Showing posts with label LAT LOADS. Show all posts
Showing posts with label LAT LOADS. Show all posts

Wednesday, 19 December 2018

HOW TO START WITH THE WIND ANALYSIS OF A BUILDING USING NSCP

The main goal of analyzing the wind load is to convert it into a concentrated force which will be applied to the frames in order for us to start computation of moments and shears using any of the methods discussed earlier:

  • Portal method
  • Cantilever method
  • Q-factor method
However, if you check the NSCP, the procedure with lead us only until  which in units  . This pressure (distributed load) is still to be converted into a force in order to be applied in the frame, thus   will then be multiplied with an area, 
. The area of which is the length of the wall being analyzed by the tributary height.






Thursday, 1 November 2018

HORIZONTAL DISTRIBUTION OF Vf TO FRAMES

After determining the shear per floor, the next step is to distribute the shear into the frames. This procedure would require the configuration of columns in each frame.

Horizontal distribution is required for the two axes - among the x-frames and the y-frames. With both axes, the same value of shear per floor will be distributed. 

The shear would be distributed accordingly: direct shear + torsional shear.

                                      

  • Direct shear, Vd,  is solved like the vertical distribution of the base shear. The procedure would only account for the stiffness of each column in a frame whereby the percentage distribution of this stiffness is used to distribute the base shear computed previously.
  • Torsional shear, Vt, on the other hand, needs a longer computation. This part of the shear would put the center of gravity and center of rigidity into account. The distance between the Cg and Cr is computed and is used in determining the torsion of the building when subjected to base shear. This distance is known the eccentricity.

Center of gravity - the point where the lateral loads is applied same as how discussion in physics are discussed.
Center of rigidity - the point where the structure will not be able to rotate, if force is applied.
Eccentricity - the distance between the center of gravity and the center of rigidity. This is taken as the moment arm to which moment is observed when a force is applied to the structure.

COMPUTATION OF DIRECT SHEAR, Vd

Going back to the previous discussion on how the base shear is vertically distributed among the floors of a structure, the procedure in distributing direct shear is similar. However, instead of floors, the components used are now the frames. Each frame may differ from the others, thus apart from the idea that the frames are not located equally from the Cg, the stiffness of each frame should be considered. Stiffness has been derived as 

                                                                [ for both ends fixed]
                                  where:
                                     I = moment of inertia
                                     L = length of column

The relative stiffness of each column in a frame is then simplified to k=I taken into account that all lengths of columns in one floor are the same. Thus the stiffness formula would be:

                                                 and          


X-Framesksx%ksxVd
4--0.----
3-- 0.----
2--0.----
1--0.----
Σ1Vf


Y-Framesksy%ksyVd
A--0.----
B-- 0.----
C--0.----
D--0.----
Σ1Vf

where: ksx and ksy = the summation of moment of inertia of columns in one frame

COMPUTATION FOR TORSIONAL SHEAR, Vt

  • For torsional shear, Vt, the center of rigidity is already considered in order to determine the eccentricity (functioning as the moment arm). This eccentricity is used to solve for the moment per floor by multiplying it with the base shear.
       NOTE: As per code the minimum eccentricity of any structure is taken as 5% of the length in the direction analyzed. Thus the standard eccentricity, ex, is computed as:
                   
                                                
  • The moment applied on the floor is then computed as the base shear on the floor, Vf, multiplied by the moment arm, ex.
                                             
  • The distribution of this moment will be as follows:

X-framesksxcc/c1ksx'M'%MApplied MVtx
4----------0.------
3-- --------0.------
2----------0.------
1----------0.------
Σ1M
                                     

Y-framesksycc/c1ksy'M'%MApplied MVty
A----------0.------
B-- --------0.------
C----------0.------
D----------0.------
Σ1M

From the column of ksx (ksy), take the location of the center of rigidity with the formula:

                                            
  • c is determined by computing the distance between the frame to the .
  • In most cases, the reference line is taken as the left-most frame or the bottom frame.  is taken as the distance of  from the reference lines. 
  • Torsional shear, 
Summary of shear for each frame is as follows:


X-FramesVdVtVtot
4------
3------
2------
1------

Y-FramesVdVtVtot
A------
B------
C------
D------

At this point, examine which of the torsional shear shall be disregarded.  This is determined by checking on the frames carrying opposite direction of moment.

RELATED ARTICLES:

Design Base Shear
Vertical Distribution of Base Shear

BASE SHEAR: Example 2 STRUCTURE WITH BASEMENT

PROBLEM: A  public school is to be built in Baguio city. Due to lack of budget, there wasn't any soil test done on site. The building should consist of a basement for carpark. The following weight and height per floor is listed below:



kN
Ground Flr1500
Second Flr to 4th Flr (typical)1800
Roof deck1000





FLOOR HEIGHTmeters
Fndtn to Basement2.50
Basement to Grnd Flr.4.00
Ground Flr to Roof Deck3.50

Determine the base shear.

SOLUTION:




Assuming the structure is a regular structure, the analysis to be used is static procedure with the following equations:

                                                      

1.  I = 1.50. For essential occupancy structures.
2. R = 8.50. For Special Reinforced Concrete Moment Frames.
3. For W: The building not being used as a warehouse, the total seismic load is the summation of the dead load above the Ground Floor.
             W= 3 (1800) + 1000  [For 3 levels of 1800 from 2Flr to 4Flr + Roof deck weight]
                = 6,400 kN
4. For T: The elastic fundamental period of the building uses the formula:
                  
              for reinforced concrete moment-resisting frames
                hn is taken from the base (gradeline) to the top [disregard stories under the gradeline].
          
