How Does Structural Design of Micropiles Perform?

Micropiles can be designed as rock socketed piles in rock formation and friction piles in weathered rocks or soils to carry either compression load or tension load. All micropiles are designed to transfer load through the shaft friction over a length of pile shaft to the founding medium. End bearing at the pile tip is generally negligible for the reasons of small base bearing areas, in which the axial load cannot be effectively transferred to the base. If to be considered, the point capacity commonly does not exceed 15 to 20 % of the side  resistance. This  design  philosophy  also  inherently demands  a founding medium with sufficient thickness to carry the imposed load from the micropile. If there is a cavity below the pile toe or the pile is socketed into a boulder, there will be some transfer of the load to the surrounding sound material by arching effect or to spread the load to the underneath soils. 


If a large pile group is involved in these founding conditions, care need to be taken to avoid punching shear failure of the rock slab or bearing failure of soils underneath the boulder causing excessive settlement under the entire pile group. The ultimate load which can be supported by a single micropile is defined by the lowest of the following:

· structural shaft resistance

 

· buckling load

· failure of the grout/ground bond.

The allowable load used is the ultimate load divided by a factor of safety. However, a lower load may be specified due to limitations of stresses and/or settlements that can be accepted by the structure being underpinned.

What is CNC File in Steel Industry?

CNC (Computer Numeric Control) is a method by which a steel fabricator sends information to specific semi-automated machinery to perform certain fabrication tasks. These tasks may include cutting members to length, drilling or punching of holes and cutting plates to size, beam copes, long slots, etc.

CNC is not new to the fabrication of structural steel. It has been provided by what is referred to as interactive methods. In the past shop drawings were sent to the fabrication shop and numeric information was entered into a computer by hand or interactively. The classical method can and does provide for the possibility of making mistakes. 

The programmer /operator, typically someone in what is called the "template shop", would then provide tapes or some other means of transferring the information to the individual CNC pieces of equipment. With this digital information the machinery would, when the material is loaded, perform the indicated operation.

In today's world of electronically produced shop drawings CNC information can be provided automatically by the detailing software. If the detailing software being used is being capable of providing CNC information, the need for a programmer in the shop to transfer the required data from the shop drawings to the computer is eliminated. CNC also reduces the possibility of an error in data transfer.

This will, for the most part, eliminate the need for a programmer in the shop, but it also means that the shop drawings must be made accurately and to scale. Furthermore, all holes, cuts, lengths, and other fabrication criteria must be incorporated electronically for inclusion with the CNC information. If shop drawings are plotted and changes are made to these plotted/hard copies, then the automatic CNC information may be rendered useless.

In today's market these hand changes are rarely performed when accurate CNC information is required. If for some reason drawings are not made to scale, the CNC information is corrupted and cannot be sent to the shops for fabrication.


CNC is a great tool providing speed of fabrication and better quality control. If fabrication information is transferred digitally from the detailing computer directly to the CNC control computer, either through a network system or stored data on some sort of digital media, there is little room for error and quality control is greatly improved.

What are Mill Tolerances in Steel Industry? What is the Importance of Mill Tolerances?

The term Mill Tolerances is used to describe permissible deviations from the published dimensions of cross-sectional profiles listed in mill catalogs, and from the thickness or lengths specified by the purchaser. Some of the variations are negligible in smaller shapes, but tend to increase and must be taken into consideration in detailing and fabricating connections for members made up from larger shapes. Other mill tolerances permit some variation in area and weight, ends out-of-square, and camber and sweep. Factors that contribute to the necessity for mill tolerances are:

•The high speed of the rolling operation required to prevent the metal from cooling before the rolling process has been completed

•The varying skill of the operators in adjusting the rolls for each pass, particularly the final pass


•The deflection (springing) of the rolls during the rolling operation 

•The gradual wearing of the rolls, which can result in some weight increase, particularly in the case of shapes

•The warping of steel in the process of cooling

•The subsequent shrinkage in length of a shape that has been cut while still hot.

