Transcript Slide 1

PCI
th
6
Edition
Fabrication Design
Presentation Outline
•
•
•
•
•
•
•
•
Planning Discussion
Stripping Process Design and Analysis
Prestress / Post Tension Effects
Handling Devices
Stripping Stress Examples
Storage Discussion
Transportation Discussion
Erection Discussion
Introduction
• The loads and forces on precast and prestressed
concrete members during production,
transportation or erection will frequently require a
separate analysis
• Concrete strengths are lower
• Support points and orientation are usually
different from members in their final position
Pre-Planning Piece Size
The most economical piece size for a project is
usually the largest, considering the following
factors:
• Stability and stresses on the element during
handling
• Transportation size and weight regulations and
equipment restrictions
Pre-Planning Piece Size
• Available crane capacity at both the plant and
the project site.
• Position of the crane must be considered, since
capacity is a function of reach
• Storage space, truck turning radius, and other
site restrictions
Planning and Setup
• Once a piece has been fabricated, it is necessary
to remove it from the mold without being
damaged.
• Positive drafts or breakaway forms should be
used to allow a member to lift away from the
casting bed without becoming wedged within the
form
• Adequate draft also serves to reduce trapped air
bubbles.
Planning and Setup
• Lifting points must be located to keep member
stresses within limits and to ensure proper
alignment of the piece as it is being lifted
• Members with unsymmetrical geometry or
projecting sections may require supplemental
lifting points and auxiliary lifting lines to achieve
even support during handling
• “Come-alongs” or “chain-falls” are frequently
used for these auxiliary lines
Planning and Setup
• When the member has areas of small cross
section or large cantilevers, it may be necessary
to add a structural steel “strongback” to the piece
to provide added strength
Planning and Setup
• Members that require a secondary process prior
to shipment, such as sandblasting or attachment
of haunches, may need to be rotated at the
production facility. In these cases, it may be
necessary to cast in extra lifting devices to
facilitate these maneuvers
Planning and Setup
• When developing member shapes, the designer
should consider the extra costs associated with
special rigging or forming, and pieces requiring
multiple handling
Stripping: General
• Orientation of members during storage, shipping
and final in-place position is critical in determining
stripping requirements
• They can be horizontal, vertical or some angle in
between
• The number and location of lifting devices are
chosen to keep stresses within the allowable
limits, which depends on whether the “no
cracking” or “controlled cracking” criteria is to be
used
Stripping: General
• It is desirable to use the same lifting devices for
both stripping and erection; however, additional
devices may be required to rotate the member to
its final position
Stripping: General
• Panels that are stripped
by rotating about one
edge with lifting devices
at the opposite edge will
develop moments as
shown
Stripping: General
• When panels are stripped this way, care
should be taken to prevent spalling of
the edge along which the rotation
occurs
• A compressible material or sand bed will
help protect this edge
Stripping: General
• Members that are
stripped flat from
the mold will
develop the
moments shown
Stripping: General
• In some plants,
tilt tables or
turning rigs are
used to reduce
stripping
stresses
Stripping: General
•
Since the section modulus with respect to the
top and bottom faces may not be the same, the
designer must select the controlling design
limitation:
•
•
•
Tensile stresses on both faces to be less than that which
would cause cracking
Tensile stress on one face to be less than that which would
cause cracking, with controlled cracking permitted on the
unexposed face
Controlled cracking permitted on both faces
Stripping: General
• If only one of the faces is exposed to
view, the exposed face will generally
control the stripping method
Rigging Configurations
• Stresses and forces occurring during
handling are also influenced by the type
of rigging used to hook up to the
member
Rigging Configurations
• Lift line forces for a two-point lift using
inclined lines are shown
Rigging Configurations
• When the sling angle is small, the components of
force parallel to the longitudinal axis of the
member may generate a significant moment due
to secondary effects
Rigging Configurations
• While this effect can and should be accounted
for, it is not recommended that it be allowed to
dominate design moments
Rigging Configurations
• Consideration should be
given to using spreader
beams, two cranes or
other mechanisms to
increase the sling angle
• Any such special handling
required by the design
should be clearly shown
on drawings
Rigging Configurations
• Using a spreader beam can also eliminate
the use of rolling blocks
• Note that the spreader beam must be
sufficiently stiffer than the concrete panel to
limit panel deflections and cracking
• Lifting hook locations, hook heights, and sling
lengths are critical to ensure even lifting of
the member
• For analysis, the panel acts as a continuous
beam over multiple supports
Stripping Design
• To account for the forces on the member
