centro de transferencia de tecnología en transportación...

Centro de Transferencia de Tecnología en TransportaciónDepartamento de Ingeniería Civil y Agrimensura

UPR-Recinto Universitario de MayagüezCall Box 9000 * Mayagüez, PR 00681

Tel. 787-834-6385 * Fax: 787-265-5695 * www.uprm.edu/prt2

2 de octubre de 2015

Instructor

Dr. Luis D. AponteDepartamento de Ingeniería Civil y Agrimensura

UPR – Recinto Universitario de Mayagüez

29 Años de Excelencia en el Adiestramiento de Oficiales de Transportación

a Nivel Municipal, Estatal, y Federal en Puerto Rico e Islas Vírgenes

LUIS D. APONTE-BERMÚDEZ, Ph.D., P.E.CATEDRÁTICO ASOCIADO – UNIVERSIDAD DE PUERTO RICO RECINTO DE MAYAGÜEZ

1

INTRODUCCIÓN AL CÁLCULO DE CARGAS DE VIENTO PARA EL DISEÑO DE UTILIDADES EN CARRETERA

Agenda

Dr. Aponte 2

• Introducción y Recuento de Huracanes Históricos en PR

• Importancia de cargas de viento en la infraestructura para carreteras

•Ejemplos de fallas

• Terminología y Definiciones de las cargas de viento según:ASCE 7-05 – Minimum Design Loads for Buildings and Other Structures

AASHTO – LTS6 – Standard Specifications for Highway Signs, Luminaries, and TrafficSignals

•Ejemplos de estructuras típicas

•Ejemplos numéricos para calcular cargas de viento e letreros

INTRODUCCIÓN AL CÁLCULO DE CARGAS DE VIENTO PARA EL DISEÑO DE UTILIDADES EN CARRETERA

INTRODUCCIÓN Y RECUENTO DE HURACANES HISTÓRICOS EN PUERTO RICO

Dr. Aponte 3INTRODUCCIÓN AL CÁLCULO DE CARGAS DE VIENTO PARA EL DISEÑO DE

UTILIDADES EN CARRETERA

Evolución de Códigos de EdificaciónREGLAMENTO DE EDIFICACION DE PR REGLAMENTO DE PLANIFICACION Nº7

Edición 1954

Edición 1968

Edición 1987

“La ultima edición (1987) requería que sediseñaran las estructura para una intensidadde viento correspondiente a un huracáncategoría 3 en la escala Saffir-Simpson, lavigencia del mismo fue hasta 1999”

4

Evolución de Códigos de EdificaciónUNIFORM BUILDING CODE 1997

UBC-97

Este fue adoptado en el año 1999

Se refiere a la guía del “American Society of Civil Engineers” ASCE 7-95 del año 1995, para el computo de cargas de vientos huracanados para una intensidad de vientos correspondientes a un huracán categoría 3

“Al momento de adopción del UBC-97 , ya había una nueva guía ASCE 7-98, la cual requería que la velocidad de diseño para Puerto Rico fuera de 145 mph, la cual corresponde a un huracán categoría 4”

5

Evolución de Códigos de EdificaciónSe refiere al International Building Code 2009 (IBC 2009)

Se refiere a la guía del “American Society of Civil Engineers” ASCE 7-05 del año 2005, para el computo de cargas de vientos huracanados para una intensidad de vientos correspondientes a un huracán categoría 4 (V=145 mph)

6

V = 145 mph

Código vigente adoptado en el 2011

Resumen: Evolución de Códigos de EdificaciónAntes del 1987 el Reglamento Nº 7 de la Junta de Planificación, en las ediciones del 1954 & 1968, establecía las presiones laterales de diseño externas, pero no las relacionaba a la escala de vientos huracanados Saffir-Simpson (la misma aún no existía)

La mayor parte de la infraestructura del país se desarrollo antes 1987

7

1987 - 1998Reglamento Nº7

V = 125 mph

1998 al 2011UBC 97 – ASCE 7-95

V = 125 mph

2011 al PresenteIBC 2009 – ASCE 7-05

V = 145 mph (~35%↑)

Evolución de la especificación de AASHTOpara el diseño de Utilidades en Carreteras

•Edición más reciente:American Association of State Highway and Transportation Officials, & Subcommittee on Bridges and Structures. (2013). Standard specifications forstructural supports for highway signs, luminaires, and traffic signals. Washington, D.C.: American Associationof State Highway and Transportation Officials.AASHTO LTS 6: 2013 (Basic Wind Speed Map ASCE 7-05)

•Previas ediciones:AASHTO LTS 5: 2010 (Basic Wind Speed Map ASCE 7-95)

AASHTO LTS 4: 2009 ((Basic Wind Speed Map ASCE 7-95)

