Chapter 2 design loads(3)

1
Chapter 2: Design Loads for
Buildings
 Chapter Objectives:
 Identify different types of structural loads acting on
steel structures.
 Calculation of structural loads acting on typical steel
buildings according to the latest edition of the ASCE
standards.

Introduction
 Loads and load combinations are to be taken from
the governing building code;
 In the absence of a building code, use the ASCE/SEI
“Minimum Design Loads for Buildings and Other
Structures” provided by the American Society for
Civil Engineers (ASCE) as stated by AISC-LRFD
Manual (pp. 16.1-10).

Design Loads on Steel Structures
ASCE 7-16 Standards AISC-LRFD Manual
For Design of Structural
Elements and Connections
Minimum Design Loads for
Buildings and Other
Structures

Design Loads on Steel Structures
• Examples of common design loads are:
Dead Load (D);
Live Load Due to Occupancy and movable equipment (L);
Roof Live Load (Lr);
Wind Load (W);
Earthquake Load (E).

Load Path and Tributary Areas
• Examples of load path on trusses

supporting
purlins
Truss
roof
The trusses transfer
their loads to the
supporting columns
Each purlin supports an area = its span x
half the distance to the purlin on either side.
The purlins transfer
their loads to the
supporting trusses
Each truss supports an area
= its span x half the distance
to the trusses on either side
Load Path on Trusses

Load Path on Slab-Beam System
Floor Beams and Girders are at the same level
 Slab Metal Decking is used: Slab transfers load along
deck ribs direction
 Slab on conventional wooden formwork: check
Llong/Lshort of slab panel

Dead Load
• It is the self-weight of all construction materials
incorporated into the building (i.e., Permanent Load).
• Examples:
Structural components (slabs, beams, girders, and
columns).
 Floors, roofs and ceilings materials.
 Exterior walls, cladding, windows, and doors.
 Interior permanent walls and partitions.
Fixed service equipments (heating, air-
conditioning, elevators, cranes, …etc.)

Dead Load
• As a result of its deterministic nature, Dead loads can
be estimated with only a small margin of error.
• Recommended minimum design dead loads in (psf)
and (kN/m2) are provided by Table C3-1 of the ASCE7-
16 Standard.
• Table C3-2, in the ASCE7-16 Standard provides
minimum densities (pcf and kN/m3) for construction
materials.

Dead Load
Example 1
The floor system of a building consists of 5 inch-thick reinforced
concrete slab resting on four steel floor beams, which in turn are
supported by two steel girders as shown in the figure. The cross-
sectional areas of the floor beams and the girders are 14.7 in2 and
52.1 in2, respectively. Determine the dead loads acting on the
beams CG and DH and the girder AD.
(A = 52.1 in2)

Dead Load
Solution
Dead load on beam CG:
from ASCE7-10, Table C3-2 (page 266): gconc = 150 lb/ft3
Based on the tributary area carried by the beam CG:
Load from the concrete slab = gconc * tslab *btributary
Load from R.C. slab = (150 lb/ft3) (5/12 ft) (10 ft) = 625 lb/ft
(A = 52.1 in2)

Dead Load
Dead load on beam CG:
from ASCE7-10, Table C3-2 (page 266): gsteel = 492 lb/ft3
Own weight of the steel beam = gsteel * Area of beam(ft2)
weight of steel beam = (492 lb/ft3) (14.7/144 ft2) = 50 lb/ft
Total uniform dead load = 625 + 50 = 675 lb/ft
(A = 52.1 in2)

Dead Load
Dead load on beam DH:
Based on the tributary area carried by the beam DH:
Load from the concrete slab = gconc * tslab *btributary
= (150 lb/ft3) (5/12 ft) (5 ft)
= 312.5 lb/ft
(A = 52.1 in2)

Dead Load
(A = 52.1 in2)
Dead load on beam DH:
Own weight of the steel beam = gsteel * Area of beam(ft2)
weight of steel beam = (492 lb/ft3) (14.7/144 ft2) = 50 lb/ft
Total uniform dead load = 312.5 + 50 = 362.5 lb/ft

