C struc design-bendapudi-sept101

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StructuralDesign
STRUCTURE magazine September 201038
Discussion on Structural Design of
Steel Pipe Support Structures
By Kasi V. Bendapudi, P.E., S.E.
This is the second of a 2 part article on the design of pipe support structures. Part 1 of this
article (February 2010 issue) discussed the effects of atmospheric temperature changes,
expansion joint requirements and the introduction to design loads. This article concludes
with the continuation of design loads, structural instability concepts and detailing for
stability requirements. (Equation and figure numbers in this part are a sequential
continuation from Part 1.)
Interaction between Pipes
and the Support Structure
In practice, pipes are not attached to
each and every pipe support in order
to restrain the forces caused by thermal
expansion or contraction. Under these
conditions, at the onset of expansion/
contraction, friction forces may develop
between the pipes and the structural steel
support. This frictional force is applied to
the top flange generally creating an eccen-
tric load, or torsion, on the supporting
wide-flange shape. The force generated by
friction (F) can be expressed as:
F = ± μ N Equation 5
where µ is the coefficient of static friction,
approximated to be 0.3 for most steel-to-
steel contact surfaces, and N the normal
force at the contact surface.
If a thermal differential between the pipe
and the supporting structure occurs, fric-
tional forces would initially be restrained
by the supporting structure. Assuming
the flexural stiffness in the longitudinal
direction of each bent of pipe rack (ref.
Figure 3, Part 1), the total force restrained
by the pipe rack bents (Ff) is given by:
Ff = ∑ k∆ Equation 6
where, ∆ is the horizontal displacement
of the pipe support bent and k is the
stiffness of the frame about its weak axis.
In this case, the pipe anchor force is also
equal to the force restrained by the pipe
bents prior to slip. Therefore, each pipe
support bent restrains a share of the
frictional force prior to slip, regardless if
the pipe is fastened to the pipe support or
is free to move longitudinally. However,
at the onset of the frictional slip the force
at the pipe anchor point, which is located
by the piping engineer, would be equal to
force P (as given in Equation 4, Part 1)
and Ff given in Equation 6.
Although friction may develop at the
contact surface from the resistance to
movement of the pipe under thermal dif-
ferentials, eventually there becomes no
correlation between the maximum fric-
tion force (F) and the force exerted by
the thermal expansion or contraction
(P) of the pipe. The maximum friction
force (F) depends upon variables such
as temperature differential and contact
surface conditions. The magnitude of the
thermal expansion force (P) is extremely
high compared to the friction force F as
assumed. This is demonstrated in the
following numerical example:
Numerical Example:
Assume:
Bay spacing = 20 ft. (~ 6m), Bay
width = 16 ft. (~ 5m). See Figure 3
(Part 1).
Atmospheric temperature variation =
80° F
Elongation per bay (∆ℓ) = E t ℓ =
(0.00065 x 80° F x 20 ft. x 12 in.)/100°
F = 0.125 in.
Change in unit stress (σ) = E € t =
(29,000,000.0 x 0.00065 x 80° F)/100°
F = 15.08 ksi.
Force imparted by restraining this ex-
pansion/contraction, P = A E € t = A x
15.08 ksi.
(Where, A is the cross sectional area of
the pipe.)
Given, a 6 in. (150 mm) diameter
Schedule 40 Std. pipe the cross sectional
area, A= 5.58 in2
Weight of the pipe
with water = 12.5 lb/ft., for 20 ft. length
(tributary length on the pipe rack.) the
weight = 230 lb
Friction force between the pipe and the
structural steel support, F = μ N
F = 0.3 x 230 lb. = 69 lb.
P is the force imparted by the pipe due
to expansion and contraction.
P = 5.58 in2
x 15.08 ksi. = 84,146 lb.
Consequently, the friction force is ex-
tremely small compared to the force
imparted by thermal expansion and con-
traction. Additionally, the increase in
magnitude of the assumed friction force
is gradual while the occurrence of slip
overcoming friction is sudden. Thus, the
maximum frictional force and eventual
slip occur at or near the onset of expansion
and contraction of the pipe. Typically,
multiple pipes are supported at any given
tier of the pipe rack. If anchor points are
staggered for each pipe, it would compli-
cate the estimation of friction forces since
these forces oppose each other; however,
these would further reduce their impact
on the supporting structure. In general,
frictional forces on the pipe racks may be
neglected, but local affects, if any, due to
the friction force (F) on the supporting
member, should be considered.
In the event that the pipes are fastened
at each pipe support location and restrain-
ing forces due to expansion/contraction
of the pipes develop, the purpose of provid-
ing any pipe anchor would be defeated.