The steel detailer should be familiar with the several tolerances, especially those of camber, sweep, depth of section and length. A more exhaustive presentation of these tolerances is found in the ASTM A6 Specification.

An important factor for the steel detailer to understand clearly is the effect of mill tolerances. The steel detailer must know when to take tolerances into account, particularly in ordering mill material and in detailing connections, especially those involving heavy rolled shapes. For instance, when detailing a moment connection the steel detailer must be cognizant of the permissible variations in the depth of the beam and out-of-square of the beam flanges in order to locate the connection material shop welded to the column.

Out-of-Plane Anchorage to Diaphragms

Diaphragms shall be defined as horizontal elements that transfer earthquake-induced inertial forces to vertical elements of the lateral-force-resisting systems through the collective action of diaphragm components including chords, collectors, and ties.

Walls shall be positively anchored to all diaphragms that provide lateral support for the wall or are vertically supported by the wall. Walls shall be anchored to diaphragms at horizontal distances not exceeding eight feet, unless it can be demonstrated that the wall has adequate capacity to span horizontally between the supports for greater distances. Anchorage of walls to diaphragms shall be designed for forces calculated using Equation (1), which shall be developed in the diaphragm. If sub-diaphragms are used, each sub diaphragm shall be capable of transmitting the shear forces due to wall anchorage to a continuous diaphragm tie. Sub-diaphragms shall have length-to-depth ratios not exceeding 3:1.  Where wall panels are stiffened for out-of-plane behavior by pilasters or similar elements, anchors shall be provided at each such element and the distribution of out-of-plane forces to wall anchors and diaphragm ties shall consider the stiffening effect and accumulation of forces at these elements. Wall anchor connections shall be considered force-controlled.

Fp = c S_XS W            (1)                

Where:
 
Fp = Design force for anchorage of walls to diaphragms

c = Factor from Table below for the selected Structural Performance Level. Increased values of χ shall be used when anchoring to flexible diaphragms

S_XS  = Spectral response acceleration parameter at short periods for the selected hazard level and damping adjusted for site class

W = Weight of the wall tributary to the anchor

Exceptions:

1. Fp shall not be less than the minimum of 400 lb/ft or 400 S_XS  (lbs/foot).

Coefficient c  for Calculation of Out-of-Plane Wall Forces
Structural Performance Level	c
	Flexible diaphragms	Other diaphragms
Collapse Prevention	0.9	0.3
Life Safety	1.2	0.4
Immediate Occupancy	1.8	0.6

FEMA 356: Earthquake Hazard Due to Ground Shaking

The seismic hazard due to ground shaking shall be defined for any Earthquake Hazard Level using approved spectral response acceleration contour maps of 5%-damped response spectrum ordinates for shortperiod (0.2 second) and long-period (1 second) response. The short-period response acceleration parameter, SS, and the long-period response acceleration parameter, S1, shall be determined as follows:

1. If the desired Earthquake Hazard Level corresponds with one of the mapped Earthquake Hazard Levels, obtain spectral response acceleration parameters directly from the maps. Values between contour lines shall be interpolated. 

2. If the desired Earthquake Hazard Level does not correspond with the mapped levels of hazard, then obtain the spectral response acceleration parameters from the available maps, and modify them to the desired hazard level, either by logarithmic interpolation or extrapolation.

3. Obtain design spectral response acceleration parameters by adjusting the mapped or modified spectral response acceleration parameters for site class effects.

4. If the desired Earthquake Hazard Level is the Basic Safety Earthquake 2 (BSE-2), obtain spectral response acceleration parameters.

5. If the desired Earthquake Hazard Level is the Basic Safety Earthquake 1 (BSE-1), obtain the spectral response acceleration.

6. Using the design spectral response acceleration parameters that have been adjusted for site class effects, develop the general response spectrum.