INTRODUCCIÓN AL CÁLCULO DE CARGAS DE VIENTO PARA EL DISEÑO DE UTILIDADES EN CARRETERA Dr. Aponte 8

ESCALA DE VIENTOS HURACANADOS SAFFIR-SIMPSON

http://www.nhc.noaa.gov/aboutsshws.php

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Escala de Vientos Huracanados Saffir-SimpsonDesarrollada a principios de la década del 70 por:

◦ Herbert SaffirIngeniero Civil Consultor

◦ Dr. Robert SimpsonDirector del Centro Nacional de Huracanes

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Huracán – Categoría 1Vientos muy peligrosos de 74 – 95 mph producirán algún daño mínimo a estructuras

Escala de Viento de Huracán Saffir-SimpsonCategoría 1 = 74 – 95 mph

Ráfaga de Viento de 3 seg. sobre tierra abierta81 – 105 mph

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Huracán – Categoría 2Vientos extremadamente peligrosos de 96 –110 mph producirán daños extensivos



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Huracán – Categoría 3Vientos devastadores de 111 – 129 mph definitivamente producirán devastadores



Huracán de diseño estructuras construidas antes del 2011 V = 125 mph (ASCE 7-95)

Velocidad de diseño estipulada en la especificación de AASHTO LTS 5: 2010

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Huracán – Categoría 4Vientos extremadamente peligrosos de 130 –156 mph producirán daños catastróficos

Escala de Viento de Huracán Saffir-SimpsonCategoría 4 = 130 - 156 mph


Huracán de diseño estructuras construidas desde 2011 V = 145 mph (ASCE 7-05)

Velocidad de diseño estipulada en la guía AASHTO LTS 6: 2013

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Huracán – Categoría 5Vientos extremadamente peligrosos de que exceden 157 mph producirán daños catastróficos

Escala de Viento de Huracán Saffir-SimpsonCategoría 5 = > 157 mph

Ráfaga de Viento de 3 seg. sobre tierra abierta> 171 mph

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Resumen de Huracanes Históricos en Puerto Rico según Escala de Vientos Huracanados Saffir-Simpson1

16

189

Categoría

Vientos

Sostenidos

(mph)

Rafaga de 3

segundos

Sobre Agua

Rafaga de 3

segundos

Sobre Tierra

Daño por Viento Ejemplo Histórico en PR

1 74-95 90-116 81-105Vientos muy peligrosos que

ocasionaran algún daño

San Nicolás (1931), Santa Clara (1956),

Hortensia (1996), Irene (2011)

2 96-110 117-134 106-121

Vientos extremadamente

peligrosos que ocasionaran

daños extensivos

San Ciprían (1932), Hugo (1989) PR,

Georges (1998)

3 111-129 135-158 122-143Daños devastadores van a

ocurrirHugo (1989) Vieques y Culebra

4 130-156 159-189 144-171Daños catastróficos van a

ocurrirSan Ciriaco (1899)

5 Mayor de 157 Mayor de Mayor de 171Daños catastróficos van a

ocurrirSan Felipe (1928)

(1) Modificado para la temporada de Huracanes del 2012

Ref.: U.S. Department of Commerce, National Oceanic and Atmospheric Administration, National Hurricane Center.

Deadliest & Costliest Tropical Cyclones (1900-2010) for Puerto Rico and the U.S. Virgin Islands.


Island or UnadjustedName Date CPA Damage ($000) DeathsSan Hipolito Aug 22,1916 Puerto Rico 1,000 36,000 1San Liborio Jul 23,1926 1 SW Puerto Rico 5,000 103,353 25San Felipe Sep 13,1928 Puerto Rico 85,000 1,757,006 312San Nicolas Sep 10,1931 1 Puerto Rico 200 4,386 2San Ciprian Sep 26,1932 1 USVI, PR 30,000 657,893 225San Mateo Sep 21,1949 St. Croix Unk - UnkSanta Clara (Betsy) Aug 12,1956 Puerto Rico 40,000 336,855 16Donna Sep 05,1960 1 PR & St. Thomas Unk - 107Eloise (T.S.) Sep 15,1975 1 Puerto Rico Unk - 44David Aug 30,1979 2 S. of Puerto Rico Unk - UnkFrederic (T.S.) Sep 04,1979 2 Puerto Rico 125,000 357,143 7Hugo Sep 18,1989 USVI, PR 1,000,000 1,824,953 5Marilyn Sep 16,1995 USVI, E. PR 1,500,000 2,254,601 8Hortense Sep 10,1996 SW Puerto Rico 500,000 737,952 18Georges Sep 21,1998 USVI & PR 1,800,000 2,512,821 0

Lenny Nov 17,1999 USVI & PR 330,000 441,201 01 Effects continued into the following day. 2 Damage and Casualties from David and Frederic are combined.