caused by form suction and impact, it is
common practice to apply a multiplier to the
member weight and treat the resulting force
as an equivalent static service load.
• The multipliers cannot be quantitatively
derived, so they are based on experience
Stripping Design
• PCI provides a table of typical values
Factor of Safety
• When designing for stripping and handling, the
following safety factors are recommended:
– Use embedded inserts and erection devices with a
pullout strength at least equal to four (4) times the
calculated load on the device.
– For members designed “without cracking,” the
modulus of rupture (MOR) , is divided by a safety
factor of 1.5.
7.5    f 'c
1.5
 5    f 'c
Stress Limits & Crack Control
• Stress limits for prestressed members during
production are discussed in Section 4.2.2.2 of
the the PCI Handbook
• ACI 318-02 does not restrict stresses on nonprestressed members, but does specify
minimum reinforcement spacing, as
discussed in Section 4.2.2.1. (PCI chapter 4
member design)
Stress Limits & Crack Control
• Members which are exposed to view will
generally be designed for the “no discernible
cracking criteria” (see Eq. 4.2.2.2), which limits
the stress to 5    f ' .
c
• In the case of stripping stresses, f′ci should be
substituted for f′c
• Whether or not the members are exposed to
view, the strength design and crack control
requirements of ACI 318-02, as discussed in
Chapter 4 of this Handbook, must be followed.
Benefits of Prestressing
• Panels can be prestressed, using either
pretensioning or post-tensioning.
• Design is based on Chapter 18 of ACI 31802, as described in Chapter 4 of this
Handbook. Further, tensile stresses should
be restricted 5to 
less
, must be
 fthan
'c
followed.
Benefits of Prestressing
• It is recommended that the average stress due
to prestressing, after losses, be within a range
of 125 to 800 psi
• The prestressing force should be concentric
with the effective cross section in order to
minimize camber, although some manufacturers
prefer to have a slight inward bow in the inplace position to counteract thermal bow
• It should be noted that concentrically
prestressed members do not camber, hence the
form adhesion may be larger than with
members that do camber
Strand Recomendation
• In order to minimize the possibility of splitting
cracks in thin pretensioned members, the
strand diameter should not exceed that
shown in the table below
• Additional light transverse reinforcement may
be required to control longitudinal cracking
Strand Recommendations
• When wall panels are post-tensioned, care must be
taken to ensure proper transfer of force at the
anchorage and protection of anchors and tendons
against corrosion
• Straight strands or bars may be used, or, to reduce
the number of anchors, the method shown may be
used
Strand Recommendation
• It should be noted that if an unbonded tendon
is cut, the prestress is lost. This can
sometimes happen if an unplanned opening
is cut in at a later date
Handling Devices
• Since lifting devices are subject to dynamic
loads, ductility of the material is a
requirement
• Deformed reinforcing bars should not be used
as the deformations result in stress
concentrations from the shackle pin
• Also, reinforcing bars may be hard grade or
re-rolled rail steel with little ductility and low
impact strength at cold temperatures
Handling Devices
• Strain hardening from bending may cause
embrittlement
• Smooth bars of a known steel grade may be
used if adequate embedment or mechanical
anchorage is provided
• The diameter must be such that localized
failure will not occur by bearing on the
shackle pin
Aircraft Cable Loops
• For smaller precast members, aircraft cable can
be used for stripping and erection purposes
• Aircraft cable comes in several sizes with
different capacities
• The flexible cable is easier to handle and will not
leave rust stains on precast concrete
Aircraft Cable Loops
• For some small precast members such as
coping, the flexible loops can be cast in ends
of members and tucked back in the joints
after erection
• Aircraft cable loops should not be used as
multiple loops in a single location, as even
pull on multiple cables in a single hook is
extremely difficult to achieve
• User should ensure that the cable is clean
and that each leg of the loop is embedded a
minimum of 48 in.
Prestressing Strand Loops
• Prestressing strand, both new and
used, may be used for lifting loops
• The capacity of a lifting loop
embedded in concrete is dependent
upon the strength of the strand, length
of embedment, the condition of the
strand, the diameter of the loop, and
the strength of the concrete
Prestressing Strand Loops
• As a result of observations of lift loop
behavior during the past few years, it is
important that certain procedures be followed
to prevent both strand slippage and strand
failure
• Precast producers’ tests and/or experience
offer the best guidelines for the load capacity
to use
• A safety factor of 4 against slippage or
breakage should be used
Strand Loops Recommendations
• In lieu of test data, the recommendations
listed below should be considered when
using strand as lifting loops.
– Minimum embedment for each leg of the loop
should be 24 in.
– The strand surface must be free of contaminants,
such as form oil, grease, mud, or loose rust, which
could reduce the bond of the strand to the
concrete
Strand Loops Recommendations