Japanese Code for Understanding and Designing of Pile Failure Due to Liquefaction

Piles are one of the most common and safe deep foundation that are practiced yet for foundation solution where bearing capacity of soil at shallow depth is poor to support a heavy or sensitive structure. But in recent years, though foundation engineering has been developed greatly, it was not determined that why piles are failing during cyclic loading that is generated by earthquake. The recent understandings about failure of pile are as follows: 

When soil gets liquefied, it loses shear strength. This liquid soil has no resistance to flow resulting soil to dragging and flow with it any non-liquefied  crust over it. Piles are dragged with the crust and deflection of pile results bending moment. When bending moment exceeds the capacity of pile it get failed. This phenomenon is sometimes called failure due to lateral spreading.

It can be concluded that the recent observation on the failure mechanism shows that piles are pushed by soil during earthquake as a consequence of liquefaction. This mechanism is sometimes proved where deformation on ground surface around pile foundation is observed.

The Japanese highway code of practice (JRA 1996) advises design engineers to consider following assumptions to provide sufficient bending strength against lateral spreading: 

1. Non-liquefied crust offers passive earth pressure to the pile

2. The liquefied soil itself offers a drag equal to 30% of total overburden pressure. 

Other codes such as NEHRP 2000 and Eurocode 8, part 5 (1998) also focus on the bending strength of the pile.

Earthquake Induced Pile Failure

So far, among the known foundation technology, Pile foundation is the safest deep foundation. But it is noticed that piles are failed during past strong earthquakes. This failure of pile is structural and almost all of these piles pass through soils that are susceptible to liquefaction. During earthquake the soils become liquefiable and the piles lose lateral confinement through its length and plastic hinges are formed that leads to structural failure. 

Bending moments or shear forces induced on the piles during earthquake exceed that predicted from Codes of practice. All current design codes apparently provide a high margin of safety (using partial safety factors on  load, material stress which increases the overall safety factor), yet occurrences of pile failure due to liquefaction are abundant.

This implies that the actual moments or shear forces experienced by the pile are many times those predicted. It may be concluded that design methods may not be consistent with the physical mechanisms  that govern the failure. In other words, something is missing. This research investigates what is missing from the current understanding of earthquake-induced pile failure by analyzing the postulated hypothesis of the existing design codes of practice, such as the Japanese Road Association Code (JRA 1996), NEHRP (2000), and Eurocode 8 (Part 5).

To avoid plastic yielding it is experienced that overall safety factor of a typical concrete pile considering the bending mechanism may range between 4 and 8. This is due to the multiplication of partial safety factors on load (1.5), material (1.5 for concrete), fully plastic strength factor (ZP/ZE = 1.67 for circular section) and practical factors such as minimum reinforcements or minimum number of bars. Therefore, one should not expect failures unless wrong failure mechanisms are postulated.

Magnitude 8.6 Earthquake is not Enough for Sumatran Earthquake!

The recent magnitude 8.6 earthquake (11/4/2012) and its aftershock of magnitude 8.2 struck Indonesian island of Sumatra. The mainshock last for five minutes and aftershock last for four minutes. Though tsunami alert was announced but no tsunami of remarkable height was not appeared any coasts of the world specially Sumatran coast.

Now the question is the magnitude 8.6 earthquake was not enough for tsunami? This earthquake off the west coast of northern Sumatra, Indonesia, occurred as a result of strike-slip faulting within the oceanic lithosphere of the Indo-Australia plate. The quake was located approximately 100 km to the southwest of the major subduction zone that defines the plate boundary between the Indo-Australia and Sunda plates offshore Sumatra. At this location, the Indo-Australia plate moves north-northeast with respect to the Sunda plate at a velocity of approximately 52 mm/yr.

Large strike-slip earthquakes, while rare, are not unprecedented in this region of the Indo-Australian plate. Since the massive M 9.1 earthquake that ruptured a 1300 km long segment of the Sumatran megathrust plate boundary in December of 2004, three large strike-slip events have occurred within 50 km of the April 11, 2012 even.These earthquakes occurred on April 19 2006 (Mw6.2), October 4 2007 (Mw6.2) and January 10, 2012 (Mw7.2). In all three cases, the style of faulting was similar. These events align approximately with fabric of the sea floor in the diffuse boundary zone between the Indian and Australian plates.