Ref. The Deadliest, Costliest, and Most Intense United States Tropical Cyclones from 1851 to 2010 (and Other Frequently

Requested Hurricane Facts) NOAA Technical Memorandum NWS NHC-6

Adjusted for Inflation 3

3 Adjusted to 2010 dollars based on U.S. Census Bureau Price Deflator (Fisher) Index for Construction

18

Ajuste de Velocidad de Viento de:Escala Saffir-Simpson Velocidad de Diseño ASCE 7

Slide courtesy of Dr. Forrest Masters, University of Florida Modified by Dr. Luis D. Aponte: University of Puerto Rico at Mayaguez

La escala de vientos huracanados Saffir-Simpson: Define vientossostenidos de 1 minuto medido a 10m de altura en unaexposición abierta al mar.

La velocidad de diseño según el ASCE 7 se define como unaráfaga de viento de 3 segundos medido a 10m de altura en unaexposición de terreno abierto

Huracanes Históricos en Puerto Rico

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Cargas de Viento ASCE 7-95 (LTS 5) vs. ASCE 7-05 (LTS 6)ASCE 7-95 (V = 125 MPH)

Estructuras diseñadas según el UBC 97

Periodo de vigencia del 1999 – 2011

Las presiones son bastante similares a las establecidas por el Reglamento Nº7 de la Junta de Planificación

AASHTO – LTS 5 – Usa la velocidad de diseño según el ASCE 7-95

ASCE 7-05 (V = 145 MPH)

Estructuras diseñadas según el IBC 2009

Periodo de vigencia del 2011 – Presente

La cargas de viento vigentes representan un aumento de ~35% en las presiones de diseño

AASHTO – LTS6 – Adopta la velocidad de diseño segn el ASCE 7-05

El ASCE 7-05 ofrece nuevos coeficientes de presión para letreros, pero estos no están en la guías de la especificación AASHTO LTS 6

20

1987 - 1998Reglamento Nº7

V = 125 mph

1998 al 2011UBC 97 – ASCE 7-95

V = 125 mph

2011 al PresenteIBC 2009 – ASCE 7-05V = 145 mph (~35%↑)

Velocidad de Diseño por LocalizaciónEl propósito de la “WINDSPEED BY LOCATION" es proporcionar a los usuarios una velocidad del viento de sitio específico que se utilizan en la determinación de las cargas de viento de diseño para edificios y otras estructuras.


http://windspeed.atcouncil.org/

http://windspeed.atcouncil.org/

Vulnerabilidad a Huracanes en PREl código vigente de edificación para cargas de vientos huracanados (ASCE 7-05), representa un aumento de 35% en las presiones de diseño

Dicho aumento representa un riesgo a todas las edificaciones construidas previo al 2011, si estas fueron diseñadas para una velocidad de vientos de 125 mph (huracán cat. 3)

El código vigente del ASCE 7-05 en Puerto Rico provee coeficientes de fuerza para letreros los cuales generan cargas de viento conservadoras en comparación con los coeficientes provisto por la especificación de AASHTO LTS-6, los valores provistos en este guía son similares a los del ASCE 7-95

23

1452 1252-

125235%=

Vulnerabilidad a Huracanes en PREl actual huracán de diseño (cat. 4), presentan un riesgo inminente a la infraestructura del país principalmente:Tendido eléctrico aéreo (i.e., postes, torres de transmisión, torres de

telecomunicaciones, etc.)

Elementos y Componentes (i.e., puertas, ventanas, cristales en edificios, etc.)

Viviendas residenciales de madera

Zonas costera susceptibles a inundación por marejadas ciclónicas (i.e., puertos, hoteles, etc.)

Zonas susceptibles a inundación por desbordamiento de ríos y quebradas

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Vulnerabilidad a Huracanes en PRCuantificación de daños:En los últimos 25 años hemos tenido el embate directo de dos huracanes

categoría 2, Hugo (1989) y Georges (1998)

Las perdidas generados por estos fenómenos ajustadas al 2010 representan $1.8 billones (Hugo) y $2.5 billones (Georges)

Si definimos la perdida total como daños generado por viento, marejada ciclónica e inundación de agua dulce. Entonces es conservador decir que dicha perdida sobrepasará los $10.0 billones para el huracán de diseño categoría 4

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Vulnerabilidad a Huracanes en PREn comunicación con el (Octubre-2014)

Dr. Peter DatinModelador de CatástrofesRisk Managment Solutions (RMS)


