28-08-2012, 11:49 AM
PRACTICAL DESIGN AND DETAILING OF STEEL COLUMN BASE PLATES
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INTRODUCTION
Preface
Steel column base plates are one of the most
~ndamental parts of a steel structure, yet the
design of base plates is commonly not given the
attention that it should by engineers. This results
in base plate details that are expensive, difficult to
fabricate and may even contribute to the hazards
of the steel erection process by not providing
stability for erection loads applied to the column.
Base plates serve two basic fianctions:
1. They transfer column loads to the supporting
member or foundation. These loads include axial
due to gravity, moments, shears and sometimes
axial due to uplift;
2. They allow the column to stand as a
temporary vertical cantilever after the lifting line is
released without having to guy off the column.
The column and base plate must withstand
temporary wind and erection loads safely.
Steel fabricators and erectors who are members of
the Structural Steel Educational Council (SSEC)
have commented that there are a variety of base
plate designs and details from engineers. Some
fabricators are critical of many of these designs
because they are difficult to fabricate, or specify
materials that are hard to obtain or that do not
exist in the sizes specified. The designs often
result in columns that are hard to erect or are
unstable without guying the column. When anchor
bolts are not properly set, expensive corrective
work is required before the column can be erected,
resulting in delays in the steel erection process.
This publication of Steel Tips attempts to address
these issues.
Organization
The focus of this issue of Steel Tips is directed
toward the practical aspects of the design and
detailing of base plates particularly as they relate
to economical fabrication and steel erection.
Section 2.0 discusses fabrication issues. Section
3.0 discusses erection and anchor bolt placement
issues. Section 4.0 discusses the "issues" involved
in the design of base plates, rather than providing
"how to" design methods or guidelines, and lists
the names of other authoritative publications
where the reader can find design formulas and
definitive procedures for design of base plates.
Section 4.0 also discusses fixed and partially fixed
column bases, for instance, moment frames which
resist wind or earthquake forces.
DESIGN GUIDELINES FOR
MATERIALS AND FABRICATION
Engineers have numerous types of steel to choose
from when designing anchor bolts and base plate
assemblies. However, materials are often specified
that are not readily available or are not suitable for
specific applications. Base plate details often are
hard to fabricate, overly complicated, call for
expensive welds and/or specify impossible welds.
The following sections provide design guidelines
for specifying suitable materials and suggestions
for details to make fabrication easier and more
economical.
Materials
According to the AISC Specification for
Structural Steel Buildings Allowable Stress Design
and Plastic Design (ASD Specifications), there are
16 ASTM designations specified for structural
applications. For specific material properties,
suitable applications and complete dimensional
information, the reader should refer to the ASTM
Specifications.
Anchor Bolts and Nuts
The most common and readily available anchor
bolt materials are ASTM A36 and A307. Smaller
bolts ge0erally are supplied in A307 and larger
diameter in A36. The material properties for these
relatively "low strength" bolts are very similar.
These two grades are weldable and should be
specified when possible.
Welding
The engineer should attempt to at least match the
thickness of the base plate with the column flange
thickness in order to prevent warping during
welding, particularly if heavy welding, such as
partial or complete penetration welds, is required
to connect the column to the base plate. Thicker
base plates without stiffeners are often more
economical than using a thinner base plate with
stiffeners. Stiffeners, if used, will have an impact
on column finish dimensions. See Section 4.7
"Architectural Issues" for further discussion.
Another common suggestion from fabricators is to
reduce weld sizes as much as possible (but account
for minimum AWS weld sizes based on material
thicknesses) and specify fillet welds in lieu of
complete penetration welds where possible.
Complete penetration welds require more labor
due to the need to bevel the end of the column and
fit up, and require extensive inspection. It is more
economical to detail larger fillet welds, even if
more weld metal is required for the fillet welds, as
a substitute for partial penetration welds.
Design for Temporary Construction
Loads
The first fianction of a base plate is to temporarily
support the column from overturning due to
temporary wind, earthquake, and erection loads,
and from the column getting bumped during
erection until the,beams are attached to "tie in" the
column. Therefore the base plates and anchor bolts
need to be at least sufficient to resist the
overturning moment and shear from these forces.
Although erectors often check the column by
assuming a one kip load applied horizontally at the
top, this does not relieve the engineer from
providing an adequate design.
If the anchor bolts and base plate are too small, for
example, with only two anchor bolts or anchor
"bolts that are too close together, the base plate
assembly may not be capable of resisting erection
loading (See Figure 3).