This quake seems to be a large earthquake within the Indian Plate and the plate has broken in a sort of lateral way. It's a sort of tearing earthquake, and this is much less likely to cause a tsunami because it's not displacing large volumes of water.

PROSPECT OF FIBER REINFORCED CONCRETE IN INDIA

As published in previous post tensile strength is a mere fraction of compressive strength. Sometimes a compression concrete member may fail when an unexpected loading like earthquake produce stress normal to axial direction to which it is designed properly. Such members which are damaged but managed to keep standing without failure need to be repaired to make structure serviceable. In this purpose, a repair technology of wrapping these members with suitable materials is required. 

This conception was developed long ago but the key materials of wrapping have found recent years. Durability, ductility and stiffness and response to creep is the key feature for a wrapper. If the  strengthening wrapping somehow  is torn the capacity of the repaired section reduced dramatically. Moreover where wrapping is exposed to environmental attack like corrosive nature, it is important to have a property of remain unchanged against any condition of corrosiveness. Considering durability fact the steel, the most common and strong material, steel is removed from the list of wrapping materials.

The purpose of wrapping is to prevent tensile cracks. Wrapped concrete element is several times stronger than non-wrapped one. Now its time to know what is the wrapping material? This is fiber reinforced concrete. Due to light weight and non-corrosive properties it can strengthen any element without increasing self weight of element or structure. Being light weight it can handle easily.  

 

It is noticed that one layer of fiber reinforced polymer concrete can increase the ultimate strength of an element by a factor of 2.5. However increase of layer doesnot incrase ultimate strength linearly. Ultimate strength increased 8 times with 8 layer of wrapping. Whereas one layer of wrap can increase the ultimate strain 6 times. These discussions indicate that with one layer increase ductility many times as ultimate strain increased many times. This feature makes fiber reinforced polymer concrete more suitable for earthquake rehabilitation projects. 

With High ultimate strain, it is required to have low stiffness. Glass fibers that have considerably lower stiffness than the carbon fibers and higher ultimate strain is desirable. The unfavorable creep behavior of glass fiber poses little adversity in earthquake resistant .applications as earthquake forces are seldom encountered. Moreover, glass fiber is much less expensive than carbon fiber. Therefore, glass fiber has been used in rehabilitation and retrofitting of structures in Gujarat. 

Wrapping can be applied to strengthen concrete beams in compression and shear composite but in this purposes fabric wraps are used in concrete beams. 

Now-a-days FRCs are getting popular in india. Indian earthquake seismic design and construction were developed rapidly after 2001 Gujarat earthquake. FRCs bring speed in repairing structures and its members. A 5000 sqm of fertilizer plant has been repaired and left for operation within 45 days. Indian manufacturer were supplied all repair materials. Now many manufacturer produce frc materials leaving great opportunity for design, construction and rehabilitation with fiber reinforced concrete.

TECTONICS OF Mw 8.6 SUMATRA EARTHQUAKE APRIL 11, 2012

Indonesian island Sumatra has jolted by a magnitude 8.6 earthquake at Wednesday, April 11, 2012 at 02:38:37 PM at local time and produced a ground shaking for up to five minutes. It was reported that after two hours an aftershock of magnitude 8.2 produced a renewal of previously superseded tsunami warning after main shock. The aftershock lasted four minutes. But finally no tsunami appeared in the coasts of the Indian Ocean. 

This earthquake off the west coast of northern Sumatra, Indonesia, occurred as a result of strike-slip faulting within the oceanic lithosphere of the Indo-Australia plate. The quake was located approximately 100 km to the southwest of the major subduction zone that defines the plate boundary between the Indo-Australia and Sunda plates offshore Sumatra. At this location, the Indo-Australia plate moves north-northeast with respect to the Sunda plate at a velocity of approximately 52 mm/yr. 