Se estiman las perdidas generadas a la infraestructura del país solo por viento

1. Hugo (1989) Cat. 2 ~ $4.00 billones

2. Georges (1998) Cat. 2 ~ $4.25 billones

3. San Ciriaco (1899) Cat. 4 ~ $10.0 billones

4. San Felipe (1928) Cat. 5 ~ $25.0 billones

Huracán Georges (1998) Categoría 2RESIDENCIAS NO REGULADAS (VILLA DEL SOL, TOA BAJA, PR)

DATOS RELEVANTES

El huracán Georges hasta la fecha representa el evento ciclónico más costoso en la Historia de Puerto Rico

27

Ref. FEMA 339 - Building Performance Assessment Team (BPAT) Report - Hurricane Georges in Puerto Rico (Hardcopy)http://www.fema.gov/media-library-data/20130726-1442-20490-0152/fema_339_2.pdf

http://www.fema.gov/media-library-data/20130726-1442-20490-0152/fema_339_2.pdf

Huracán Georges (1998) Categoría 2COMUNIDAD A LAS AFUERAS DE LOIZA

PERDIDA DE TECHO EN LA MAYORÍA DE LAS ESTRUCTURAS DE MADERA

28

Huracán Georges (1998) Categoría 2EROSIÓN Y PERDIDA DE CAPACIDAD DE SUSTENTACIÓN DEL SUELO

EROSIÓN CAUSADO POR CRECIDA DE RÍO EN JAYUYA (EDIFICIO: IZQUIERDA ES UNA ESCUELA Y DERECHA CASA)

29

Huracán Georges (1998) Categoría 2EROSIÓN COSTERA POR MAREJADA CICLÓNICA

COLAPSO DE PUENTES DEBIDO A CRECIDA DE RÍO (ADJUNTAS, PR)

30

Huracán Georges (1998) Categoría 2DESLIZAMIENTOS

DAÑOS OCASIONADOS POR PERDIDA DE VENTANAS EN EDIFICO COMERCIALES

31

Huracán Georges (1998) Categoría 2CASA DE HORMIGÓN SIN DAÑO ESTRUCTURAL (ADJUNTAS, PR)

CASA DE MADERA CON DAÑO ESTRUCTURAL MAYOR

32

IMPORTANCIA DE CARGAS DE VIENTO EN LA INFRAESTRUCTURA PARA CARRETERAS

33INTRODUCCIÓN AL CÁLCULO DE CARGAS DE VIENTO PARA EL DISEÑO DE


Guidelines for the Installation, Inspection, Maintenance and Repair of Structural Supports for Highway Signs, Luminaries, and Traffic SignalsAmerican Association of State Highway and Transportation Officials, & Subcommittee on Bridges and Structures. (2013). Standard specifications for structural supports for highway signs, luminaires, and traffic signals. Washington, D.C.: American Association of State Highway and Transportation Officials.

Web link: https://www.fhwa.dot.gov/bridge/signinspection.cfm

“The primary reason for compiling this guidance is increasing problems with wind induced vibration, fatigue, and even structural collapse of these support systems…”

Additional Reference:

Dexter, R. J., & Ricker, M. J. (2002). Fatigue-resistant design of cantilevered signal, sign, and light supports. Washington, D.C: National Academy Press.

34

https://www.fhwa.dot.gov/bridge/signinspection.cfm

Efectos de Cargas “Load Effects”For most structures, design will be governed by wind loads.

The primary loads applied to sign, signal, and luminaire structures are due to natural winds. The structure must not only have sufficient strength to withstand the maximum expected wind loads, normally a once in 50 year maximum, but also the fatigue effects of fluctuating winds of lower force. There are four wind-loading phenomena that can lead to vibration and fatigue:Natural wind gusts

Truck-induced gusts

Vortex shedding

Galloping

Each type of structure (e.g., signal, sign, or luminaries) is susceptible to either two or three of the four wind loads, as is shown in Table 2. The interaction of the support structure with the wind is dependent on the structure's stiffness and shape. For instance, galloping which can occur in cantilever signal structures, does not occur in bridge support configurations.


Efectos de Cargas “Load Effects”


Note: X indicates structure is susceptible to this type of loading* Vortex shedding has occurred in monotube bridge supports and can occur in cantilevered structures if the sign or signal is not attached, such as could occur during installation.

Efectos de Cargas “Load Effects”The high flexibility and low damping of cantilevered support structures makes them susceptible to resonant vibration in the wind. The flexible cantilevered structures have low natural frequencies of about 1 Hz (period of vibration of 1 second), which is in the range of typical wind gust frequencies. The closeness of the wind gust frequencies to the natural frequency causes resonance or dynamic amplification of the response. In addition, typical damping ratios in these structures are extremely low (less than one percent of critical damping). The low damping increases the amplification of the wind-induced vibrations.