Dead Load
Dead load on girder AD:
The distributed dead load on girder AD is due to its own weight only
Own weight of the girder = gsteel * Area of beam(ft2)
= (492 lb/ft3) (52.1/144 ft2) = 178 lb/ft
(A = 52.1 in2)

Dead Load
(A = 52.1 in2)
10,770 lb10,770 lb
Dead load on girder AD:
The dead load acting on BF is the same as for CG
So, the reactions from BF and CG are the same = 8100 lb

 In general, Live Load varies in magnitude with time
 It is the load resulting from all non-permanent
installations in the building;
 The Live Load value varies from one location to
another inside the building.
 Examples of Live Loads:
 building occupants.
 furniture.
 office equipment.
 movable room partitions,…etc .
Live Load

• Live load on buildings can be divided into two main
components:
1) Sustained Live Load (SLL): weight of relatively
permanent fixtures and furnishings.
2) Transient Live Load (TLL): weight of occupants who
enter and leave the space.
Live Load

 In general, live loads are prescribed in building
codes based on occupancy, location, and
importance of the building;
 Due to the uncertainty associated with the
expected value of Live loads, they are estimated
with a much larger margin of error than in dead
load.
 Forms of application of live loads:
I. Uniformly Distributed Live Load;
II. Concentrated Live Load.
Live Load

27
Live Load
 Live loads due to vehicular traffic on highway bridges
the American Association of State Highway and
Transportation Officials (AASHTO) Speciﬁcation
deﬁnes two systems of standard trucks, H trucks and
HS trucks, to represent the vehicular loads for design
purposes.

 Impact Loads: Moving vehicles may bounce or sidesway
as they move over a bridge, and therefore they impart
an impact to the deck. The percentage increase of the
live loads due to impact is called the impact factor, I.
28
Live Load
 Live loads for railroad bridges are speciﬁed by the
American Railway Engineering and Maintenance of
Way Association (AREMA) in the Manual for Railway
Engineering.

 Tables 4-1 and C4-1 in the ASCE7-16 Standard
provide minimum values for live loads (Lo) in (psf)
and (kN/m2);
 Live load values (Lo) provided in the tables may be
reduced depending on the tributary area supported
by the structural element under consideration;
 What is the “Tributary Area” and how may it differ
from a structural element to another in the same
building?
Live Load

A tributary area for a member is the area that, when
loaded, causes a stress change in the member.
Example (1): Tributary Area for Column Load
LiveLoad

Example (2): Tributary Area for Beam and Column Loads
Live Load

Example (3): Tributary Area for Beam and Column
Loads
Live Load

• Reduction in Live Load: A reduced live load may be
design according to the following
) (S.I. Unites)
where:
L is the reduced design live load per ft2 (m2) of area
supported by the member.
) (Imperial Unites)
K A
15
( 0.25 
LL T
o
used in the
formula:
L  L
K A
4.57
LL T
oL  L ( 0.25 
Live Load
(KLLAT ) > 400 ft2 (37.16 m2 )

where:
Lo is the unreduced design live load per ft2 (m2) of area
supported by the member (from table 4-1 or C4-1)
AT is the tributary area in ft2 (m2).
KLL is the live load element factor (from table4-2)
Live Load

(A = 52.1 in2)
Live Load
Example 1
The plan below shows a floor system of the offices area
in an office building. Use the ASCE7-16 standards to
calculate the live loads acting on the beams CG and DH
and the girder AD.

KLL AT= 2 * 240 = 480 ft2 > 400 ft2 (reduction is needed)
Solution
Live load on beam CG:
From ASCE7-10, Table 4-1, Lo = 50 psf
From ASCE7-10, Table 4-2, KLL for interior beam CG = 2
ATcarried by the beam CG = 10 ft x 24 ft = 240 ft2
Live Load

Live load on beam CG (cont’d):
Total uniform live load = wL = L * bT = 46.75 * 10 = 467.5lb/ft
15 15
480 



  50*0.935  46.75 psf
 
 
 