• Continued:
– The diameter of the hook or fitting around
which the strand lifting eye will be placed
should be at least four times the diameter
of the strand being used
– Do not use heavily corroded strand or
strand of unknown size and strength.
Strand Loops Recommendations
• In the absence of test or experience, it is
recommended that the safe load on a
single 1/2 in. diameter 270 ksi strand
loop satisfying the above
recommendations not exceed 8 kips
• The safe working load of multiple loops
may be conservatively obtained by
multiplying the safe load for one loop by
1.7 for double loops and 2.2 for triple
loops
Strand Loops Recommendations
• To avoid overstress in one loop
when using multiple loops, care
should be taken in the fabrication to
ensure that all strands are bent the
same
• Thin wall conduit over the strands in
the region of the bend has been
used to reduce the potential for
overstress
Strand Loops Recommendations
• When using double or
triple loops, the
embedded ends may
need to be spread apart
for concrete
consolidation around
embedded ends without
voids being formed by
bundled strand
Threaded Inserts
• Threaded inserts can have NC
(National Course) or coil
threads
• Anchorage is provided by loop,
strut or reinforcing bar
• Inserts must be placed
accurately because their safe
working load decreases sharply
if they are not perpendicular to
the bearing surface, or if they
are not in a straight line with the
applied force
Threaded Inserts
• Embedment of inserts close to an edge will
greatly reduce the effective area of the
resisting concrete shear cone and thus
reduce the tension safe working load of the
embedded insert
• When properly designed for both insert and
concrete capacities, threaded inserts have
many advantages
• However, correct usage is sometimes difficult
to inspect during handling operations
Threaded Inserts
• In order to ensure that an embedded insert acts
primarily in tension, a swivel plate as indicated in
should be used
• It is extremely
important that
sufficient threads
be engaged to
develop the
strength of the bolt
Threaded Inserts
• For straight tension loads only, eye bolts or
wire rope loops provide a fast method for
handling precast members.
• Do not use either device if shear loading
conditions exist.
Proprietary Devices
• A variety of castings
or stock steel
devices, machined
to accept
specialized lifting
assemblies are used
in the precast
industry
Proprietary Devices
• These proprietary devices are usually
recessed (using a “pocket former”) to provide
access to the lifting unit. The recess allows
one panel to be placed against another
without cutting off the lifting device, and also
helps prevent spalling around the device
• Longer devices are used for edge lifting or
deep precast concrete members
• Shallow devices are available for thin precast
concrete members.
Proprietary Devices
• The longer devices
usually engage a
reinforcing bar to
provide greater pullout
capacity, and often have
holes for the bar to pass
through as shown to the
left
Proprietary Devices
• These units have a rated capacity as
high as 22 tons, with reductions for thin
panels or close edge distances
• Supplemental reinforcement may be
required to achieve these values
• Shallow units usually have a spread foot
or base to increase pullout capacity
Proprietary Devices
• Reinforcing bars are required in two
directions over the base to fully develop the
lifting unit, as shown in Figure below
• These inserts are
rated up to 8 tons
Proprietary Devices
• Some lifting eyes do not swivel, so rotation
may be a concern
• In all cases manufacturer recommendations
should be rigorously followed when using any
of these devices
Wall Panel Example
• This example and others in Chapter 5
illustrate the use of many of the
recommendations in this chapter
• They are intended to be illustrative and
general only
• Each manufacturer will have its own preferred
methods of handling
Wall Panel Example
Given:
A flat panel used as a loadbearing wall on a two-story
structure, as shown on next slide
Section properties (nominal dimensions are used for
design):
Solid panel
Panel with openings
A = 960 in2
A = 480 in2
Sb = St = 1280 in3
Sb = St = 640 in3
Ix = 5120 in4
Ix = 2560 4 in4
Unit weight @ 150 pcf = 100 psf = 0.100 ksf
Total weight = 35.2 kips (solid panel)
= 29.2 kips (panel w/ openings)
Wall Panel Example
Wall Panel Example
• Stripping method:
– Inside crane height prevents panel from being
turned on edge directly in mold, therefore, strip flat
• Handling multipliers:
– Exposed flat surface has a smooth form finish with
false joints. Side rails are removable. Use
multiplier of 1.4
Wall Panel Example
• f′ci at stripping = 3000 psi
• Allowable tensile stresses at stripping and lifting:
5    f 'c 
5  (1.0)  3000psi
1000
 0.274ksi
Problem:
Check critical stresses involved with stripping.
Limit stresses to 0.274 ksi.
Compare Simple Solution to Mechanics Solution
Solution Steps
Step 1 – Determine section properties
Step 2 – Select number of pick points and
determine maximum stress
Step 3 – Determine stress from mechanic
approach
Step 4 – Check panel with opening
Step 5 – Check rolling block solution
Step 6 – Check transverse bending
Step 7 – Check secondary effects
Step 1 – Determine Section Properties
Solid panel dimensions
a = 10 ft, b = 35.2 ft, a/2 = 5 ft = 60 in.
S for resisting section (half of panel width)
b  h2 60 in  (8 in)2