Usually, the greater the length of the cantilevered mast arm, the more susceptible the support structure will be to wind-induced vibration. In recent years, the span length of the cantilevered mast arms has increased significantly. One example is a 22-m (72-ft) long cantilevered signal support structure near the University of Tampa, which vibrates frequently due to its extreme flexibility.


Hurricane Winds:Hurricane Sandy damage to highway signs


Harriman-based engineers assess damage | Monroe Woodbury NY | Local News. (n.d.). Retrieved October 2, 2015, from http://www.thephoto-news.com/apps/pbcs.dll/article?AID=/20121106/NEWS01/121109968/Harriman-based-engineers-assess-damage

http://www.thephoto-news.com/apps/pbcs.dll/article?AID=/20121106/NEWS01/121109968/Harriman-based-engineers-assess-damage

Natural Wind GustsThe most common type of wind-induced vibration in support structures is from ordinary wind gusts or fluctuations of the wind velocity.

Natural wind gusts exert a fluctuating force that is primarily horizontal, and the resulting motion of the mast arm is also primarily horizontal, although there is often a significant vertical motion as well.

In the 2001 Specifications, the natural wind gust pressure is applied horizontally to the projected frontal area of all surfaces, including the structural members, as well as the sign and signal attachments.


Natural Wind GustsFatigue cracking caused by natural wind gusts, cracking developed over a period of at least several years. In light poles, the connection of the arm supporting the lights to the pole top has cracked.

In cantilevered sign and signal support structures, the cracks will usually manifest at the connection of the mast arm to the pole, with cracks forming along the sides of the connection.

Natural wind gust loading has also caused cracks at truss connections and at the base of the pole, at the weld joining the pole to the base plate, at the top of the stiffeners, at hand holes, or at the anchor rods.

Support structures in certain areas of the country that are very windy are particularly susceptible to vibration and fatigue from natural wind gusts. Typically, these are places where the mean annual wind velocity is greater than 5 m/s (10 mph).


Natural Wind GustsPlaces where there are frequent constant winds, support structures are at risk for fatiguecracking due to natural wind gust loading, are not the same places with the maximum peak windvelocities that are identified on wind maps, such as those in ASCE 7 Design Wind Speed Maps.These maps show the maximum wind speed that occurs over a 50-year period that is used forstrength design. It is noteworthy that there are very few fatigue failures of cantilevered sign andsignal supports after hurricanes or other large storms. These once-in-a-lifetime wind gusts havelittle influence on fatigue.

The fatigue-limit-state natural wind gust loads in the 2001 Specifications are derived for windsthat are exceeded about 2 hours each year. Such areas, surprisingly, are not necessarily thosewith occasional very high winds such as hurricanes or tornadoes. For example, the four places inthe United States with the greatest wind velocities that are exceeded at least 2 hours per yearare:Tatoosh Island, Washington: 28 m/s (62 mph)Clayton, New Mexico: 24 m/s (53 mph)Point Judith, Rhode Island: 24 m/s (53 mph)Cheyenne, Wyoming: 24 m/s (53 mph)


Cantilever Sign Structure Failure


Guidelines for the Installation, Inspection, Maintenance and Repair of Structural Supports for Highway Signs, Luminaries, and Traffic Signals - Structures - Bridges & Structures - Federal Highway Administration. (n.d.). Retrieved October 2, 2015, from https://www.fhwa.dot.gov/bridge/signinspection.cfm

https://www.fhwa.dot.gov/bridge/signinspection.cfm

Truck-Induced Wind Gusts

The passage of trucks beneath support structuresinduces both horizontal and vertical gust loads on thestructure, creating a motion that is primarily vertical butmay also include a significant horizontal component aswell. The magnitude of the horizontal truck gust isnegligible relative to the natural wind gust and istherefore ignored in the specifications. The vertical truckgust load in the specifications is an equivalent staticpressure range applied to the underside of the mast armor truss and any attachments. Due to the large depth ofvariable-message signs (VMS) in the direction of trafficflow (up to 1.2 m for older signs), the support structuresare the most susceptible to truck-induced wind gustfatigue because they present a large area in thehorizontal plane, as seen in the Figure


Truck Passing Under variable-message signs VMS

Vortex SheddingVortex shedding is the shedding of vortices on alternate sides of asymmetric member (i.e., one without any attachments). Vortexshedding can result in resonant oscillations of a pole in a planenormal to the direction of wind flow as shown in Figure. In the 2001Specifications, the vortex shedding pressure range for fatigue designis applied horizontally to the projected area of one side of the poleto calculate the fatigue stress ranges in the details.

Only luminaries support structures (i.e., light poles) have exhibitedproblems from vortex shedding. However, other simple cantileverpoles, such as camera poles may also be susceptible. Luminairesupport structures may occasionally exhibit excessive vibration fromnatural wind gusts. However, if there is a vibration problem withluminaire supports, it is typically due to vortex shedding.


