Launching Girders

There are a number of constructive methods to construct the bridge superestructure. Girders can be launch or cast in situ.

In the case of launching girdgers, we have 2 methods: Underslung and Overhead.

Underslung Method

For long spans, an alternative is cantilever segmental construction. Beam is started in the pier and built up as cantilever in two ways. In this way, cantilevers must be symmetrical. Segements can be cast in situ or precast and erect it on site.

Segmental Construction

Another type is the incremental launching. A factory is built at the beggining of the bridge where the girder is constructed and launched as a cantilever. It is not required any prop since at the tip of the deck is placed a beam that gives support to the deck that comes behind.

Incremental Launching

References:
http://www.nbmcw.com/articles/bridges/2586-launching-systems-for-segmental-bridges.html

Damping

The seismic waves are amplified in the structure, because of the pulses of the wave. However, the structure tries to relief these efforts according to the damping, which is the capacity of the structure to dissipate energy. Therefore, structures after an earthquake continue moving after the struck.

Usually a 5% of damping and a natural period of T=n/10 is related to buildings (n is the number of stories).

Response Spectrum Accelerations (RSA) can be constructed in different ways:

Single parameter scaling:

a)      RSA is anchored to the PGA at zero period. RSA=C/T^n

b)      RSA is anchored by the maximum response spectrum acceleration RSAmax. RSA=C/T^n.

c)      RSA is scaled by the PGV (a=PGV/750 in Australia, AS1170.4, 1993)

Dual parameter scalling, that is to say, RSA is scaled by SaT=0,3s y SaT=1,0s. RSA=SaT1/T

Multiple parameter scalling. From data obtained for a range of periods.

Peak Ground Acceleration PGA, PGV, PGD

Peak ground acceleration (PGA) is the maximum acceleration of the ground by earthquake. However, the acceleration in the structure is higher because of dynamic amplification. In fact, for an average building the acceleration could be increased up to 2.5 to 3.0 PGA (Melbourne). Rigid buildings are controlled by acceleration. PGA can be well measured using strong motion accelerometers placed close of the epicenter of the earthquake. The response of the structure depends on the duration of the pulse and the time history as a whole, and not just the PGA.

Peak ground velocity (PGV) gives more information about the response of structures, being a better indicator of damage than PGA. The difference between PGV and the velocity in the center mass of structure is less than PGA and a, that is to say, the velocity (V) is 1.8 to 2.0 PGV. Buildings less rigid are controlled for velocity. PGV can be measured by seismometers place far away of the epicenter, which could be quite complex. PGV is also related to MMI intensity and with the PGA = PGV/750 (All the values for Rock in Melbourne)

Peak ground displacement (PGD) is related to the drift of the building. In this case, the displacement of the center mass is 1.4 to 1.5 PGD. However, it is not possible to measure directly, but could it be calculated using advanced seismometers, like tele-seismometers from a long distance. Flexible buildings are controlled by displacement.

Finally, the duration of a shaking depends of the fault size and duration of the rupture. Nonetheless, the duration can be prolonged by features in the wave transmission a geological site. For instance, soil could amplify the seism and increasing the dynamic properties of the earthquake, such as this happened in the Mexican Earthquake of 1985.

World Seismicity and Earthquake Hazards

Earthquakes are caused by relative movements between tectonics plates. The stress increase up until bond friction is broken and rupture occurs realizing energy in form of waves through the earth.

The point where is originated the fault is called focus, and the point immediately upper in the ground is called epicenter. Other terms are hypocentral distance, epicentral distance.

Faults can be generated by a combination of strike-slip and dip-slip. Strike slip is a horizontal movement between plates, which could it be strike left or right. If the fault is vertical, then it is called dip-slip, which could it be reverse if that rock moves up the fault and normal if it moves downward. Thrust fault when the dip angle is shallow, usually in subduction zones.

Magnitude of Earthquakes is measured using the Richter scale.  The Richter scale measured the amplitude of the wave by means of seismograph, and corrected using a logarithmic scale. For instance, an increase in 1 degree means an increase in 10 times in energy released.

Earthquakes Hazards could it be: shaking due to ground vibrations, mudflows, liquefaction of soils, tsunamis, floods, fire.

Introduction Earthquakes

The analysis of structures that resist wind loads could it be similar to earthquakes loads, because both are mainly lateral loads. However, deep differences are present in their design.

Design for combined Gravity and wind load is based in verifies that loads are less than capacity of structural elements, trying to keep an elastic behavior. However, design of structures for combined gravity and earthquake is based in deformations and damage rather than only strength. Inelastic behavior is expected, and the design allows reach less strength than an elastic analysis. Moreover, design lead to ductile structures and a design by displacements instead of strength.

