Structural Stability (Continued)
Ironworkers Setting Column
(Click to play video)
Inadequate anchor rod and bolt installation has been identified as a primary contributing factor to structural collapses. To prevent structural collapse, it is critical erectors
use the proper use of anchor rods (anchor bolts) to ensure column stability. This section is covered by OSHA Standard 1926.755.
When performing steel erection operations, it is very important to follow these best practices:
- All columns must 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, must be designed to resist a minimum eccentric gravity load of 300 pounds located 18 inches from the extreme outer face of the column in each direction at the top of the column shaft.
- Columns must be set on elements that adequately transfer the construction loads, such as the following:
- level finished floors
- pre-grouted leveling plates
- leveling nuts
- shim packs
- All columns must be evaluated by a competent person to determine whether guying or bracing is needed. If guying or bracing is needed, it must be installed.
Anchor Rods (Anchor Bolts)
Approval by the project structural engineer of record is required before anchor rods (anchor bolts) can be:
Before the erection of a column, the controlling contractor must provide written notification to the steel erector if there has been any of the following:
- modification of the anchor rods of that column
Beams and Columns
Inappropriate or inadequate connections of beams and columns is hazardous and can lead to collapses and worker fatalities. This section describes requirements for connecting beams and columns, in
order to minimize the hazard of structural collapse during the early stages of the steel erection process. The requirements of this section are
detailed in OSHA Standard 1926.756.
Releasing the Load
During the final placing of solid web structural members, the load must not be released from the hoisting line until:
- The members are secured with at least two bolts per connection, of the same size and strength as shown in the erection drawings.
- These bolts are drawn up wrench-tight.
- The equivalent, as specified by the project structural engineer of record, except as specified in
OSHA Standard 1926.756b.
Cantilever arms on a suspended truss bridge
(Source: U.S. Marines)
(Click to Enlarge)
A cantilever is a rigid structural element, such as a beam or a plate, anchored at only one end to a (usually vertical) support from which it is protruding.
When subjected to a structural load, the cantilever carries the load to the support where it is forced against by a moment and shear stress. Click on the image to the right to view the cantilever
arms on a suspended truss bridge.
Cantilever construction allows for overhanging structures without external bracing, in contrast to constructions supported at both ends with loads applied between the supports, such as a
simply supported beam found in a post and lintel system.
A competent person must determine if more than two bolts are necessary to ensure the stability of cantilevered members. If additional bolts are needed, they must be installed.
Solid web structural members used as diagonal bracing must be secured by:
- at least one bolt per connection drawn up wrench-tight
- the equivalent, as specified by the project structural engineer of record
Double Connections at Columns
When two structural members on opposite sides of a column web, or a beam web over a column, are connected sharing common connection holes:
- At least one bolt with its wrench-tight nut must remain connected to the first member, or
- A shop-attached, or field-attached seat or equivalent connection device is supplied with the member in order to secure the first member and to prevent the column from being displaced.
(See Appendix H of the OSHA Standard for examples of equivalent connection devices.)
If a seat or equivalent device is used:
- The seat (or device) must be designed to support the load during the double connection process.
- Before the nuts on the shared bolts are removed to make the double connection, it must be adequately bolted or welded to both:
- a supporting member
- the first member
Each column splice must be:
- designed to resist a minimum eccentric gravity load of 300 pounds
- located 18 inches from the extreme outer face of the column in each direction
- located at the top of the column shaft
Perimeter columns must not be erected unless:
- They extend a minimum of 48 inches above the finished floor, to permit installation of perimeter safety cables prior to erection of the next story.
- They have two sets of holes, or other devices that:
- are 42-45 inches above the finished floor, and also at the midpoint between the finished floor and the top cable
- permit installation of perimeter safety cables as required by 29 CFR 1926.760(a)(2)
In multi-story structures, when holes in the column web are used for perimeter safety cables, the column splice should be placed sufficiently high so as not to interfere with any attachments
to the column necessary for the column splice.
- Column splices are recommended to be placed at every other or fourth levels as design allows.
- Column splices at third levels are detrimental to the erection process and should be avoided if possible.
EXCEPTION: Where constructability does not allow, the above requirements can be waived.
(See Appendix F of 1926.750.)
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