Vortex Shedding - VideosVideo References:

1. AAWE :: The American Association for Wind Engineering - Video Gallery. (n.d.). Retrieved October 2, 2015, from http://www.aawe.org/gallery/video.php?i=5

2. Horrifying video shows motorway lampposts shaking violently in storm. (n.d.). Retrieved October 2, 2015, from http://www.dailymail.co.uk/news/article-2931551/Horrifying-video-shows-motorway-lampposts-shaking-violently-snowstorm-phenomenon-experts-call-vortex-shedding.html

3. vortex shedding - YouTube. (n.d.). Retrieved October 2, 2015, from https://www.youtube.com/watch?v=YbZE_dgAqkc


http://www.aawe.org/gallery/video.php?i=5

http://www.dailymail.co.uk/news/article-2931551/Horrifying-video-shows-motorway-lampposts-shaking-violently-snowstorm-phenomenon-experts-call-vortex-shedding.html

https://www.youtube.com/watch?v=YbZE_dgAqkc

Vortex Shedding – Video Ref. 2


GallopingGalloping is different than vortex shedding but also results in large-amplitude, resonant oscillations perpendicular to the direction of wind flow as shown in Figure 4(B). Unlike vortex shedding, galloping occurs on asymmetric members (i.e., those with signs, signals, or other attachments) rather than circular members. Therefore, it is the mast arms rather than the poles that are susceptible to galloping. Galloping has caused mast arms to move up and down with a range greater than 1 m. According to NCHRP Report 469, a large portion of the vibration and fatigue problems that have been investigated for cantilevered sign and signal support structures were caused by gallopin


Galloping- VideosVideo References:

1. vortex shedding - YouTube. (n.d.). Retrieved October 2, 2015, from https://www.youtube.com/watch?v=YbZE_dgAqkc

2. Tacoma Narrows Bridge


https://www.youtube.com/watch?v=YbZE_dgAqkc

Galloping - Video


TERMINOLOGÍA Y DEFINICIONES DE LAS CARGAS DE VIENTO SEGÚN: ASCE 7-05



ASCE 7•Engineers seeking guidelines to design modern low-rise buildings resistant to wind loads usually turn to the American Society of Civil Engineers Minimum Design Loads for Buildings and Other Structures (ASCE 7) for guidance

•ASCE 7 is referenced by most major building codes, including the◦ International Building Code

◦ AASHTO - LTS

ASCE 7-05 Wind Load Provisions•Structural Systems

◦ Main Wind Force Resisting System (MWFRS)◦ Components and Cladding (C&C)

•Velocity Pressure◦ Exposure◦ Topography◦ Directionality◦ Basic Wind Speed◦ Importance

•ASCE 7 Terminology

•Design◦ Method 1: Simplified Procedure◦ Method 2: Analytical Procedure◦ Method 3: Wind Tunnel Procedure

INTRODUCCIÓN AL CÁLCULO DE CARGAS DE VIENTO PARA EL DISEÑO DE UTILIDADES EN CARRETERA 52

Methods• Three Methods

6.4 Method 1—Simplified Procedure

6.5 Method 2—Analytical Procedure

6.6 Method 3—Wind Tunnel Procedure

• Method 1 is a “simplified” version of Method 2, so we begin here

Method 2: Analytical Method•Generally C&C loads are higher because of localized high pressures

•MWFRS receives pressures from multiple surfaces. Correlation and spatial averaging reduces loads

Method 2: Velocity Pressure

•The starting point is to calculate the velocity pressure to evaluate pressures on the MWFRS and the C&C

•Form of the dynamic pressure = ½rV2

•See Section 6.5.10

(psf)00256.0 2IVKKKq dztzz

=

Method 2: Velocity Pressure

•Numerical coefficient collapses constants


=

0.00256 = [ 0.5 ] [ 0.002367 slug / ft3 ] [ 2.5111 (ft/s)/mph ]2

Method 1&2: Terrain “Exposure” Coefficient

•Defined in 6.5.6, provided in Table 6-3

•ASCE provides three terrain exposures

B Urban and suburban areas

C Open terrain with scattered obstructions

D Flat, unobstructed areas outside hurricaneprone regions


=

Methods 1&2: Exposure B•Urban and suburban areas, wooded areas, or other terrain with numerously closely spaced obstructions having the size of a single-family dwellings or larger

•Prevails in the upwind direction for a distance of at least 2600 ft or 20H

•For a building with a mean roof height ≤ 30 ft, this distance may be reduced to 1500 ft

Methods 1&2: Exposure B

From the Commentary

Methods 1&2: Exposure BFrom the Commentary

Methods 1&2: Exposure C•Open Exposure

•Open terrain with scattered obstructions having heights generally less than 30 ft