Ductility can be defined as the ratio between dmax and the drift expected. The ductility will keep constant in an Elastic behavior. Because the inelastic behavior is complex, then it is assumed that ductility of structures are defined always in the elastic range.

Due to the ground accelerations, then inertia forces appear in the structure, which distribution depends of mass distribution through the structure. Magnitude of the response is depending of the dynamic properties of the structure. However, the drift between floors could generate plastic hinges, which modify the structure stiffness, thus the dynamic response of the whole structure is modified.

Capacity Design principle leads to ensure enough dissipation of energy under large earthquakes, and this system has to be reliable for the duration of the Earthquake. Capacity design ensures not collapse of the structure. Therefore CD assume that establishing strength hierarchies within the structure and detailing weak zones to respond in a ductile way, then the structure will remain stable during a large earthquake.

• Selection of suitable structure for inelastic response, i.e. regular structure
• Selection of suitable location for inelastic deformations.
• Insurance, that inelastic behavior does not occur in undesirable locations.

Prescriptive Criteria

A new approach is moving from prevent collapse to controlled damage, because of the huge costs involved in repairing or rebuild structures, which will increase dramatically.

A design based in performance can be stated as how much damage can be resisted by a given building in a given level of earthquake. This depends of the performance that we want for a certain building. We have to link the performance of a building for a certain level of earthquake. To control the damage, we have to understand that displacements are more important than just keep a strength performance. Topics like limit the drift between floors, controlled deflection to reduce damage of non-structural elements who cannot resist vertical loads has to be analyzed.

Performance Design, procedure: Select performance objectives, Develop preliminary design, Asses this against performance objectives. The performance objectives can be presented in the matrix as follow:

HIGH RISE STRUCTURES. INTRODUCTION

In the design of high-rise buildings, different sectors and specialists gather in workshops to sort out an optimum design in terms of aesthetics, creativity, built ability and economy. The main elements involved in the design are shear core, footings, beams, floors and columns. In terms of the total cost of HB, floor is 40% and core a 30%. Therefore, an economic design has to reduce the floor or wall thick, because just a couple of millimeters imply a huge decrease in the total cost. Designs can be improved learning of the other’s mistakes. First, the design of short beams cannot be analyzed thinking that its behavior is similar to a beam under bending moment, which implies a minimum reinforcement. The right design should be made considering a truss model. Second, when is required a construction joint, the grout should be a concern thus the strength should be rise at least the same strength resistant of the concrete. Third, pedestrian comfort is a topic usually not thought. High Rise Buildings modify the airflow, thus the wind force is moved not only to the sides, in fact is also spread downwards to the pedestrians. When this force increases could be quite uncomfortable for pedestrians who are walking in the ground floor. Also the design of buildings should be simple. In other words, the vertical loads should be transferred to the ground in a simple way. The lower levels cannot be weaker than the upper. Nowadays, our technology let us built buildings of 2km height, without problem. The fire control however is a limitation. The requirements lead to let escape to people in just a minutes. Lift can not be used, parachutes are not a safety solution, therefore we have to expect that someone who lives in the last floor escaped alive. High-rise design has three main characteristics that have to be considered: strength, stability and serviceability (deflections). Lateral loads, like wind and earthquake controlled the behavior of these structures, whereas short buildings are governed by strength of individual elements. Therefore the HR design has to be focus in to assess the magnitude of wind and earthquake loads and lateral displacements or drift to limit the formation of plastic hinges. The engineer has to be aware of the effects on non-structural elements, like dry walls, which cannot resist vertical loads. It is required a analysis of second order effects like P-D, creep movements and differential movements. A analysis of overall stability against overturning and sliding and the assessment of soil properties are also required. The optimum design has to be lead to reduce the core size, which is quite complex and require the participation of the all sectors, not only the structural engineer. The reason is that the core is not included in the net lettable area.

Listado de perfiles

Con el objeto de diseñar de forma rápida perfiles de acero, en ocasiones es necesario tener un listado de ellos ya sea de manuales ICHA o CINTAC, siendo los más utilizados. Es por ello que pongo a su disposición una planilla excel con un listado de perfiles, como por ejemplo: L, C, T, I, H, IN, UP, etc.

Lo bueno de esta planilla, es que está realizada con macros por lo que además es posible mediante una interfase muy sencilla buscar el perfil que buscamos. Lo importante es que la dirección de la carpeta de imágenes se encuentre correctamente redireccionada, pues de lo contrario aparecerá un mensaje de error.

La planilla entrega propiedades de inercia, áreas, radios de giro, etc. Espero les guste. Saludos.

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