Column Design Lines

Column design is performed in this section.

Ultimate forces on columns are calculated according to tributary analysis. Load take down is the transferring process of the loads cumulatively to the vertical members below.

Load Take Down Based On Tributary Analysis

Area Loads On Slabs

Load take down of area loads which are in the same load class is calculated as follows;

→ The loading area within the boundaries of the tributary area of column is determined at each floor ( if exists )

→ This intersection areas are multiplied by the loading value at each floor (in psf)

→ The lumped forces (in kips) are reported cumulatively at each floor depending on their load class in Tributary Forces spreadsheet

Line Loads On Slabs

Load take down of line loads which are in the same load class is calculated as follows;

→ The length of line load within the boundaries of the tributary area of column is determined at each floor (if exists )

→ This length is multiplied by the loading value at each floor (in k/ft)

→ The lumped forces (in kips) are reported cumulatively at each floor depending on their load class in Tributary Forces spreadsheet

Point Loads On Slabs

Point loads modelling on slabs mostly occurs at regions where column load transfer from columns above to the slab/beam below.

Before the load take down, distribution factors for the columns near to point load must be calculated.

Distribution factors are determined as follows;

→ Tributary areas of columns near to point load are determined as illustrated in Figure 1

Figure 1

→ A second tributary area calculation is performed by assuming the point load as a column as illustrated in Figure 2

Figure 2

→ The intersection areas of the tributary area of point load and the first tributary areas of the columns are determined as illustrated in Figure 3

Figure 3

→ Intersection ratio is calculated as the intersection area divided by the tributary area of point load

→ The distribution factor for each column is equal to intersection ratio

Load take down of point loads which are in the same load class is calculated as follows;

→ Distribution factor of the column is multiplied by the value of the point load at each floor ( if exists )

→ The forces (in kips) are reported cumulatively at each floor depending on their load class in Tributary Forces spreadsheet

Live Load Reduction Based On Alternative Method

Live load reduction is performed according to NYCBC - 2014 Chapter 16 Section 1607.9.2.

Live loads which are classified as “L,L_Heavy or L_Passenger” are permitted to be reduced in alternative method.

Figure 4

L

Live loads not exceeding 100 psf and are other than public assembly or parking vehicle garages are classified as “L” live load.

The design live load of any structural member supporting 150 square feet or more is permitted to be reduced.

Live load reduction along column design line is calculated as follows;

→ The tributary area of each column at column design line is determined.

→ The live load area/ tributary area ratio of each live load class (if any) at each column is determined.

→ Each tributary area is multiplied by the live load area ratio of each live load class (if any) to determine the reducible live load area at each floor along the column design line.

→ Cumulative reducible area of live load classes(if any) at each column is determined along the column design line.

→ Reduction in percent is a function of cumulative tributary area where;

R= 0.08*(At -150)

→ For one way slabs, tributary area, At, shall not exceed the product of slab span and a width normal to slab span of 0.5 times slab span.

→ Maximum reduction in percent for columns can not be more than %60 or %23.1*(1+D/L) where D is dead load per square foot of area supported and L is unreduced live load per square foot of area supported.

→ Live load reduction factor is equal to (100-Rmax)/100

L_Heavy

Live loads exceeding 100 psf are considered to be heavy live loads. Live load reduction for L_Heavy is calculated as follows;

→ If column supports 1 floor, live load reduction factor is 1.0

→ If column supports two or more floors, live load reduction factor is 0.8

L_AssemblyGroupA

Live Loads which are classified as “L_AssemblyGroupA “ are not permitted to be reduced in alternative method. Thus live load reduction factor for L_AssemblyGroupA is always 1.0.

L_ParkingVehicleGarages

Live loads which are classified as “L_ParkingVehicleGarages” are permitted to be reduced as following;

→ If column supports 1 floor, live load reduction factor is 1.0

→ If column supports 2 or more floors, live load reduction factor is the maximum of live load reduction factor explained in “L” class live loads and 0.8.

Lr

Roof live loads which are classified as ”Lr” is considered to be unreducible live loads.

Figure 4

Click “Run Tributary Analysis” as shown in Figure 4 to start analysis.

Figure 5

Ultimate forces written in blue in column schedule are updated after the analysis as shown in Figure 5.

Figure-6

If there is discontinuity in colum design lines, a red point is created on the cell in spreadsheet. Move cursor over the red point to see load distribution factors of vertical members near to discontinuous column as illustrated in Figure 6.

The distribution factors are calculated by assuming the total force supported by the column as point load.