Sismos Periodo Largo contra periodo Corto.
El periodo corto (1 seg) afecta a edificios pequeños.
El periodo largo (2 seg) afecta a los altos.
Frecuencia de resonancia 4 Hz afecta los más altos.
Conforme aumenta la frecuencia los más altos son menos afectados, pero los medianos entran en resonancia, por ejemplo 6.35 Hz
Las estructuras más bajas peligran a más altas frecuencias 11.35 Hz
3 minutos, doce veces más que el terremoto de Kove, en ciudad de México el periodo largo duro dos segundos.
1.5 Coeficiente sísmico, NTCDS CDMX.
El coeficiente sísmico, c, es el cociente de la fuerza cortante horizontal que debe considerarse que actúa en la base de la edificación por efecto del sismo, Vo, entre el peso de la edificación sobre dicho nivel, Wo.
Con este fin se tomará como base de la estructura el nivel a partir del cual sus desplazamientos con respecto al terreno circundante comienzan a ser significativos. Para calcular el peso total se tendrán en cuenta las cargas muertas y vivas que correspondan, según las Normas Técnicas Complementarias sobre Criterios y Acciones para el Diseño Estructural de las Edificaciones.
El coeficiente sísmico para las edificaciones clasificadas como del grupo B en el artículo 139 del Reglamento se tomará igual a 0.16 en la zona I, 0.32 en la II, 0.40 en las zonas IIIa y IIIc, 0.45 en la IIIb y 0.30 en la IIId (ver tabla 3.1), a menos que se emplee el método simplificado de análisis, en cuyo caso se aplicarán los coeficientes que fija el Capítulo 7 (tabla 7.1). Para las estructuras del grupo A se incrementará el coeficiente sísmico en 50 por ciento.
Tabla 3.1 Valores de los parámetros para calcular los espectros de aceleraciones
Zona c ao Ta 1 Tb 1 r
I 0.16 0.04 0.2 1.35 1.0
II 0.32 0.08 0.2 1.35 1.33
IIIa 0.40 0.10 0.53 1.8 2.0
IIIb 0.45 0.11 0.85 3.0 2.0
IIIc 0.40 0.10 1.25 4.2 2.0
IIId 0.30 0.10 0.85 4.2 2.0
1 Periodos en segundos
Seismic Analysis Parameters (US Code UBC97, Obsoleto)
Parameters of a structure seismic analysis depend on a seismic code used during calculations of a structure influenced by seismic loads. Select the US seismic code UBC 91 ("The Uniform Building Code 1991") in the New Case Definition dialog or click Parameters in the Analysis Type dialog.
To complete the seismic analysis according to the rules given in a code, define the following parameters:
· Seismic Zone:
Zone = 0, seismic zone coefficient Z = 0
Zone = 1, seismic zone coefficient Z = 0.075
Zone = 2A, seismic zone coefficient Z = 0.15
Zone = 2B, seismic zone coefficient Z = 0.20
Zone = 3, seismic zone coefficient Z = 0.30
Zone = 4, seismic zone coefficient Z = 0.40
· Soil:
Sa - Hard rock
Sb - Rock
Sc - Very dense soil or soft rock
Sd - Stiff soil
Se - Soil
Sf - UBC 1629.3.1
If the site geotechnical evaluation has not been performed, the soil SD should be used.
· Seismic source type.
· R - Response modification factor: values from the interval: from 2.8 (light steel braced frames) to 8.5 (special steel or reinforced concrete structures and dual systems).
· Distance from the closest seismic source.
Seismic acceleration is defined on the basis of Figure 16-3 (design response spectrum) in UBC97 code. In the lower part of the dialog, the Direction definition and Filters buttons are located. There is the additional button Base Shear; click it to open the Base shear dialog.
The details concerning this structure analysis method can be found in the mentioned code. Parent topic: Seismic Analysis Parameters (Dynamics)
SEISMIC AMPLIFICATION FACTOR.
Base Shear Terms
In this section, the various terms of the static base shear equation are examined in more detail.
Z : seismic zone factor.
· Effective peak ground accelerations with 10% probability of being exceeded in 50 yrs.
· Given as a percentage of acceleration due to gravity.
· For example, consider zone 4, where Z = 0.4g horizontal ground acceleration is predicted at 0.4g at bedrock.
· Doesn’t account for building dynamic properties or local soil conditions.
· ’97 UBC Figure 16.2? seismic zone map.
· Table 16.1? Z values as given below:
Zone Z
0 0 1 0.075 2A 0.15 2B 0.20 3 0.30 4 0.40
I : importance factor.
· Classifying buildings according to use and importance.
· Essential facilities, hazardous facilities, special occupancy structures, standard occupancy structures, miscellaneous structures.
