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Ultimate forces on columns are calculated based on 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 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 area is multiplied by the loading value (in psf)
→ The calculated loads on each floor(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 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 (in k/ft)
→ The calculated loads on each floor(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
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Figure 1
→ A second tributary area calculation is performed by assuming the point load as a column as illustrated in Figure 2
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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
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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 the alternative method.
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Figure 4
The design live load of any structural member supporting 150 square feet or more is permitted to be reduced.
Reducible live load area is calculated as described below;
→ The tributary area of each column at column design line is determined.
→ The intersection area of the column tributary and the reducible load area is calculated at each floor.
→ Cumulative reducible area is calculated at each floor.
→ Reducible area of the transfer column ( if exists ) is multiplied by the distribution factor and added to cumulative reducible live load area at the transfer floor.
Live load reduction along column design line is calculated as follows;
→ Total reducible area of live load class L (if any) at each column is determined along the column design line as described before.
→ Reduction in percent is calculated based on the following equation;
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 can not be more than %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.
L Load Reduction
Live loads not exceeding 100 psf and are other than public assembly or parking vehicle garages are classified as “L” live load.
→ Maximum reduction in percent for columns can not be more than %60.
→ Live load reduction factor is equal to maximum of (0.6, 1- Rmax/100).
L_Heavy Load Reduction
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 the maximum of ( 0.8, 1-Rmax/100 )
L_AssemblyGroupA Load Reduction
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 Load Reduction
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 (0.8, 1-Rmax/100).
Lr Load Reduction
Roof live loads which are classified as ”Lr” is considered to be unreducible live loads.
Column Schedule
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Figure 4
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Figure 4’de görülen “Run Tributary Analysis” butonuna tıklanarak tributary analizi gerçekleştirilir. |
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