A B
B
A
54321
(~ 5 m)
16’- 0
W12x26 (TYP)
DENOTES BEAM
TO COLUMN
MOMENT
CONNECTION
SECTION
PLAN
(APPROX16m)
50’-0
(~5m)
16’-0
(~ 6 m)
20’- 0
(~ 6 m)
20’- 0
(~ 6 m)
20’- 0
(~ 6 m)
20’- 0
A B
B
A
5432
(~ 5 m)
16’- 0
W12x26 (TYP)
DENOTES BEAM
TO COLUMN
MOMENT
CONNECTION
SECTION
PLAN
(APPROX16m)
50’-0
(~5m)
16’-0
(~ 6 m)
20’- 0
(~ 6 m)
20’- 0
(~ 6 m)
20’- 0
(~ 6 m)
20’- 0
Figure 5.
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This is primarily due to the fact that each pipe
support bent is providing a restraint for the
pipe against its expansion/contraction force
equal to its tributary length of the pipe support.
In addition, pipe restraint against expansion and
contraction defeats the purpose of providing
the expansion loops, U-shaped attachments
intended to flex with pipe expansion/contrac-
tion, and the pipe anchorages for thermal affects.
Such a system is not only impractical, but is
also not economical.
As illustrated in the numerical example, the
friction forces are very small in comparison to
the forces imparted due to the expansion and
contraction of the pipe material. The general
practice of not fastening the pipes against the
forces of expansion/contraction of the pipes is
the most practical approach.
Stability
Stability of the frames is essential in the design
of pipe support structures. Frame instability
occurs due to initial eccentricities, fabrication
and erection tolerances, dead loads, and the
elastic deformations. In addition to the bracing
required for the applied loads, frame stability
bracing should be provided as shown:
where:
Ab = Area of brace required for stiffness and
frame stability (in2
),
P = Actual axial load in column (kips),
∑P = Axial loads in all columns braced by the
brace being designed (kips),
LB = Bay spacing or the distance between the
columns (inches),
LC = Unbraced length of the column (inches),
and
E = Modulus of Elasticity (ksi).
The minimum area of brace, Ab, is required
for frame stability without consideration of
any applied lateral loads. Structures will be
subjected to instability without this minimum
brace in addition to the brace required for
applied loads. Therefore, the total brace area
Ab(total) required would be Ab plus the brace
area required for the applied lateral loads. This
total area of the brace should be provided at or
near the center of thermal stiffness as shown in
Figure 3 (Part 1).
The number of braced bays should also be
symmetric with respect to the center of thermal
stiffness. In any symmetrical and uniform
structure, the center of thermal stiffness
could be assumed to coincide with the cen-
troid of the structure.
Detailing for Stability
Oversized and slotted holes should be avoid-
ed. Expansion joints are not required in any
pipe rack of less than 500 feet long. However,
at intersecting pipe racks an additional frame
could be located at the intersection if the in-
tersecting pipe rack is a long stretch (Figure 4,
Part 1).
The spacing of these adjoining frames at the
expansion joint need not be greater than the
required spacing for the installation of anchor
bolts and the connections of the structural steel.
Typically, a two foot gap between the columns/
frames is adequate. This would allow both the
columns at the expansion joints to be placed
on one common foundation. Alternatively,
depending upon the length of the intersecting
pipe rack, longitudinal beams could be con-
nected directly to the intersecting pipe rack
with oversized or slotted holes. For short runs
of intersecting pipe racks, no slotted or over-
sized holes for the bolts would be required. In
all cases, only the longitudinal exterior beams of
the pipe racks should be connected to the inter-
secting pipe rack columns. Ideally, if the vertical
bracings and the centers of thermal stiffness are
in the proximity of the intersecting pipe racks,
they could be directly connected with the
standard bolted connections.
Columns are generally oriented with their
strong axis along the length of the pipe rack
without transverse bracing. If transverse bracing
is provided, as shown in Figure 5, column ori-
entation with its strong axis along the transverse
direction provides a more economical design.
Bracing for Stability
Primary bracing systems include:
1) Transverse braces (Figure 5) in the plane
of the bents,
2) Longitudinal braces (Figures 3 and 4;
Part 1) along the length of the pipe
rack, and
3) Horizontal bracing or plan bracing, as
shown in Figure 4 (Part 1).
Plan bracing, as shown in Figure 4 (Part 1), is
not always necessary but should be considered
for pipe racks located in regions of high seismic
risk with weak or soft story pipe rack frames.
Ab = Equation 7
LB
LC
( )
2
E
2[1+ ] ∑PLB
LC
( )
2 3/2
Figure 6: T-Support Column.
CL
CL CL
CL
TYP.
A.B.
NUT WASHER
TYP (NF FF)
FINISHSTIFFENER
GROUT2”
FINISHCOLUMN
1”
A.B. A.B.
SHEAR LUG AS REQUIRED
CONC. FOUND.
HOLES FOR A.B.
TYP (NF FF)
NOTE: BRACE COLUMN IN ITS WEAK AXIS
CL
CL CL
CL
TYP.
A.B.
NUT WASHER
TYP (NF FF)
FINISHSTIFFENER
GROUT2”
FINISHCOLUMN
1”
A.B. A.B.
CONC. FOUND.
