Shoring Basics
Tips for shoring system design, bracing, erection, and removal
BY BOB RISSER
ne of the contractor’s
many responsibilities is
the design and construction of shoring systems,
which support the formwork system and construction loads. Improperly designed or constructed
shoring systems can be extremely
h a z a rd o u s, leading to damaged
concrete or even catastrophic failure. Qualified and experienced
formwork designers should develop shoring plans and schedule
shore removal. Frequently the designer is a professional engineer
who works for the contractor or
shoring manufacturer. When necessary, the project engineer or architect reviews these plans for
structural adequacy, especially for
complex multistory construction.
O
Designing Shores
The shoring system is a temporary structure designed to carry
both vertical and horizontal loads
during construction. These loads
include the weight of the fresh conc re t e, forms, shores and other
falsework; construction and equipment loads; and wind loads and
other loads. Ho ri zontal shores,
such as adjustable beams, trusses,
or a combination of both, require
special design attention because
they distribute greater loads to few-
Figure 2. Determination of l/d ratio for a 3x4 wood shore with different bracing
plans. Bracing may be required in both directions to adequately reduce the l/d
ratio below the maximum of 50. The larger ratio governs maximum load limits.
er vertical supports.
ACI 347 (Ref. 1) recommends using a minimum construction live
load of 50 pounds per square foot
(psf) for shore load calculations (75
psf if motorized carts are used to
transport concrete). The minimum
design load for combined dead and
live loads should be 100 psf, or 125
psf if motorized carts are used. The
weight of formwork and shores is
typically assumed to be 10% of the
dead load of the slab. If unusually
heavy equipment or construction
material loads are anticipated, include these loads in the analysis for
shoring requirements.
Reference 2
Figure 1. Bearing area between the shore and the horizontal formwork may be
less than the total end area of the shore.
Bearing loads or crushing of the
shore ends against joists, stringers,
or other supported hori zo n t a l
formwork also must be considered
in the shoring system design. Excessive deflections from bearing
loads could make it very difficult to
meet strict tolerance requirements.
The allowable bearing stress at the
interface of lumber shores and horizontal formwork may also govern
the shore design rather than the capacity of the shore itself. The size of
the loaded contact area between
the shore and the supported formwork will determine the maximum
allowable load. In some cases, the
bearing area may be considerably
less than the actual dimensions of
the shore (Figure 1). Ha rd w o o d
caps or metal bearing plates can
distribute end loads from timber
shores so full structural capacity of
the shore can be utilized.
Simplified design, typically satisfactory for standard shoring systems, assumes that each shore or
scaffold leg supports an area which
extends halfway to adjoining sup-
p o rt s. Wood shores are treated as
simple columns, with allowable
loads based on the slenderness ratio l/d, where l is the unsupported
length, in inches, and d is the net
dimension of the wood face under
consideration, in inches. For wood
shores, l/d must not exceed 50.
Ho ri zontal bracing is often used
to reduce the size of the l/d ratio.
When this is the only purpose of
horizontal bracing, it is more properly called lacing. Figure 2 shows
how slenderness ratios are calculated and adjusted by using lacing.
The allowable load for simple wood
shores is based on the magnitude
of the l/d ratio and the compressive
strength of different lumber types.
Design round wood shores by assuming a square shore with the
same cross-sectional area.
For other shoring systems such
as steel tubes, adjustable shore s,
combinations of wood and steel,
and tubular steel scaffold-type
shoring, follow manufacturer recommendations on allow a b l e
loads. Load limits should be based
on actual load tests conducted on
the products and devices under
s t a n d a rd i zed conditions. Fo r
TABLE 1. TYPICAL RANGE
OF
Type of Foundation
Material
Massive bedrock
many patented systems, the analysis of allowable loads becomes so
complex that only load tests will
provide sufficient accuracy. Follow
all manufacturer’s recommendations for bracing, assembly, and
construction.
Bracing Shores
Adequate lateral bracing is extremely important to stability and
safety in shoring construction, but
all too often it is treated carelessly
or even omitted entirely (Ref. 2).
Shoring systems must be designed
to carry all anticipated lateral
forces due to wind, cable tensions,
inclined supports, concrete placement, and starting and stopping of
equipment. ACI 347 (Ref. 1) recommends a minimum assumed horizontal load of 100 pounds per linear foot of floor edge, or 2% of the
total dead load on the form,
whichever is greater.