Large strike-slip earthquakes, while rare, are not unprecedented in this region of the Indo-Australian plate. Since the massive M 9.1 earthquake that ruptured a 1300 km long segment of the Sumatran megathrust plate boundary in December of 2004, three large strike-slip events have occurred within 50 km of the April 11, 2012 even. These earthquakes occurred on April 19 2006 (Mw6.2), October 4 2007 (Mw6.2) and January 10, 2012 (Mw7.2). In all three cases, the style of faulting was similar. These events align approximately with fabric of the sea floor in the diffuse boundary zone between the Indian and Australian plates.

Sumatra Earthquake 11 April 2012 and Bangladesh

A quake of magnitude 8.6 struck Sumatra at Wednesday, April 11, 2012 at 02:38:37 PM at local time and produced a ground shaking for up to five minutes. It was reported that after two hours an aftershock of magnitude 8.2 produced a renewal of previously superseded tsunami warning after main shock. The aftershock lasted four minutes. 

This quake was unlike those seen off Indonesia in recent years, where ground had been pushed under the continental plate, "flipping up" the seabed. It seems to be a large earthquake within the Indian Plate and the plate has broken in a sort of lateral way. It's a sort of tearing earthquake, and this is much less likely to cause a tsunami because it's not displacing large volumes of water. Still no reports of damage or casualties are declared by local government.

The main shock was centered at a depth of 33km (20 miles), about 495km from Banda Aceh, the provincial capital.

Among many aftershocks(so far 18 aftershocks have been recorded) the major shock 8.2 struck 16km (10 miles) beneath the ocean floor and 615km from Banda Aceh. 

Singapore, Thailand, Sri Lanka, Malaysia, Bangladesh and India are also felt a fraction of this massive quake. 

There were no immediate reports of casualties or damage in Bangladesh. According to Meteorological Department the earthquake was felt at 2:38:30pm in Bangladesh. 

The response of Bangladeshi people expressed in Modified Mercalli Intensity (MMI) scale and distance from epicenter are listed below:

Country	Region	MMI	Distance from Epicenter
Bangladesh	Chittagong	3	2218
Bangladesh	Dhaka	3	2383
Bangladesh	Noakhali	2	2277
Bangladesh	Narayanganj	3	2373

Aerogel: An Incredible Insulator

In 1931 Aerogel was first produced from silica gels. After that it were derived from variety of materials like alumina, carbon, chromia and tin oxide. The gel goes under supercritical drying to keep the structure of the gel intake even after drying. The supercritical drying is performed under intense heat and pressure. The resulting aerogel is porous, having lattice composed of more than 90% air. This property make it ultra-lightweight. 

Importance:
 

Aerogel is porous but solid material that shows a group of extreme properties. This materials are extremely costly and can only be used at expansive properties with environmentally friendly features. But in coming years as aerogel becomes cheaper, it is expected to use in most structures as an insulating materials. The physical properties that make aerogel lucrative are as follows: 

1. Aerogel is the lowest density solid in the world. This is unbelievable that some sorts of aerogel composed of more than 99% air.

2. Being such low density, it still performs as solid

3. Aerogel can support many times load of its selfweight

4. One cubic inch of aerogel may have a surface area like a football area. This poor conductive large surface area makes it extreme insulation properties.

Aerogel as an Insulator:

Aerogel can almost fully reduce heat transfer as the gaseous pores are poor heat conductor. The heat transferring methods, like conduction, convection and radiation, almost do not work through aerogel.

What is Acid Rain? and What are its impacts on structure?

From 18^th  century acid rain has been observed in many places of world industrial emissions are above pollution level. Chemical chemical precursors of sulfuric and nitric acids combine with natural sources of acidic particles results acid rain. Precursors are nitrogen oxide (NOx) and sulfur dioxide (SO2). Acidic particles are from volcanoes and decaying vegetation. This mixture when reacts with oxygen, water and other chemicals the acid carried to soil and groundwater through rain snow, frost and mist.