 640 in3
6
6
Step 2 – 4-point pick
Figure 5.36.1.1(a) (page 5-5)
My  0.0107  w  a  b2
  
 0.0107 0.10 10 35.2ft
 121.4 223kip in
2
Step 2 – Check Stresses
4 Point Stresses
ft  fb 
223 kip  in
3
640 in
 0.348 ksi  0.274 ksi
Not Good try 8 point pick
Step 2 – 8 Point Pick
Figure 5.3.1.1(b) (Page 5-5)
My  0.0027  w  a  b2
  
 0.0027 0.10 10 35.2ft
 121.4 56.2kip in
2
Step 2 – Check Stresses
8 Point Stresses
ft  fb 
56.2 kip  in
3
640 in
 0.088 ksi  0.274 ksi
Step 3 – Mechanics of Materials
 
w  l 2 1.0  3.65
Mmax 

2
2
 6.7ft  kip
2
Mmax  area under shear diagram
 
1
10.3ft
 6.7  5.15
2
2
 6.7kip  ft
ft 
  
6.7 1.4 12
1280
 0.088 ksi  0.274 ksi
Step 4 – Panel With Openings
ft 
  
6.8 1.4 12
1280
 0.089ksi
or ft 
  
5.9 1.4 12
640in3
 0.155 ksi  0.274 ksi O.K.
Step 5 – Rolling Blocks
• If using a rolling block for handling as shown below,
the panel cannot be analyzed with the previous
method
• Each leg of continuous cable over a rolling block
must carries equal load
Step 5 – Rolling Block
w  29.2kips
RL 
   13.9 kips
29.2 8.4
9.3  8.4
13.9
R1  R 2 
 7.0 kips
2
R R  29.2  13.9  15.3 kips
R1  R 2 
15.3
 7.6 kips
2
Step 6 – Transverse Bending
• Consider lower portion of panel with openings
• Note that Figure Without the concrete in the area of the
opening, the weight is reduced and unevenly distributed.
Also, the resisting section is limited to a width of 4.7 ft.
Step 6 – Transverse Bending
Section through lifters:
   
w2  4.7 0.67 0.15  0.47 kips
ft
From continuous beam
analysis, load carried by
bottom two anchors is 7.2
kips, therefore:
w1 
    0.97 kips
ft
2 2.5
7.2  0.47 5
Mmax  2.1 ft  kips  25.2 in  kips
Step 7 – Secondary Effects
Check added moment due to sling angle
Using recessed proprietary lifting anchor
e = 3.5 in
sling angle   60o
7.2 kips
P
 3.6 kips
2
 3.5 
Mz  3.6  
 7.3 kips
o
 tan 60 
Mtotal  25.3  7.3  32.5 in  kips
Step 7 – Secondary Effects
Resisting Section