•Includes flat open country, grasslands and all water surfaces in hurricane prone-regions

•Applies where B and D do not

Methods 1&2: Exposure CFrom the Commentary

Methods 1&2: Exposure D•Flat, unobstructed areas and water surfaces outside hurricane-prone regions

•Smooth mud flats, salt flats, unbroken ice

•Prevails in the upwind direction for a distance of at least 5000 ft or 20H

Methods 1&2: Exposure D

From the Commentary

0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.90

30

50

100

150

200

250

300

Exposure Coefficient Kz

Hei

ght (

ft)

CASE 2

CASE 1Low-Rise

ASCE 7 Table 6–3

Exposure coefficient Kz


Table 6-2 Values to evaluate Kz


Method 2: Wind Direction and Sectors•Determine ground surface roughness within each 45º sector

•Use the exposure that gives the higher load◦ NE Load = larger of Cases 1 and 2

◦ N Load = larger of Cases 8 and 1

•For each wind direction considered, wind loads for the design of the MWFRS shall be based on the exposure categories

•For low-rise buildings, use the worst-case scenario

•C&C pressure loads taken from the worst case roughness

EAST

NORTH

1

2

3

45

6

7

8

Methods 1&2: Range of Exposure Categories

z0 = 0.001 0.01 0.1 1.0

EXPOSURE Dz0 ≤ 0.01

z0 = 0.005 m

EXPOSURE C0.01 ≤ z0 ≤ 0.15

z0 = 0.02 m

EXPOSURE B0.15 ≤ z0 ≤ 0.7

z0 = 0.3 m

Method 2: Topographic Factor

•Defined in 6.5.7 and provided in Figure 6-4

•Buildings situated on the upper half of a hill or escarpment may experience significantly higher wind speeds than buildings situated level on the ground


=

HILL ESCARPMENT

FASTER FASTER


•Defined in 6.5.7 and provided in Figure 6-4

•Buildings situated on the upper half of a hill or escarpment may experience significantly higher wind speeds than buildings situated level on the ground

•Topographic effect of escarpment only applies when the upwind terrain is free of topographic features for a distance of 100·H or 2 miles, whichever is smaller


=


•Formula

Kzt = ( 1 + K1 K2 K3 )2

•Three considerations are made

K1 Relates to the shape of the topographic feature and the

maximum speedup near the crest

K2 Accounts for the reduction in speed-up with distance

upwind or downwind of the crest

K3 Accounts for the reduction in speed-up with heightabove the local ground surface


=


K1 Relates to the shape of the topographic feature and the

maximum speedup near the crest

Increases as H / Lh Increases


=

Lh H Lh H

Lh is the distance upwind of the crest where the difference in ground elevation is half of the height of the hill or escarpment


K2 Accounts for the reduction in speed-up with distance

upwind or downwind of the crest

Decreases as x/Lh Increases


=

x xUPWIND DOWNWIND

x xUPWIND DOWNWIND

Lh Lh



K3 Accounts for the reduction in speed-up with heightabove the local ground surface

Decreases as z/Lh Increases


=

z

Lh Lh

z



•Defined in 6.5.7 and provided in Table 6-3

•Accounts for wind speed-up over hills and escarpments

•Topographic effect of escarpment only applies when the upwind terrain is free of topographic features for a distance of 100H or 2 miles, whichever is smaller


=

Method 2: Directionality Factor

•Accounts for the reduced probability of◦ Maximum winds coming from any direction

◦ Maximum pressure coefficient occurring from any wind direction

•In ASCE 7-95, Kd was incorporated into the Combined Factored Loads using Strength Design


=

ASCE 7-95 1.2D + 1.3W + L + 0.5(Lr or S or R)1.6 X 0.85, Kd did not exist

ASCE 7-98+ 1.2D + 1.6W + L + 0.5(Lr or S or R)1.6 X 1.00, Kd = 0.85

Method 2: Directionality Factor

•As new research becomes available, Kd can be directly modified without altering the load factor

0.85 for C&C and MWFRS for most buildings

0.90-0.95 for towers, chimneys, tanks and similar structures


=

Methods 1&2: Basic Wind Speed

•3-second gust at 33 ft over an open exposure

•50-year mean recurrence interval

•Assumes that a ~500-year speed might reasonably represent an approximate ultimate limit state event

•Tornado winds are not included

•Figure 6-1 provides a series of wind maps with 10 mph isotachs(lines that connect points of constant velocity)

•Linear interpolation is allowed


=

BASIC WIND SPEED (ASCE 6.5.4)•The basic wind speed V is defined as the 3-s peak gust at 33 ft (10 m) above ground in open terrain with sufficient long fetch in all directions (i.e., terrain with exposure C), with mean recurrence intervals (MRIs) defined as follows:

