Test (ASTM D 4263)

– is a simple qualitative test, where an 18 by 18 inch [450 by 450 mm] square plastic sheet is taped tightly to the concrete and left in place for a at least 16 hours. The presence of moisture under the plastic sheet is a positive indication that excess moisture is likely present in the slab. However, a negative indication is not an assurance that the

slab is acceptably dry below the surface.

Mat Test

– where the adhesive intended for use is applied to a 24 by 24 inch [600 by 600 mm] area and a sheet vinyl flooring product is placed face down on the adhesive and sealed at the edges. A visual inspection of the condition of the adhesive is made after a 72-hour period. This test is no longer favored since it can produce false negative results.

Test Strip

– in which a test strip of the proposed primer or adhesive is evaluated for 24 hours to predict its behavior on the floor. This procedure is not very reliable.

Moisture meters

– Measure electrical resistance or impedance to indicate slab moisture. Electronic meters can be useful survey tools that provide comparative readings across a floor but should not be used to accept or reject a floor because they do not provide an absolute measure of moisture conditions within the slab.

Gravimetric

– This is a direct and accurate method of determining moisture content by weight in the concrete slab. Pieces of concrete are removed by chiseling or stitch-drilling and dried in an oven to constant weight. The moisture content is then calculated as a percentage of the dry sample weight. This is rarely recommended by floor covering manufacturers.

Nuclear Density and Radio Frequency

– This nondestructive test instrument is relatively expensive and can take a long time to properly correlate correction factors for each individual project. The instrument has a radioactive source and therefore requires licensed operators.

Anhydrous Calcium Chloride Test (ASTM F 1869)

– is specified by most floor covering manufacturers for pre installation testing. A measured amount of anhydrous calcium chloride is placed in a cup sealed under a plastic dome on the slab surface and the amount of moisture

absorbed by the salt in 60 to 72 hours is measured to calculate the moisture vapor emission rate (MVER).

Maximum limits of vapor transmission generally specified are 3 to 5 pounds of moisture per 1000 square feet per 24 hours. This test is relatively inexpensive, and yields a quantitative result. However, it has some major shortcomings: it determines only a portion of the free moisture at a shallow depth of concrete near the surface of the slab. The test is sensitive to the temperature and humidity in the building. It provides only a “snapshot in time” of current moisture conditions and does not predict if the sub-slab conditions will cause a moisture problem later in the life of the floor.

Relative Humidity Probe (ASTM F 2170)

This procedure involves measuring the relative humidity of concrete at a specific depth from the slab surface inside a

drilled or cast hole in a concrete slab. The relative humidity is measured after allowing 72 hours to achieve moisture equilibrium within the hole. Typically a relative humidity of 75% to 80% is targeted for installation of floor coverings. Relative humidity probes can determine the moisture profile from top to bottom in a slab, conditions below the slab,

and can monitor the drying of a slab over time, leading to predictions of future moisture conditions. These instruments have been used for many years in Europe and are becoming more popular in the

United States.