K A
L  L 0.25    50 0.25 
LL T
o
wL = 467.5lb/ft
5610 lb 5610 lb
C G
Live Load

Live load on beam DH:
From Table 4-2, KLL for edge beam DH without cantilever slab = 2
ATcarried by the beam DH = (5 ft) (24 ft) = 120 ft2
KLL AT= 2 * 120 = 240 ft2 < 400 ft2 (No reduction is needed)
Live Load

Live load on beam DH (cont’d):
L = Lo= 50 psf
Total uniform live load = wL = L * bT = (50 psf) (5 ft) = 250lb/ft
wL = 250lb/ft
3000 lb 3000 lb
D H
Live Load

1515
720 
 
  50*0.809  40.45 psf

 

 
K A
L  L 0.25    50 0.25 
LL T
o
PL=? PL=?
Live load on girder AD:
From Table 4-2, KLL for edge girder AD without cantilever slab = 2
ATcarried by the girder AD = (30 ft) (24/2 ft) = 360 ft2
KLL AT = 2 * 360 = 720 ft2 >400 ft2 (reduction is needed)
Live Load

Live load on girder AD (cont’d):
PL is the reaction from Beams CG and BF on Girder AD
From the Floor Beam CG Solution:
For a reduction factor of 0.935  PL = 5610 lb
From the Girder AD Solution:
For a reduction factor of 0.809  PL = ???? lb
Live Load

Live load on girder AD (cont’d):
Having:
For a reduction factor of 0.935  PL = 5610 lb
For a reduction factor of 0.809  PL = ???? lb

 
0.935
P  5610  0.809   4854 lbL
PL=4854 lb 4854 lb
Live Load

Reduction in Flat, Pitched or Curved Roofs Live Load:
Lr = Lo R1 R2 ;
Lr = Lo R1 R2;
where12 < Lr < 20 psf ,or
where 0.58 < Lr < 0.96 kN/m2
where Lr is the reduced roof live load applied to the
horizontal projection of the roof area.
b
a
Roof Live Load

R1 and R2 are reduction factors that are determined as
follows:
 1 for At  200 ft2
structural member considered.

1 t
R 

.2  0.001A for 200  A  600 ft2
1


t
for At  600 ft2
for At  18.58 m2
0.6


in which At is the tributary area supported by the
t1 t
for At  55.74 m2
0.6
for 18.58  A  55.74 m2R 

.2  0.011A
1
1
Roof Live Load

where for pitched roofs:
F = Slope x 12 (i.e., = (a/b) x 12)
and, for arches or domes:
F = (rise-to-span ratio)x32

 for F  4
for 4  F 12
for F  120.6
1
2
R   .2  0.05F
1
b
a
Roof Live Load

 The reduced live load must not be less than 50
percent of Lo for members supporting one floor or a
section of a single floor, nor less than 40 percent of Lo
for members supporting two or more Floors;
 For a column or beam supporting more than one
floor, the term AT represents the sum of the tributary
areas from all floors;
 Reduction in live load is not permitted for public
assembly areas or when the live load is high (>100
psf).
Live Load

Roof Live Load
Example 3
For the three-story building shown in the figures below.
Calculate the design live load supported by the interior
column C located in the first story. Assume a 50 lb/ft2
design live load, Lo, on all floors including the roof.

Solution
Roof live load on Column C:
Lr = Lo R1R2
AT = 20(24) = 480ft2
R1 = 1.2 − 0.001 (480) = 0.72
R2 = 1.0
Lroof = LoR1R2
= 50(0.72)(1.0) = 36.0 psf
Roof Live Load

Roof Live Load
Solution
Floor live load on Column C:
the tributary area for the remaining two floors
AT= 2(480) = 960 ft2
KLL AT= 4 * 960 = 3840 ft2 > 400 ft2
(reduction is needed)

Solution
Total live load on Column C:
Since 24.6 lb/ft2 > 0.4 × 50 lb/ft2 = 20 lb/ft2 (the lower limit)
So, use L = 24.6 lb/ft2.
Load to column = AT(Lroof) + 2AT(Lfloor)
= 480(36.0) + 960(24.6)
= 40,896 lb = 40.9 kips.
Roof Live Load

Chapter 2 design loads(3)

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