OFFICE BUILDING
STRUCTURAL SYSTEMS
Overview
Floor Framing Systems
Lateral Load Resisting Systems
Overview
Floor Framing Systems
Lateral Load Resisting Systems
American Airlines Center
Questions??
Floor Framing Systems
Concrete vs. Steel – advantages,
disadvantages
Concrete Systems Conventionally Reinforced
Post - tensioned
Structural Steel Systems Beam Camber
Floor flatness and levelness
Concrete creep and shrinkage
Concrete vs. Structural Steel
Structural Steel
Advantages
Speed of frame erection
Smaller column sizes
Long span capabilities
Ease of modification of floor framing
Lighter building self-weight on
foundations
Concrete vs. Structural Steel
Structural Steel
Disadvantages
Time to purchase, fabricate,
and deliver steel – may require
fast tracking
Deeper structural depth typically
required
Steel costs tend to fluctuate
More difficult to achieve level floors
Reduced serviceability – floor
vibration and frame stiffness
Concrete vs. Structural Steel
Concrete
Advantages
Can bid job and begin construction
immediately
Shallow structural depths
Predictable cost of construction
Excellent serviceability – floor vibration
and frame stiffness
Concrete vs. Structural Steel
Concrete
Disadvantages
Slower frame construction
Large column sizes at the bottom of
building
Cost penalties for very long spans
Difficult to modify existing floor
framing
Heavier building self-weight on
foundation
Concrete
Longer
span
Shorter
span
One-way Systems
Slab spans in one direction,
between beams
Main reinforcing placed
Direction
perpendicular to beams
of load
Loads distributed in one
distribution
direction only
Two-way Systems
Load distribution in both
directions
Works best for approximately
equal spans
Primary reinforcing
Concrete
Maximum
recommended spans for
standard joists
Pan Joists
Uses reusable metal pan
forms
Span between larger
girders
Standard joists < 36” on
center
Skip joists > 36” on
center
Commonly used in office
buildings
Spans can be increased
by post-tensioning
(joists and girders)
35’
35’
distribution rib
2’-0, 3’-0, …, 6’-0
8” to 24”
5, 6, 7”
12
1
Section
Concrete
Beam and girder
Slab spans between
main girder
intermediate beams
which are carried
by larger girders
Wide range of loads
and can be supported
by varying slab and
beam spacings
Spans can be increased
by post-tensioning beams
or girders
One-way slab
Intermediate
beam
Typical spans
= 15-30’
Concrete
Waffle Slab (Two-way Joist)
Uses reusable metal
pan forms
Supports heavy loads
Spans up to 40’
Solid slab at columns to
resist punching shear
Often left exposed with
lighting integrated into
the coffers
Expensive
3’-0, 4’-0, 5’-0
8” to 24”
6, 7, 8”
12
1
Section
Concrete
Flat Plate
Used for mostly
rectangular bays
Orthogonal reinforcing
Shallow structure =
smaller floor-to-floor height
Short to medium spans
and moderate live loads
Primarily used in hotels and
condos. Used in office
buildings in certain parts of
the country
(i.e. Washington D.C.)
Two-way system
Flat Plate;
Maximum
span = 25’
capitals and
drop-panels
for Flat Slab
systems;
Maximum
spans up to
35’
Concrete
Flat Slab
with drop
panels
Flat Slab
Similar to flat plate
Uses drop panels, minidrops, or column capitals
for heavier loads, longer
spans, and to control
punching shear
Two-way system
Flat Slab;
Maximum
span = 35’
Flat Slab
with minidrops
Standard ACI drop panel
Mini-drop panel
Steel
Steel Joists
Span between beams or
bearing walls
Floor joists usually spaced
2’-4’ on center and covered
with metal deck slabs
Roof joists spaced 4’-6’ on
center and covered with
roof/form deck and roofing
material
Openings in joists for MEP
to pass through
Columns or
Bearing wall
supports
Open Web Steel Joist System
Steel
girder
Steel Beams and Girders
Wide-flange I-beams
Beams 8’-15’ on center
Typical beam spans 30’-45’
Typical girder spans 25’-30’
Standard shapes can be 70’+
Girders supported by wide
flange columns
Floors constructed with metal
deck slabs
beams
Open Web Steel Joist System
Beam and girder system
Composite Slab Construction
Two or More Materials Acting as One Unit
Composite steel deck with concrete slab and steel
beams
Headed studs for load transfer between materials
Headed stud
anchors
Structural Steel Floor Beam Camber
Cold bending of steel beams to induce an upward
deflection
Camber, 1” to 3”
Camber is used to offset beam deflection resulting
from placement of concrete slab on unshored
construction. Goal is for beam to be “flat” after
concrete placement
The issue of camber can be a confusing one unless
proper quality control is utilized
Check upon arrival to site
Check after erection, before concreting
Check after concreting
Floor Flatness and Levelness
Flatness refers to the “waviness”
of a slab surface
12”
12”
Levelness refers to the “tilt” of a
slab surface
10’-0”
Floor Flatness and Levelness
F - numbers
The Flatness F-number, FF measures
“waviness”, and is primary a function of
finishing operations after strike off
The Levelness F-number, FL measures
levelness and is mainly a function of accuracy
of formwork and initial strike off
FF 25/ FL 20 are typical criteria specified for
office buildings
Concrete Creep and Shrinkage
Creep – concrete continues to deform with
time under sustained loads
Floor beam deflection
Column shortening
Shrinkage – concrete shrinks as it cures and
dries. Shrinkage is unrelated to load
application
Floor beam deflection
Random floor cracking
60% to 70%
Deformation
90 days
Time
Overview
Floor Framing Systems
Lateral Load Resisting Systems
American Airlines Center
Questions??
Lateral Load Resisting Systems
Concrete Systems Rigid frame
Shear walls
Structural Steel Systems Rigid frame
Braced frame
Mixed systems
Concrete Rigid Frame
floor
diaphragm
Lateral wind or
seismic loads
Rigid frames which
consist of building
beams and columns
work well for buildings
up to 20 to 25 stories
Most common system
utilized during the
recent commercial
office market
construction in Dallas
lateral load
resisting frame
Concrete Shear Walls
floor
diaphragm
Lateral wind or
seismic loads
Shear walls are
typically
introduced for
buildings greater
than 25 stories
lateral load
resisting shear
walls
Structural Steel Systems
Field welded
beam-column
connections
Rigid Frame Elevation
Knee-braced Frame Elevation
Expensive
Inefficient
Structural Steel Systems
X-braced Frame Elevation
K-braced Frame Elevation
Efficient
Most commonly used
Mixed Systems
Steel braced Frame
Steel Rigid Frame
Mixed Systems
Concrete Shear Walls
Concrete or Steel Rigid Frame