· Essential facilities mean that the building must remain functioning in a catastrophe.
· Essential facilities include: hospitals, communication centers, fire and police stations.
· Design for greater safety.
· ’97 UBC Table 16-K.
· I = 1.25 for essential and hazardous facilities.
· I = 1.0 all others.
T : building’s fundamental period of vibration.
Fundamental period of vibration is the length of time, in seconds, it takes a structure to move through one complete cycle of free vibration in the first mode.
There are two methods to estimate T:
· Method A:
· Method B: (an iterative approach not generally used in regular structures)
Using Method A, the fundamental period of vibrations for masonry buildings is estimated at:
Height (ft) Period (seconds)
20 0.19 40 0.32 60 0.43 120 0.73 160 0.90
· Ca and Cv : seismic dynamic response spectrum values.
· Accounts for how the building and soil can amplify the basic ground acceleration or velocity.
· Ca and Cv are determined from respectively ’97 UBC tables 16-Q and 16-R as a function of Z, underlying soil conditions, and proximity to a fault.
· Using method A,
· Soil profile type:
· The soil layers beneath a structure effects the way that structure responds to the earthquake motion.
When the period of vibration of the building is close to the period of vibration of the underlying soil, the bedrock motion is amplified. The building experiences larger motions than that predicted by Z alone. The following are generalizations about building response as a function of building flexibility and underlying soil stiffness.
· Building Description Soil Description Induced Seismic Force
· Flexible (Large T’s) Soft (big S) Higher
· Flexible Stiff Lower
· Stiff Soft Higher
· Flexible Stiff Lower
· The soil profile types are:
· Description Type
· Hard Rock SA
· Rock SB
· Very dense soil and soft rock SC
· Stiff soil SD
· Soft soil SE
· See ’97 UBC 1629.3.1 SF
Specific details about each type can be found in ’97 UBC Table 16-J and ’97 UBC 1629.3.1.
· In the absence of a Geo-technical site investigation, use SD. This is in accordance with ’97 UBC 1629.3
· Do not confuse this requirement with the one stated in ’97 UBC 1630.2.3.2 which applies ONLY when using the simplified design base shear
procedures of ’97 UBC 1630.2.3. This web site is NOT using these simplified procedures, but is using 1630.2.1.
· R = response modification factor.
· A judgement factor that accounts for building ductility, damping, and over-strength.
· Ductility = ability to deform in the inelastic range prior to fracture:
Damping = resistance to motion provided by internal material friction.
· Over-strength : the extra or reserve strength in the structural system. It comes from the practice of designing every member in a group according to the forces in the most critical member of that group.
· Structural systems with larger R = better seismic performance.
· In ’97 UBC Table 16-N, R range from 2.8 (light steel frame bearing walls with tension bracing) to 8.5 (special SMRFS of steel or concrete and some dual systems).
· For bearing wall systems where the wall elements resist both lateral and vertical loads:
· Wood shear panel buildings with 3 or less stories: R = 5.5
· Masonry shear walls: R = 4.5.
· Nv and Na : near source factors that are applicable in only seismic zone 4. They account for the very large ground accelerations that occur near the seismic source (the fault).
· Nv is generally used with Cv for structures located < 9.3 miles (15km) from the fault.
· Nv is found in ’97 UBC Table 16-T
· Na is used with Ca for structures located < 6.2 miles (10 km) from the fault.
· Na is found in ’97 UBC Table 16-S.
· Both Na and Nv are based upon the type of seismic source, A-C. This source type, and location of fault, must be established using approved geotechnical data like a current USGS survey.
Nunca un edificio va a corresponder al 100% con sus planos de proyecto.
Errores o cambios en el proceso constructivo.
Asentamientos.
Reparaciones, modificaciones o ampliaciones.
Por lo tanto. Ni el alineamiento, ni la nivelación serán armónicas, por lo tanto, su comportamiento idóneo estar afectado.
Descripcion. MoE(Gpa) Poisson Ratio. Density.(kg/m3)
Wcb1 0.167 0.213 2.67
1:4 mix 2.65 0.165 0.444
1:6 mix 2 0.117 23.12
1:8 mix 1.167 0.119 0.251
EIGENVALUES:
Cuando hay múltiples eigenvalues, es decir frecuencias de vibración in el caso de análisis modal.
Aplica siempre la verificación de Sturm.
Resultados de Columnas.
Zona de compresión marcada en amarillo.
Zona de tensión marcada en gris.
Rd/Sd
Sd – Longitud del vector de carga.
Rd – Longitud del vector de carga que corresponde al estado limite último de la sección transversal, cuando la carga actúa en una excentricidad dada.
MRd/ MSd
Muestra el campo del facto de seguridad para una combinación seleccionada de la lista.