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CROSS Safety Report

Steel column erection anchoring connection failure injured worker

Report ID: 992 Published: 10 May 2021 Region: CROSS-US


Overview

This report describes how a construction worker was injured during the erection of a steel column due to shortcomings in erection procedures and inadequacies in a column base connection.

Key Learning Outcomes

For all built environment professionals:

  • Be mindful that accidents caused by instability are not uncommon in structural steel erection

  • The erector is, generally, responsible for the method and sequence of erection. Be aware that in unusual cases, design contract documents may prescribe or control certain aspects of erection methods and sequence.

  • Some industry standards, particularly OSHA 1926, have specific requirements for steel erection

For steel erectors:

  • It is good practice to review and assess the capability of column base connections

For structural design engineers:

  • It is good practice for design contract documents to be clear in defining responsibility for steel column base details. This includes defining design responsibilities for both in-service conditions and erection conditions. Be aware that responsibilities may vary from project to project.

For fabricators and their detailers:

  • It is good practice to understand the project requirements for column base connections, including definition of responsibility for the connection design

Full Report

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The column was a TS 6x6 (15x15 cm tube), two-stories tall. The design of the anchoring system consisted of four 3/4 in (1.9 cm) diameter all-thread bolts embedded into the concrete foundation and welded to a leveled base plate. The column had a 1 in (2.5 cm) thick base plate welded to its end. This base plate had two 1-1/4 in (3.2 cm) (nominal 1 in) holes to receive two 5/8 in (1.6 cm) diameter bolts that the contractor field welded to the leveled baseplate. The column was temporarily secured with 1-3/4 in (4.4 cm) diameter by 1/8 in (3 mm) thick washers and 5/8 in (1.6 cm) nuts. The column base detail is shown in Figure 1.

From top to bottom: The TS 6 by 6 column is shop welded to a 1” thick base plate. The base plate is secured to a 3/4” thick “leveled plate” immediately below by two 5/8” diameter all thread bolts welded to the leveled plate. The leveled plate bears on a concrete footing and is anchored to it by four ¾” diameter all-thread anchors embedded in the concrete. Figure drawn by REM Arch LLC.
Figure 1: Column base detail showing base plates and anchors

During steel erection, while a worker was halfway up the column for the purpose of connecting a beam, the column fell over (possibly pulled down by the crane) injuring the worker not only from the fall, but the column landed on his legs.

During steel erection, while a worker was halfway up the column for the purpose of connecting a beam, the column fell over

There are multiple ways this accident could have been avoided. First, the steel erector did not secure the column with bracing. Second, the project was in an area of the country where the structural engineer does not design steel connections. However, the design drawings contained a detail that provided the contractor with the option to field weld the two 5/8 in diameter erection bolts to the embedded plate. Thus, the engineer accepted some responsibility for the design of the connection and for the contractor means and methods of erection. The provided shop drawings, reviewed by the engineer, contained the suggested detail without sufficient information to evaluate its capacity. The responsibility for determining the adequacy of the connection fell into the gap between the engineer and the fabricator/erector. The failure of the bolt weld was the weak link in the connection detail. Moreover, both the fabricator/erector and engineer failed to recognize the OSHA requirement that ‘all columns shall be anchored by a minimum of 4 anchor rods (anchor bolts).’ While four anchor bolts may not have prevented the accident, the designer of the connection should have been aware of the OSHA requirement.

Expert Panel Comments

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Steel erection accidents like the one reported herein are not uncommon. Responsibilities amongst designers, fabricator/detailers, and erectors can be complex and may be unclear. Consequently, written standards of practice such as OSHA 1926 and the AISC Code of Standard Practice (ANSI/AISC 303-16) have become more detailed and definitive with time.

Responsibilities amongst designers, fabricator/detailers, and erectors can be complex and may be unclear

Generally, and unless the contract design documents prescribe otherwise, responsibility for erection means and methods resides with the erector. Where erection stability depends, in part, on the strength and configuration of connections, as in the case of column base connections, responsibility can be obscured. There is insufficient detail in the report to precisely ascertain the responsibilities for the final column base detail design amongst the structural designer, the fabricator, and erector in this case. While CROSS has a no-blame culture, numerous learning outcomes can be drawn.

OSHA 1926.750 has extensive provisions for steel erection. 1926.755(a) requires that:

  • ‘All columns shall be anchored by a minimum of 4 anchor rods (anchor bolts).’

  • ‘Each column anchor rod (anchor bolt) assembly, including the column-to-base plate weld and the column foundation, shall be designed to resist a minimum eccentric gravity load of 300 pounds (136.2 kg) located 18 inches (0.46m) from the extreme outer face of the column in each direction at the top of the column shaft.’

  • ‘Columns shall be set on level finished floors, pre-grouted leveling plates, leveling nuts, or shim packs which are adequate to transfer the construction loads.’

  • ‘All columns shall be evaluated by a competent person to determine whether guying or bracing is needed; if guying or bracing is needed, it shall be installed.’

If the design of a column base connection is shown fully on the design documents, the connection must have four anchor bolts as required by OSHA. Normally then, the structural engineer will design the connection, as a minimum, for the eccentric gravity load required by OSHA, as well as the in-service loads. If the structural engineering design professional intends for the fabricator/detailer or its structural engineer to design the connection, then the contract documents must clearly state so, including specification of the required design capacity. The fabricator/detailer must also design the connection for erection conditions, again, as a minimum for the OSHA prescribed eccentric gravity load.

In the case where the fabricator/detailer designs the connection, final responsibility for the connection’s adequacy depends, again, on the requirements of the contract documents. Often responsibility for a connection’s design adequacy remains with the structural engineering design professional regardless of who performs the function, but this is not always the case. See, for example, Option 3 of AISC/AISC 303-16 referenced above, in which connection design is delegated to another registered professional engineer who works on behalf of the fabricator/detailer.

Clarity of responsibility for column base connection design in contract documents is critical.

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