•Outside hurricane-prone regions, V is defined as the speed with nominal 50-year (MRI)

•In hurricane-prone regions, V is no longer defined as the speed with an MRI of 50-year. Rather, it is defined as the hurricane wind speed with an MRI of 500 years, divided by √(1.5) (ASCE 7-05 Commentary C6.5.4)

BASIC WIND SPEED (ASCE 6.5.4)


•The basic wind speed is 145 mph in Puerto Rico


=


•How do we apply a different mean-recurrence interval? See Table C6-7


=

MRI Conversion for ≥ 100 mph in Hurricane-Prone Regions500 1.23200 1.14100 1.0750 1.0025 0.8810 0.74 (minimum of 76 mph)5 0.66 (minimum of 40 mph)

Methods 1&2: Importance Factor

•Adjusts the level of structural reliability to be consistent with the ASCE 7 Building Classifications listed in Table 1-1


=

Occupancy Category

Nature of Occupancy Importance Factor

I Agrarian, temporary and storage facilities 0.77II All buildings that do not fall in Categories I, III and IV 1.00III Risk of substantial risk of life or major socioeconomic

impact. Schools, power-generating stations1.15

IV Essential facilities, such as hospitals, police departments, aviation control towers, emergency operation centers

1.15

Method 2: Gust Effect Factor (G)•Accounts for loading effects in the along-wind direction

•Not for across-wind, vortex shedding, flutter, galloping and dynamic torsional effects

•Rigid Buildings◦ Accounts for wind turbulence-structure interaction

◦ 0.85 permitted without calculation

◦ Or designer can calculate from a “suggested” procedure

•Flexible Buildings◦ Accounts for dynamic amplification

Method 2: Flexible Gust Effect Factor (Gf)

•Section 6.5.8.2 provides a more elaborate methodology to calculate the gust effect factor for flexible or dynamically sensitive structures

•Note addition of subscript “f”

•Rational analysis may also be used for either gust factor, but not to replace any (GCp) given in the figures and tables

Method 2: Analytical Method

6.5.12Design Wind Loads on Enclosed and Partially

Enclosed Buildings

6.5.12.2Main Wind Force

Resisting Systems

6.5.12.2.1Rigid Buildingsof All Heights

Figs 6-6, 6-7, 6-8

6.5.12.2.2Low-RiseBuildingsFig 6-10

6.5.12.2.3Flexible

BuildingsFigs 6-6, 6-8

6.5.12.3Design WindLoad Cases

Fig 6-9

6.5.12.4Components and

Cladding

6.5.12.4.1Low-Rise Buildings and

Buildingswith h ≤ 60 ft

Figs 6-11 thru 6-16

6.5.12.4.2Buildings with

h > 60 ftFig 6-17

6.5.12.4.3Alternative for Buildings with60 ft < h < 90 ft

Figs 6-11 thru 6-17

6.5.13Design Wind Loads on Open Buildings with

Monoslope, Pitched, or Troughed Roofs

6.5.14Design Wind Loads

on Solid Freestanding Walls and Solid Signs

6.5.15Design Wind

Loads on Other

Structures

ASCE 7-05 Sec. 6.5.14 Design Wind Loads on Solid Freestanding Walls and Solid SignsLos coeficientes de fuerza presentados en esta sección son mucho más conservadores que los valores provistos en las versiones previas del ASCE 7, y los valores especificados en AASHTO LTS 6 y versiones previas para letreros solidos


Coeficientes de presión y fuerzaLos coeficientes de presión externa o , y el coeficiente de fuerza , según aplique por ejemplo: el coeficiente se utiliza para cálculo de presiones en la estructura, el coeficiente se utiliza para cálculo de presiones en los C&C y el coeficiente se utiliza para cálculos de fuerza generada por el vientos para otras estructuras, estos coeficientes de se determinan según las secciones 6.5.11.2 y 6.5.11.3, respectivamente.

La sección 6.5.11.2 se utiliza para los coeficientes de presión externa

En la sección 6.5.11.2.1 se proveen los para el MWFRS en las figuras 6-6 a la 6-8, el efecto combinado del factor de ráfaga y el coeficiente de presión externa , se presentan en la figura 6-10

En la sección 6.5.11.2.2 se proveen los para los C&C, el efecto combinado de factor de ráfaga y el coeficiente de presión externa , se presentan en las figura 6-11 a la figura 6-17.

La sección 6.5.11.3 se utiliza para determinar los coeficientes de fuerza

En las figuras de la 6-20 a la 6-23


Figure 6-20


Figure 6-22


Figure 6-23


TERMINOLOGÍA Y DEFINICIONES DE LAS CARGAS DE VIENTO SEGÚN: AASHTO LTS 6 : 2013



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