2
56 8
bt
S

 597in3
6
6
Mtotal  handling mult.
32.5 1.4
f

S
597in3
 0.08 ksi  0.274 ksi
2

   
Therefore Section is OK
Prestressed Wall Example
Given:
Same wall
panel as
previous
example
Prestressed Wall Example
Problem:
Determine required number of 1/2 in diameter, 270 ksi
strands pulled to 28.9 kips to prevent cracking in window
panel. Assume 10% loss of prestress.
From previous example, tensile stress is 0.431 ksi. The
desired level of tensile stress is 5  3000 psi or 0.274 ksi
Solution Steps
Step 1 – Determine additional
compressive Required
Step 2 – Determine the number of strands
required based on stress
Step 3 – Calculate the number of strands
Step 1 – Additional Compressive
Compressive stress required
0.431 – 0.274 = 0.157 ksi
Step 2 – # Of Strands Based On Stress
From previous the max moment/stress occurs at lifting
points (-M). This results in tensile stresses on the top
face.
   
2 30 7.25 


 526 in
A  2 30 7.25  435 in2
2
St
P

A
3
6
no. of strand 28.9 0.90
    0.060 no. of strandksi

435in2
 7.25

no. of strand 28.9 0.90 
 4
 2

  

M

S
526
 0.019 no. of strand ksi

 
Step 3 – Number of Strands
0.060(no. of strands) – 0.019(no. of strands) = 0.157 ksi
No. of strands = 3.8
Add four strands to panel (two on each side of opening)
Storage
• Wherever possible, a member should be
stored on points of support located at or near
those used for stripping and handling
• Where points other than those used for
stripping or handling are used for storage, the
storage condition must be checked
Storage
• If support is provided at more than two points,
and the design is based on more than two
supports, precautions must be taken so that
the element does not bridge over one of the
supports due to differential support settlement
Storage
• Warpage in storage may be caused by
– temperature or shrinkage differential between
surfaces
– creep
– storage conditions
• Warpage can only be minimized by providing
• Where feasible, the member should be
oriented in the yard so that the sun does not
overheat one side
Storage
• By superposition, the total instantaneous
deflection, ymax , at the maximum point can
be estimated by:
y max

1.875 w  sin   a4 l 4 

  
Eci
 Ic Ib 
Ic , Ib = moment of inertia
of uncracked section in
the respective directions
for 1 in. width of panel
Storage
• This instantaneous deflection should be
modified by a factor to account for the time
dependent effects of creep and shrinkage
• ACI 318-02 suggests the total deformation
yt, at any time can be estimated as:
 
y t  y max  1  
Storage
• λ = amplification due to creep and shrinkage as a
function of ′ρ (reinforcement ratio for non-prestressed
compression
reinforcement,
As/b·t)
Transportation
• The method used for transport can affect the
structural design because of size and weight
limitations and the dynamic
• Except for long prestressed deck members,
most products are transported on either flatbed
or low-boy trailers
• Trailers deform during hauling
• Size and weight limitations vary from one state
to state
• Loads are further restricted on secondary roads
• The common payload for standard trailers
without special permits is 20 tons.
Transportation
• Low-boy trailers permit the height to be
increased to about 10 to 12 ft.
• However they have a have a shorter bed
length.
• This height may require special routing to
avoid low overpasses and overhead wires
Transportation
• Erection is simplified when members are transported
in the same orientation they will have in the structure
• For example, single-story wall panels can be
transported on A-frames with the panels upright
• A-frames also provide good lateral support and the
desired two points of vertical support
Transportation
• Longer units can be transported
on their sides to take advantage
of the increased stiffness
compared with flat shipment
Transportation
• In all cases, the panel support locations should be
consistent with the panel design
• Panels with large openings sometimes require
strongbacks, braces or ties to keep stresses within
the design values
Transportation
• For members not symmetrical with respect to
the bending axis, the following expressions
can be used for determining the location of
supports to give equal tensile stresses for
positive and negative bending moments
Transportation
• For one end cantilevered…