Unless the shoring system has
enough internal X-bracing to provide internal rigidity, the shoring
and formwork system should
transfer all horizontal loads to the
ground or to already-completed
construction. Place diagonal brac-
MAXIMUM ALLOWABLE BEARING PRESSURES
Maximum Allowable Bearing
Pressure, lb. per sq. ft.
200,000
Foliated or laminated rock
including sound limestone,
schist, slate
40,000-80,000
Sedimentary rock including
shale and sandstone
20,000-50,000
Soft or broken rock
10,000-20,000
Hardpan
16,000-20,000
Gravel soils
8,000-20,000
Sands
4,000-13,000
Gravelly sands
6,000-16,000
Silt and clay soils, inorganic,
medium to stiff
1,200-10,000
Soft and organic deposits
(Reference 2)
Values below 500 psf are common
ing in both vertical and horizontal
planes where needed to provide
stability to individual shoring
members. Even though horizontal
lacing is used to decrease the l/d ratio, increasing the load capacity of
the individual shores, some diagonal bracing is desirable in all
shoring systems to increase the stability of the shores and resist lateral
loads. Braced bays, bracing lines, or
bracing tied to completed walls are
three common schemes used to
provide stability and lateral support (Figure 3).
Do not mix different types of
shoring systems or individual
shores of different materials on the
same level. The different characteristics of each system can lead to
nonuniform deflections under the
imposed load, creating stresses in
the structure and shoring system
for which they are not designed.
Follow manufacturer recommendations for bracing patented
shoring systems. Although bracing
may not be specifically recommended by the manufacturer to increase the system’s load-carrying
capacity, it may be needed to increase the safety and stability of the
entire formwork assembly.
Mudsills for Shoring
The shoring system must be capable of carrying the concrete and
construction loads without excessive deflections. If shores are supported on soil, the load the soil can
support must be known or conservatively estimated. Table 1 shows
typical allowable bearing pressures
for foundation materials. Because
new soils tests can be costly, some
contractors choose to use larger
mudsills based on very conservative estimates of soil strength. One
rough estimate of soil strength is
the “heel test.” An ave ra g e - s i ze d
man walking on compacted soil
without leaving heel marks indicates a soil-bearing strength of at
least 3,000 psf.
Mudsills are typically used to distribute shore loads to the soil.
Where the soil’s bearing capacity is
low or uncertain (Figure 4), mudsills
(a)
(b)
Figure 3. Braced bays (a) require no exterior guys or anchors. Bays marked with dashed X-lines have complete X-bracing
system on vertical lines in both directions as well as horizontal X-bracing. Distance between braced bays depends on such
factors as bay size, weight on form, form height, and live load. Center shores are tied with strut bracing.
Completed columns or walls (b) can be used for bracing. Dashed lines indicate one line of strut braces to columns cast
earlier. Solid lines represent X-bracing also tied into existing columns. Intermediate lines are needed if shores are spliced.
must be capable of evenly distributing shore load over a large enough
area to avoid unequal or excessive
settlement. Do not place mudsills
on frozen ground, recently placed
backfill, or where rainwater will flow
over the area. To determine the required area of the mudsill, divide
the total load on the shore by the allowable soil-bearing pressure.
Even when care is taken to properly design the mudsill for the shore
load, formwork is typically set
slightly higher than specified to accommodate a small amount of settlement and reduce the size of final
adjustments with wedges and
jacks. If the soil is very poor or likely to become unstable during construction, other techniques such as
t e m p o ra ry concrete sills, piles, or
soil stabilization may be re q u i re d
to support the shore loads.
Erecting Shores
Before erecting the formwork
support system, check to see that
the appropriate shores are being
used. This is especially important
when using rented equipment.
Load ratings can vary considerably
based on bracing configurations
and assumptions of the extended
height of the shore post or scaffoldtype shoring system. One contractor matches serial numbers on
equipment orders to those on de-
l i ve red materials to ensure proper
load-carrying capacity.