             

5. For Cv:

  • Baguio city is located at Zone 4.
  • Lack of soil analysis, Sd
                              
              where:
                  Seismic source type = A
                  Distance to known seismic source <=2km [since location is not found on map, and many faults surround the city, it can be assumed that source is very near.]
                  
          Thus:
                 
                


Therefore:








CHECK WITH Vmax AND Vmin:

1.    

                    where:
                           with Zone 4 and Soil Profile Type Sd
                                      for <2km and Seismic source type A
                         

                                     
                                     
Thus:


 

  [Design V is ok]

2. 
      
              [Design V is ok]

3.   
         
             [Design V is ok]

RELATED ARTICLES:

Design Base Shear


Wednesday, 31 October 2018

ELASTIC FUNDAMENTAL PERIOD OF STRUCTURES

Basically, every system has a set of frequencies in which it responds to vibrate when set in motion by some disturbance such as seismic or wind event. This response is based on the mass and stiffness of the system. The shortest form of frequency is more known as the natural frequency and it is just the inverse of the fundamental building period.

In seismic analysis and design, if a structure's frequency is close to the frequency of the earthquake, more energy is introduced in the structure. Shorter fundamental periods attract higher seismic vibrations.

The elastic fundamental period of structures, T, can be computed in two ways.

METHOD A: APPROXIMATE FUNDAMENTAL PERIOD

The most straightforward method for determining the building period involves using empirical formulas based on information from several instrumented buildings.

                                        

where: Ct for steel- moment-resisting frames =0.0853
           Ct for reinforced concrete moment-resisting frames and eccentrically braced frames  = 0.0731
            Ct for all other buildings = 0.0488
            Ct for structures with concrete or masonry shear walls =


hn = total height of the building from the support or base to the top in each direction.

  where the value of  shall not exceed 0.90.

METHOD B. PROPERLY SUBSTANTIATED ANALYSIS USING EIGENVALUE ANALYSIS AND RAYLEIGH'S METHOD

Fundamental period, T, may be calculated using the structural properties and deformational characteristics of the elements.
          

                                     

where: fi = any lateral force distributed approximately 


RELATED ARTICLES:

Design Base Shear
Solving for Total Seismic Design Load
How to Determine the Coefficient of Over Strength and Ductility Capacity, R

Identifying the Importance Factor, I
What do Seismic Zones mean?
Philippine Seismic Source Types
Near Source Factors and Seismic Coefficient
Soil Profile Types

SOIL PROFILE TYPES

Soil shall be classified into one of the categories shown in the code. With the soil profile type, the appropriate site-dependent design spectrum can be defined.

Site categorization schemes of the seismic codes use different descriptions of geological and geotechnical parameters to define the soil classes. The most commonly used parameter is the    which is the average shear wave velocity of the top 30m of the soil profile.This was introduced in post 1994 in US seismic codes (the 1994 and 1997 edicitons of NEHR and also the 2000 IBC) as the main categorization parameter.

                                        
where:

di  = thickness of layer i in meters
Vsi = shear wave velocity in layer i in m/s

Soil Penetration Test (SPT) is one of the procedures to determine the soil bearing capacity. This test is economical, used to identify surface information on land and offshore.  Standard penetration blow count   is used.

                                       
where:
di  = thickness of layer i in mm
ds = the total thickness of cohesionless soil layers in the top 30m
Ni  = the standard penetration resistance of soil layer from standards

And lastly, the undrained shear strength  may also be used to characterize the top 30m of the soil.
                 
                                       

where: 
dc  = the total thickness of cohesive layers in the top 30m.
Sui = the undrained shear strength in the standards, not to exceed 250 kPa.

SOIL PROFILE TYPES


SOIL PROFILE TYPEGENERIC DESCRIPTIONSHEAR WAVE VELOCITY VS,30SPT, N (BLOWS/300mm) UNDRAINED SHEAR STRENGTH SU (kPa)
SaHard rock> 1500
SbRock760 to 1500
ScVery dense soil and soft rock360 to 760> 50> 100
SdStiff soil profile180 to 36015 to 5050 to 100
SeSoft soil profile< 180< 15< 50
Sf

NOTE:
  • Soil Profile Sf is noted as soil requiring site-specific evaluation.
  • Soil Profile Type Se also includes any soil profile with more than 3.0m of soft clay defined as a soil with plasticity index > 20, and Su<24kPa.
NSCP reserves the exception:

"When the soil properties are not known or with insufficient detail , Type Sd shall be used." Soil profile type Se or Sf need not be assumed unless the building official determines that type Se or Sf may be present at the site or in the event that type Se or Sf is established by geotechnical data."


RELATED ARTICLES:

Design Base Shear
Solving for Total Seismic Design Load
How to Determine the Coefficient of Over Strength and Ductility Capacity, R
Identifying the Importance Factor, I
What do Seismic Zones mean?
Philippine Seismic Source Types
Near Source Factors and Seismic Coefficient
Elastic Fundamental Period of Structures