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Anchor Block Design Made Easy
Conference Paper · June 2013
DOI: 10.1061/9780784413012.048
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Anchor Block Design Made Easy
Chris Sundberg, S.E., SCWI (Senior Certified Welding Inspector)
Principal Structural Technologist
CH2M HILL, Inc., 1100 112th Avenue NE, Suite, Bellevue, WA 98004
Phone: (425) 453-5000, Fax: (425) 468-3100, Email: csundber@ch2m.com
Abstract
Anchor blocks are a key component of many above ground pipelines. Anchor Blocks are typically used in
water-works applications where thrust restraint of a pipeline is desired. Some design guidance can be
obtained from several sources including Steel Penstocks, Second Edition, ASCE Manuals and Reports on
Engineering Practice No. 79, however the analysis and design of anchor blocks for water-works
applications could use some improvements for easy use in the design office.
This paper will address the fundamentals of designing anchor blocks for elevated welded steel pipe and
will explore the geometry and mathematics with emphasis on design simplicity. Significant
computational advantage can be obtained by application of vector algebra to resolve forces that often
act along the pipeline axes. Further computational simplification is possible by approximating the center
of gravity of the anchor block, recognizing a volume is calculated by revolution of a planar section and
application “Pappus’s Centroid Theorem”. A common design issue is what to do when stability cannot be
achieved economically by adding more concrete weight? Adding rock anchors to provide stability is
typically more cost-effective than adding weight. The goal of this paper is to provide a useful tool to
pipeline designers who may have struggled in the past with design issues related to anchor block design.
An example problem is included.
Figure 1 indicates an idealized anchor block for an elevated welded steel penstock where geometry of
the bend is defined. The anchor block is constructed of reinforced concrete and the steel penstock
passes through it as indicated in Section A. Often for constructability and forming purposes, concrete
anchor blocks are built with multiple straight sided surfaces with tangents that extend beyond the
points of tangency of the bend. Analysis of the component prisms that make up an anchor block can
make calculations of volume and center of gravity very tedious and complex. In order to simplify
calculations, an idealized anchor block is substituted as shown in Figure 1 where a planar section is
rotated about an axis forming a geometric solid of revolution. With this figure, simple calculations can
be performed to compute volume and center of gravity with reasonable accuracy, recognizing that a
precise model requires more computational effort.
Typical anchor block forces include (1) the axial forces acting along the penstocks reaches both
upstream and downstream of the anchor block and include gravity loads from penstock and water,
friction loads from pipe supports, friction loads from expansion joints, pressure loads acting on ends of
expansion joints and internal pressure, (2) the fixed end beam moments and shears acting at the faces
of the anchor block where the penstock penetration occurs, (3) the gravity load from the anchor block
itself that includes components of weight from concrete, water and steel within the anchor block.
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Figure 1-Anchor Block Detail
To properly evaluate all the effects of all the above forces requires very detailed calculations. However,
the author believes that reasonable preliminary design accuracy can be obtained by selecting the
primary loads only that include (1) destabilizing force from internal pressure force acting at the anchor
Pipelines 2013 © ASCE 2013
block along the bend bisector together with (2) stabilizing force from the anchor block weight.
Resolution of forces and moments about the base of the anchor block is necessary to calculate, sliding
and overturning stability and bearing stresses. The selection of a plane to perform the analysis should be
based upon an understanding of the geology of the site however, Line “A” in Figure 1, located near the
base of the anchor block at right angle to the bend bisector has been chosen for illustration. The midpoint of Line “A” is the location where summing of forces and moments is performed. Typically the
anchor block alone is inadequate to provide stability and the addition of weight in the form of prestressed rock anchors will assist in attaining stability.
Steps to Anchor Block Analysis and Preliminary Design;
1. Assume plan/profile layout geometry of anchor block where pipeline anchorage is desired.
2. Find CG of anchor block (steel pipe, water and concrete) using Pappus’s Centroid Theorem
where anchor block is assumed to be a planar section rotated about a constant radius and
central point that results in a volume and center of gravity.
3. Determine magnitude and direction of primary thrust load. This is usually the pressure
component of thrust which, for a non-reducing bend, is of magnitude T=2*PA*sin(delta/2),
acting along the bisector of bend. The dynamic portion of thrust also acts along the bisector of
the bend, however, this force is usually small when compared to the pressure component of
thrust and can be ignored. The validity of this statement is supported by the example problem
that demonstrates the dynamic thrust is about 1% of the pressure thrust.
4. Determine magnitude and direction of other secondary loads. These include axial pipeline forces
(gravity loads from penstock and water, friction loads from pipe supports, friction loads from
expansion joints, pressure loads acting on ends of expansion joints) and anchor block external
forces, from beam shear and beam bending moments, acting at faces of block. For simplicity of
presentation, secondary loads are not included in this paper as they have only a minor effect on
the example problem results however, in any detailed design, secondary loads should be
included.
5. Resolve all forces on a plane normal to anchor block bend bisector.
6. Determine required pre-stress force necessary to keep base entirely in compression and check
heel and toe pressures to verify rock or soil bearing capacity is adequate. Rock anchors that are
based upon a 1.375 inch diameter threaded bar per ASTM A722, Grade 150 pre-stressing steel,
develop a design force of approximately 100 kips each (45% of the tensile strength for normal
design conditions) and, for constructability, should be configured on a 3 ft x 3 ft. pattern as
necessary to engage adequate rock or soil weight and to fit available anchor block footprint
space.
7. Check base shear and overturning moment safety factors to verify they are adequate for all
hydraulic conditions. For Normal conditions a sliding and overturning safety factor greater than
1.5 is desirable.
8. Check other hydraulic conditions (Intermittent, Emergency and Exceptional) to verify design
9. Alter layout geometry and repeat steps 1 through 8 as necessary to arrive at optimal anchor
block geometry.
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Summary
This paper has addressed the fundamentals of design of anchor blocks for elevated welded steel pipe
and including detailed geometry and mathematics with emphasis on design simplicity. Application of
vector algebra to locate critical points and resolve pipeline forces has been presented. The
computations necessary to find the location of center of gravity of the anchor block is substantially
simplified when compared to conventional methods of taking components of the block and their
respective moment arms to determine the CG, recognizing a volume is calculated by revolution of a
planar section and application “Pappus’s Centroid Theorem”. Dynamic thrust has been demonstrated to
be a small fraction of pressure thrust in most waterworks applications and can be ignored in typical
anchor block analysis without incurring a significant error. A common design issue regarding stability
that cannot be achieved economically by adding more concrete has been addressed by addition of rock
anchors. This paper has provided useful tools to pipeline designers who may have struggled in the past
with design issues related to anchor block design. An example problem demonstrating simple
techniques has been included.
References
ASCE (2012). Steel Penstocks, Second Edition, ASCE Manuals and Reports on Engineering Practice No. 79.
AWWA (2005). AWWA C200, Steel Water Pipe-6 In. (150-mm) and Larger.
Calculus, Thomas Finney, Ninth Edition, 1998, by Addison-Wesley Publishing
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