In addition to checking for the
appropriate materials, check that:
• Single-post shore members are
straight and true without twists
or bends
• Metal shores are free of excessive corrosion which reduces
structural capacity
• All metal shore connections are
sound
• All locking devices, coupling
pins, and pivoted cross-braces
TABLE 2. MINIMUM TIME SUPPORTS SHOULD REMAIN IN PLACE
(If no concrete strength requirements are provided)
Less Than
Structural
Design
Dead Load
More Than
Structural
Design
Dead Load
Arch centers
14 Days
7 Days
Joist, beam, or girder soffits
Under 10 ft. clear span between
structural supports
7 Days
4 Days
10 to 20 ft. clear span between
structural supports
14 Days
7 Days
Over 20 ft. clear span between
structural supports
21 Days
14 Days
4 Days
3 Days
10 to 20 ft. clear span between
structural supports
7 Days
4 Days
Over 20 ft. clear span between
structural supports
10 Days
7 Days
Where Structural
Design Live Load is:
One-way floor slabs
Under 10 ft. clear span between
structural supports
* Assumes 50° F ambient temperature around concrete.
(Reference 1)
on prefabricated
shoring systems are
working
Follow the shoring
layout plan closely,
keeping a copy on the
jobsite at all times.
Changes made to the
layout or timing of
shore removal should
be approved by the
shoring designer, engineer, or architect. Keep
extra shores or other
necessary accessories
Figure 4. Spread-type wood mudsills are recommended for fair and poor bearing or heavy loads.
on the jobsite for use in
an emergency. Plumb
them during concrete placement.
termine the earliest time to remove
all shore posts or scaffold legs to
Adjustable shores and other
shores and forms.
minimize lateral loads and signifipatented devices usually contain a
Concrete strength gain va ri e s
cant reductions in vertical load cajack or screw-type mechanism for
widely depending on job condipacity.
making elevation adjustments. Foltions. ACI 347 recommends removMulti-tier shoring with singlelow manufacturer recommendaing supports from horizontal conpost shores is considered dangertions on maximum extension of
crete beams and slabs only after the
ous and is not recommended by
these shores, since load ratings are
concrete has gained at least 70% of
ACI 347. Field constructed butt or
usually based on these configuraits design strength, unless earlier
lap splices of timber shores are altions. Additional diagonal bracing
support removal is approved by the
so not recommended by ACI unless
may be required to extend the
engineer-architect.
splices are made with approved
shore further.
If strength tests are used to deterfabricated hardware (Ref. 1). OSHA
mine shore removal, be sure all
requires vertical alignment of the
Removing Shores
parties involved in the construction
spliced shores, splicing to pre ve n t
Shores cannot be removed from
understand:
misalignment, splices on each
a slab or beam until concrete is
• The tests to be used
shore face (three splices for round
strong enough to hold its own
• Appropriate handling and testshores), perpendicular bracing at
weight. Reshores may be needed to
ing procedures
the splice level, and diagonal braccarry approved construction loads
• Who will make concrete speciing in two directions.
after the shores are removed. Almens
Where slab forms are continuous
though design, construction, and
• How many specimens will be
across several supporting shores,
safety aspects of the formwork are
made and who will test them
concrete placed in one area can cause
the contractor’s responsibility, the
• The minimum required conuplift of the formwork in an adjacent
required concrete strength and
crete strength
area. Positively attach the shores to
time period before shore removal
Cure concrete specimens at the
deck forms to resist this uplift and to
should be specified by the engijobsite under conditions no better
keep shores from falling out.
neer-architect.
than those the concrete in the
Vertical shores must bear firmly
Removing shores and other constructure are cured under to obtain
against the formwork they support
crete formwork supports should not
a conservative estimate of the inand be unable to tilt under conproduce excessive deflections, displace concrete strength. Se ve ra l
struction loads. Jacks or wedges altortions, or damage to the concrete.
nondestructive testing techniques
low positive adjustment for formRemove shores in a sequence that
are also recommended by ACI 318
work settlement and final
does not produce stresses in the
(Ref. 3) for determining in-place
alignments prior to placing conconcrete for which it is not designed.
concrete strength: penetration rec re t e. These devices also facilitate
For typical slabs and beams, begin
sistance, pullout strength, and mastripping the forms after the conshore removal in the middle of the
turity measurement.