1
x  1
2

yb 

 1
yt
yt 

yb
Where
yb = distance from the bending axis to the bottom fiber
yt = distance from the bending axis to the top fiber
Transportation
• For two ends cantilevered…
x
1

yt 

2  1  1 
yb 



Where
yb = distance from the bending axis to the bottom fiber
yt = distance from the bending axis to the top fiber
Erection
• Precast concrete members frequently must
be reoriented from the position used to
transport to its final construction position
• The analysis for this “tripping” (rotating)
operation is similar to that used during other
handling stages
• In chapter 5 in the PCI handbook, maximum
moments for several commonly used tripping
techniques are illustrated
Tripping Design Guide
Erection
• When using two crane lines, the center of
gravity must be between them in order to
prevent a sudden shifting of the load while it
is being rotated
• To ensure that this is avoided, the stability
condition shown must be met:
2
2
l
l b
e 
a
2
2
Erection
• The capacities of lifting devices must be
checked for the forces imposed during the
tripping operation, since the directions vary
• When rotating a panel with two crane lines,
the pick points should be located to prevent
the panel from an uncontrolled roll on the
roller blocks can be done by slightly offsetting
the pick point locations to shift the weight
toward the upper crane line lift points, or by
using chain drags on the rolling block
Erecting Wall Panels Example
Given:
The wall panels with
openings used on
previous examples
Problem:
Determine appropriate
procedures for erecting
the wall panels with
openings, panel will be
shipped flat
Erecting Wall Panels Example
Assumptions
• Limit stresses to 5 f 'c (0.354 ksi).
• Crane has main and auxiliary lines.
• A telescoping man lift is available on site.
Solution:
• Try three-point rotation up using stripping inserts and
rolling block: To simplify, conservatively use solid panel
(no openings) to determine moments.
Erecting Wall Panels Example
W  1.0kips / ft
RB 
 35.17 
35.17 1 
 2 

 