HOLES FOR A.B.
TYP (NF FF)
CL CL
CLA.B.
NUT WASHER
TYP (NF FF)
FINISHSTIFFENER
GROUT2”
FINISHCOLUMN
1”
A.B. A.B.
CONC. FOUND.
HOLES FOR A.B.
TYP (NF FF)
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The purpose of plan bracing is primarily to
transmit horizontal applied loads resulting
from pipe anchorages or guides. Guides are
restraints attached to the support bents to pre-
vent lateral displacements of the pipes. Plan
bracing may be provided in heavy seismic areas
if the contents of the pipes carry hazardous
material. Plan bracing would also function
as a collector element and would provide a
horizontal diaphragm to transmit the loads.
This would also permit the transverse bents to
share the lateral loads as described in Part 1
of this article. For long spans, such as at the
roadway crossings of pipe bridges, plan brac-
ing is essential to prevent torsional instability.
Transverse frames and the fastening system
(attachments) of the pipes should be designed
to resist the wind loads without any plan brac-
ing. Therefore, plan bracing is not necessary to
resist the wind forces, particularly if the pipe
rack heights do not exceed approximately 50
feet (15 meters). All interior hanger or trapeze
type pipe supports should be braced in both
orthogonal directions for seismic loads.
All T supports, as shown in Figure 6, require
stability in both the longitudinal and transverse
directions. In the longitudinal direction, vertical
bracing with struts should be provided. In the
lateral or transverse direction of the T support,
the stability of the system depends upon the
base fixity of the T support; that is, the transla-
tion and the rotation of the T support at the
connection of its base must be restrained. In
structural steel W shapes, the flanges can ide-
ally be assumed to resist the flexural demand
of the column and the web may be assumed to
resist the shear force. The stress distribution in
W shapes is shown in Figure 7.
Figure 7: Stress Distribution in Wide Flange Shapes.
COMPRESSION
TENSION
MOMENT SHEAR
Kasi V. Bendapudi, P.E., S.E. is the Chief
Civil, Structural, and Architectural Engineer
with BEK Inc., at Houston, Texas. He can
be reached at kasib46@yahoo.com.
References
1. American Institute of Steel Construction, Specification for Structural Steel Buildings,
Chicago, IL , March 9, 2005.
2. Steel Construction Manual, Thirteenth Edition, American Institute of Steel Construction,
Chicago, IL, April 2007.
3. Yura, J.A., and Helwig, T.A., Bracing for Stability©, Structural Stability Research Council,
AISC, May, 1995.
4. Perry, D.C., “Lecture #1: The Concept of Stability”, Georgia Institute of Technology
(unpublished), 1973.
5. Levy M., and Salvadori M., Why Buildings Fall Down, W.W. Norton Company, New
York, NY, 1992.
6. Bendapudi, K.V., Structural Design of Industrial Facilities, in seminar notes, presented
on September 21-22, 2006; Manchester, NH. Sponsored by American Society of Civil
Engineers (ASCE), Reston, VA.
7. Bjorhovde, R., Columns: From Theory to Practice, AISC Engineering Journal, 1st
Qtr. 1988,
Chicago, IL, (pp 21-34).
8. Bendapudi, K.V., Practical Approaches in the Design of Mill Building Columns Subjected to
Heavy Crane Loads, AISC, Engineering Journal, 4th
Qtr., 1994, Chicago, IL, Vol. 31, No.
4, pp.125-140.
9. International Building Code®
, International Code Council, Inc.®
, Country Club Hills, IL.
10. Process Industry Practices (PIP)©, Construction Industry Institute, Austin , TX, September
2007.
Therefore, in order to provide an elastic, mo-
ment resisting connection, the flanges should
be fastened as shown in Figure 6. The base
plate connection with 4 anchor bolts is similar
to an end plate connection and there would be
a rotational slip depending upon the stiffness
of the base plate and the rotational restraint
offered by the foundations. T-support bases
with two bolts along the strong axis of a
column (Figure 6) are structurally unstable
without the bolt cages connecting to the column
flanges in combination with the longitudinal
bracing with struts. For W shape T-support
columns, the flanges should be restrained in
order to provide for a moment-resisting con-
nection. Such non-seismic connections are
shown in the suggested details of column base
plates in part 4 of the American Institute of
Steel Construction’s (AISC) Manual of Steel
Construction. OSHA requirements necessitate
a minimum of 4 bolts be placed at all the
column bases.
All column bases should be finished and
field-welded to restrain the horizontal shear
at the column bases. Full penetration welds at
the column bases are uneconomical and need
not be used just to resist the horizontal shears
at the column bases. In practice, two C-shaped
fillet welds (between the inside of the column
flanges and along the web) would be adequate.
The transfer of horizontal shear could be
achieved by providing a shear lug at the base-
plate. Structural shapes are not economical or
practical to be used as shear lugs. Flat plates
are very effective as shear lugs and the welds
should be balanced to account for reversal of
stresses and eccentricities.▪
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