crete has attained sufficient
bay or beam, working out toward
If no strength requirements are
strength. Wedges can be used eisupporting walls or columns. This
provided by the engineer or local
ther at the top or the bottom of the
ensures that the slab or beam will
codes, refer to Table 2, which gives
shore post, but not in both areas.
deflect and be loaded as designed.
the minimum time shores should
After the wedges are in position,
To maximize form reuse, a reliremain in place. The table indicates
toenail them to the shore to secure
able system needs to be used to dethe total number of days needed,
given an ambient temperature
around the concrete of at least 50°
F. The times can be shortened as
approved by the engineer-architect
if high-early-strength concrete is
used to speed construction. If temperatures below 50° F occur following concrete placement, the engineer may decide to lengthen the
minimum requirements.
Contractors often use reshores to
maximize form reuse and limit sag
or creep in the new concrete. When
reshores are used, forms and
shores are removed after the concrete has sufficient strength to carry its own weight. After the beam or
slab achieves its initial deflection,
the reshores are placed under the
concrete to carry additional construction loads or prevent further
creep deflections.
Place reshores as soon as possible after the forms are stripped, but
at least by the end of the day the
forms are removed. Reshores must
be placed snugly without altering
the deflected shape of the concrete.
The engineer-architect should approve all reshoring plans.
Multistory Construction
In multistory work, the partially
completed structure becomes part
of the shoring support. In planning
the shoring/reshoring system, the
designer uses information on the
structural design loads, usually obtained from the project drawings.
Multistory construction presents
special challenges for shoring proc e d u re s, especially in terms of removing the shores to maximize
form reuse. Se ve ral stories need to
be supported by shores and
reshores to handle the dead load of
the concrete and the construction
loads before the concrete gains full
design strength. Reshores are used
after the formwork is stripped to
distribute any further loads among
the older, stronger slabs.
Typical shoring and re s h o ri n g
schemes for multistory construction use a rule of thumb or local
custom: Contractors, for example,
may build two stories of shoring for
e ve ry one story of reshoring. Con-
tractors find these traditional
schemes acceptable on most projects for safely distributing concrete and construction to older,
stronger floors. In some cases,
howe ve r, these shoring and
reshoring plans will not safely carry
the constructions loads, and the
contractor should use one or more
of the following methods to ensure
safe construction:
• Increase the number of floors of
shoring and reshoring
• Use a longer cycle time for
each floor
• Either use a concrete mix or create curing conditions that produce faster concrete strength gain
Calculation of the loads imposed
on each floor during multistory
construction can be complex. The
shoring and reshoring plan should
be approved by the project engineer or architect.
This is particularly important for
construction of commercial office
buildings or other structures designed for relatively low live loads.
If the ratio of the design live load to
concrete dead load is low, the
s t ru c t u re has very little reserve
strength to carry the imposed construction loads and weight of fresh
c o n c re t e. Often the construction
live load and weight of new concrete are greater than the live load
for which the slab is designed. De-
pending on rate of construction,
many floors may need to be interconnected with shores and
reshores to safely carry the imposed load until the concrete
reaches design strength.
In multistory construction, the
most heavily loaded shores are
those at ground level which carry
the load of all floors concreted before first-level shores have been removed. Depending on the construction sequence, loads on these
shores can be several times the
dead load of the slab itself.
Place shores in the same location
on each floor to avoid developing
reversed bending or punching
shear stresses the slab cannot carry.
If offset placement is needed, recalculate the stresses to ensure that
the slab has adequate re i n f o rc i n g
in these areas to handle the reversed bending stresses that can
develop (Figure 5).
References
1. ACI Committee 347, “Guide to
Formwork for Concrete,” (ACI 347R88), American Concrete Institute, Detroit, 1988.
2. M.K. Hurd, Formwork for Concrete,
SP-4, fifth edition, American Concrete
Institute, 1989.
3. ACI Committee 318, “Building Code
Requirements for Reinforced Concrete,” (ACI 318-92), American Concrete Institute, 1992.
Figure 5. Improper positioning of shores from floor to floor can create bending
stresses for which the slab is not designed. Calculate for reversal stresses when
reshores do not match shores above. Be sure shores resist uplift and cannot fall.
PUBLICATION #C940856, Copyright © 1994, The Aberdeen Group, All rights reserved