0.292 
35.17  0.604 
2 

 23.4 kips
R B  R B  11.7 kips
1
2
Erecting Wall Panels Example

 
R T  35.17 1  23.4kips  11.8kips
In Horizontal Position
MMAX  69.6kip  ft  835.2kip  in
1.2(835.2kip  in)
f
 1.58 ksi  0.354 ksi
635
Therefore, 3 point pick not adequate
Erecting Wall Panels Example
• Knowing from the stripping analysis
that a four-point pick can be used,
the configurations shown here may
be used
• However, this rigging may become
unstable at some point during
tripping, i.e., continued rotation
without tension in Line A
• Therefore, the lower end of the
panel must stay within inches of the
ground to maintain control.
Erecting Wall Panels Example
• Because the previous configuration
requires six rolling blocks and can be
cumbersome, the method shown on the
following slide may be an alternative
Erecting Wall Panels Example
Erection Bracing Introduction
• This section deals with the temporary bracing
which may be necessary to maintain
structural stability of a precast structure
during construction
• When possible, the final connections should
be used to provide at least part of the
erection bracing, but additional bracing
apparatus is sometimes required to resist all
of the temporary loads
Erection Bracing Introduction
• These temporary loads may include wind,
seismic, eccentric dead loads including
construction loads, unbalanced conditions
due to erection sequence and incomplete
connections Due to the low probability of
design loads occurring during erection,
engineering judgment should be used to
establish a reasonable design load
Erection Bracing Responsibilities
• Proper planning of the construction process is
essential for efficient and safe erection
• Sequence of erection must be established
early, and the effects accounted for in the
bracing analysis and the preparation of shop
drawings
• The responsibility for the erection of precast
concrete may vary as follows:
– (see also ACI 318-02 Section 10.3)
Erection Bracing Responsibilities
• The precast concrete manufacturer supplies
the product erected, either with his own
forces, or by an independent erector
• The manufacturer is responsible only for
supplying the product, F.O.B. plant or jobsite
• Erection is done either by the general
contractor or by an independent erector
under a separate agreement
Erection Bracing Responsibilities
• The products are purchased by an
independent erector who has a contract to
furnish the complete precast concrete
package.
• Responsibility for stability during erection
must be clearly understood.
• Design for erection conditions must be in
accordance with all local, state and federal
regulations. It is desirable that this design be
directed or approved by a Professional
Engineer
Erection Bracing Responsibilities
• It is desirable that this design be directed or
approved by a Professional Engineer
• Erection drawings define the procedure
• on how to assemble the components into the
final structure
• The erection drawings should also address
the stability of the structure during
construction and include temporary
connections
Erection Bracing Responsibilities
• When necessary, special drawings may be
required to include shoring, guying, bracing
and specific erection sequences
• It is desirable that this design be directed or
approved by a Professional Engineer
• Erection drawings define the procedure
• on how to assemble the components into the
final structure
Erection Bracing Responsibilities
• The erection drawings should also address
the stability of the structure during
construction and include temporary
connections
• When necessary, special drawings may be
required to include shoring, guying, bracing
and specific erection sequences
Erection Bracing Responsibilities
• For large and/or complex projects, a pre-job
conference prior to the preparation of erection
drawings may be warranted, in order to
discuss erection methods and to coordinate
with other trades
Handling Equipment
• The type of jobsite handling equipment
selected may influence the erection sequence,
and hence affect the temporary bracing
requirements
• Several types of erection equipment are
available, including truck-mounted and crawler
mobile cranes, hydraulic cranes, tower cranes,
monorail systems, derricks and others
• The PCI Recommended Practice for Erection
of Precast Concrete provides more information
on the uses of each.
Surveying and Layout
• Before products are shipped to the jobsite, a
field check of the project is recommended to
ensure that prior construction is suitable to
accept the precast units
• This check should include location, line and
grade of bearing surfaces, notches,
blockouts, anchor bolts, cast-in hardware,
and dimensional deviations
• Site conditions such as access ramps,
overhead electrical lines, truck access, etc.,
should also be checked
Surveying and Layout
• Any discrepancies between actual conditions
and those shown on drawings should be
addressed before erection is started
• Surveys should be required before, during
and after erection:
– Before, so that the starting point is clearly
established and any potential difficulties with the
support structure are determined early.
– During, to maintain alignment.
– After, to ensure that the products have been
erected within tolerances.
Loads on Structure
• The publication Design Loads on Structures
During Construction (SEI/ASCE 37-02)
provides minimum design loads, including
wind, earthquake and construction loads and
load combinations for partially completed
structures and structures used during
construction
• In addition to working stress or strength
design using loads from the above
publication, the designer must consider the
effect of temporary loading on stability and
bracing design
Temporary Loading Examples
• Columns with eccentric loads from other framing
members produce sidesway which means the
columns lean out of plumb
• A similar condition can exist when
cladding panels are erected on one
side of a multistory structure
Temporary Loading Examples
• Unbalanced loads due to partially complete
erection may result in beam rotation
• The erection drawings should address these
Conditions
Temporary Loading Examples
• Some solutions are:
– Install wood wedges
between flange of tee
and top of beam
– Use connection to
columns that prevent
rotation
– Erect tees on both
sides of beam
– Prop tees to level
below
Temporary Loading Examples
• Rotations and deflections of framing members may
be caused by cladding panels. This may result in
alignment problems and require connections that
allow for alignment adjustment after all panels are
erected
Temporary Loading Examples
• If construction equipment such as concrete
buggies, man-lifts, etc., are to be used,
information such as wheel loads and spacing
should be conveyed to the designer of the
precast members and the designer of the
erection bracing
Factors of Safety
• Suggested safety factors are shown
Bracing inserts cast into precast members
3
Reusable hardware
5
Lifting inserts
4
Bracing Equipment and Materials
• For most one-story and
two-story high
components that
require bracing, steel
pipe braces similar to
those shown are used
Bracing Equipment and Materials
• Proper anchoring of the braces to the precast
members and deadmen must be considered
• When the pipe braces are in tension, there
may be significant shear and tension loads
applied to the deadmen
• Properly designed deadmen are a
requirement for safe bracing
• Cable guys with turnbuckles are normally
used for taller structures
Bracing Equipment and Materials
• Since wire rope used in cable guys can resist
only tension, they are usually used in
combination with other cable guys in an
opposite direction
• Compression struts, which may be the
precast concrete components, are needed to
complete truss action of the bracing system
• A number of wire rope types are available
• Note that capacity of these systems is often
governed by the turnbuckle capacity
General Considerations
• Careful planning of the erection sequence is
important
• This plan is usually developed by a coordinated
effort involving the general contractor, precast
erector, precaster production and shipping
departments and a structural engineer
• A properly planned erection sequence can
reduce bracing requirements
• For example, with wall panel systems a corner
can first be erected so that immediate stability
can be achieved
General Considerations
• Similar considerations for shear wall
structures can also reduce bracing
requirements
• All parties should be made aware of the
necessity of closely following erection with the
welded diaphragm connections
• This includes the diaphragm to shear wall
connections
General Considerations
• In order for precast erection to flow smoothly:
–
–
–
–
–
The site access and preparation must be ready
The to-be-erected products must be ready
Precast shipping must be planned
The erection equipment must be ready
Bracing equipment and deadmen must be ready
Questions?