The scenario of USA are- 2/3 of all SO2 and 1/4 of the NOx emissions in the atmosphere are generated from coals that burn in power plants and agricultural and vehicular equipment that use gasoline as fuel. It can be said that some wear on building structures in industrialized regions might be occurred due to acid rain. But in some arid regions where dry depositions containing acidic pollutants, forms on buildings can suffer more sticking. 

In this arid regions when rain or snow falls, subsequent wet deposition of nitric and sulfuric acids becomes more acidic. These acidic components than wash into soil and subsequently recharge aquifers.

streaking and discolorization of bridges and commercial or industrial metal buildings have been observed from many years due to impact of acid rain. This is due to corrosion of bronze, zinc, nickel, carbon-steel and copper.

When limestone and marble are used as building stone, the acidic chemicals of acid rain dissolve them easily. All minerals, paint and road overlay are affected to some degree due to acid element of acid rain.

Soil-Shaft Side Friction

Side friction is developed when a shaft is provided to resist Vertical loads. The side friction between soil and shaft results relative movements between them. The maximum side friction is often developed after relative small displacements less than 0.5 inches. Side friction is limited by the adhesion between the shaft and the soil or else the shear strength of the adjacent soil, whichever is smaller. 

Side friction often contributes the most bearing capacity in practical situations unless the base is bearing on stiff shale or rock that is much stiffer and stronger than the overlying soil.

Side friction is hard to accurately estimate, especially for foundations constructed in augered or partially jetted holes or foundations in stiff, fissured clays.

Vibro Concrete Columns (VCC): A Ground Improvement Technique for Cohesive and Organic Soils

The Bottom Feed Vibro-Replacement System represented a major advance in ground improvement technology with its dry construction of high integrity compacted stone columns in weak cohesive soils. Now the bottom feed concept has been extended to construct Vibro Concrete Columns (VCC) through very weak cohesive and organic soils which are unsuitable for the proper applications of conventional Vibro-Replacement techniques. Since being developed in 1976, one application of the VCC technique has been as a ground modification technique beneath embankments or structures with large floor loads.

The VCC technique combines the ground improvement advantages of the vibro systems with the load carrying characteristics of piles. Beneath large area loads such as embankments or structures with significant floor loads, the VCC technique is used to reduce settlement, increase bearing capacity and. if necessary, increase slope stability. This vibro-displacement technique will density granular soils and transfer loads through soft cohesive and organic soils.

The analysis and design of the VCC improved site is essentially the same as would be performed for a pile foundation, except that the improved soil parameters are used. For large area loads the VCC system can be overlain by a granular mat, sometimes reinforced by a geogrid,to evenly dis­tribute the structural loads to the VCC system.
  

 At the ground surface a slight mushrooming of the concrete column occurs which also assists with the load transfer. Therefore, the planned structure can be designed with a standard shallow foundation system or, in the case of embank­ments, with a uniform bearing pressure. The addition of the granular mat is not needed if a suffi­cient thickness of surface granular soil is present, which will be densified as a result of the VCC construction and act to distribute the loads to the VCC system. 

In general, the Concrete Columns are used without reinforcement. However, if required, single bars for uplift loads or short cages (<20 ft) can be vibrated into the completed columns for lateral load conditions.

Grouting Considerations For Groundwater and Aquifer

What is Groundwater?

Groundwater is the water in the zone of saturation. The upper limit of this zone is referred to as the water table. The depth to the water table may vary considerably depending on site conditions. The groundwater may be found either in continuous bodies or in several separate strata, and the thickness may vary considerably. Local saturated zones that may occur above the main water table are termed perched water.

Groundwater Imapact

Since groundwater conditions have an important effect on design and construction, the regional and local conditions must be studied during the investigation stages so that potential problems may be evaluated. The grouting program should be designed for the existing groundwater conditions as well as for postconstruction conditions. Different methods and procedures may be employed, depending on the formation permeability, the depth to water table, and the type of aquifer present (confined and unconfined). These conditions affect the type of grout, the grouting procedure, the depth and extent of treatment, the spacing of holes, the need for a multiple- or single-line grout curtain, and the pressures that should be used.

If there have Aquifer

Aquifer conditions also have a direct bearing on the need for and type of drainage required. The chemistry of the groundwater should be considered with respect to the materials to be used in the proposed structure and to the grout to be used. Samples should be tested for pH and the chemistry analyzed. Springs in the construction area may require special treatment, including special grouting methods.

How to Identify Collapsible Soils?

Collapsible soils will settle without any additional applied pressure when sufficient water becomes available to the soil. Water weakens or destroys bonding material between particles that can severely reduce the bearing capacity of the original soil. The collapse potential of these soils must be determined for consideration in the foundation design.

Identification:

;
Many collapsible soils are mudflow or windblown silt deposits of loess often found in arid or semiarid climates such as deserts, but dry climates are not necessary for collapsible soil. Typical collapsible soils are lightly colored, low in plasticity with LL < 45, PI < 25 and with relatively low densities between 65 and 105 lbs/ft3 (60 to 40 percent porosity). Collapse rarely occurs in soil with porosity less than 40 percent. 

Potential Collapse. The potential for collapse should be determined from results of a consolidometer test. The soil may then be modified as needed using soil improvement methods to reduce or eliminate the potential for collapse.

Scouring Around Foundation

Foundations such as drilled shafts and piles constructed in flowing water will cause the flow to divert around the foundation. The velocity of flow will increase around the foundation and can cause the flow to separate from the foundation. A wake develops behind the foundation and turbulence can occur. Eddy currents contrary to the stream flow are the basic scour mechanism. The foundation must be constructed at a sufficient depth beneath the maximum scour depth to provide sufficient bearing capacity.

(1) Scour Around Drilled Shafts or Piles in Seawater. The scour depth may be estimated from empirical and experimental studies. Refer to Herbich, Schiller and Dunlap (1984) for further information.

(a) The maximum scour depth to wave height ratio is £ 0.2 for a medium to fine sand.

(b) The maximum depth of scour  S_U  as a function of Reynolds number Re is (Herbich, Schiller and Dunlap 1984)

S_U=0.00073R_e^0.619      
          

where  S_U  is in feet.

(2) Scour Around Pipelines. Currents near pipelines strong enough to cause scour will gradually erode away the soil causing the pipeline to lose support. The maximum scour hole depth may be estimated using methodology in Herbich, Schiller, and Dunlap (1984).

(3) Mitigation of Scour. Rock-fill or riprap probably provides the easiest and most economical scour protection.

AASHTO Requirements for Gusset Plate

Gusset plates are steel plates that connect columns to beams and girders. Fasteners like bolts, rivets or welding or all of three can be used to fasten gusset plates to other members. Gusset plates not only serve as a method of joining steel members together but they also strengthen the joint. They can be used in bridges and buildings along with other structures.

The lateral dimensions of a gusset plate are determined principally by the fastener requirements of the members, which leaves only  the thickness to be based on other considerations.

According to the AASSHTO specifications, “Gusset plates shall be of ample thickness to resist shear, direct stress, and flexure, acting on the weakest or critical section of maximum stress.” This is all very good, but the only practicable method of estimating these stresses is based on the assumption that the elementary formulas for beams apply, and these formulas are valid only for beams whose span is more that twice the depth and at cross sections not closer to concentrated loads than about half the depth.

The ordinary gusset plate falls considerably short of these requirements, so the results obtained by the application of beam formulas are of questionable value and may be misleading.

In case of truss, Compressive stresses may develop parallel to and at the edge of gusset plates. This is because deflection of a truss tends to change the angles between its members. Therefore, the width of the top edge must not be too large, compared with the thickness, or the plate may buckle. Bending of this kind is called local buckling. The AASHTO specifications require than an unsupported edge of a gusset plate be stiffened if it is longer than 11,000/√F_y.psi times the thickness.