For years, the steel joist industry standard has been that loads either need to be applied at panel points, or a member brace (strut) must be added in the field. In response to inquiries from structural engineers and other members of the Building Team, the Steel Joist Institute recently issued a new rule for loads not exceeding 100 pounds within any given panel.
The full text of the SJI rule reads as follows: “For nominal concentrated loads between panel points, which have been accounted for in the specified uniform design loads, a strut to transfer the load to a panel point on the opposite chord shall not be required, provided the sum of the concentrated loads within a chord panel does not exceed 100 pounds and the attachments are concentric to the chord.”
Bruce Brothersen, engineering manager for Vulcraft-Utah, Brigham, Utah, says, “The rule is an attempt to better define what is acceptable and put some quantitative numbers to it.”
Tim Holtermann, corporate engineering manager for Canam Steel in Washington, Mo., says that even with the new rule, manufacturers remain concerned that “people may be hanging things in excess of 100 pounds away from panel points and not field-adding struts, as they’re required to do.”
Holtermann says one option for structural engineers is to ask the manufacturer to provide a specially designed joist that can take point loads over 100 pounds without the added members. “It’s not uncommon to have a design requirement for, say, 250 pounds or 400 pounds,” he says. “We can readily do that as long as the magnitude of the loads and this design requirement are specified on the contract drawings.”
DON’T WAIT FOR THE FINAL PUNCH LIST
Often the problem with the joist loads isn’t discovered until the project is completed and the final walkthrough is taking place, says Joe Pote, director of research and development for New Millennium Building Systems, Hope, Ark.
“The inspector notices ductwork or sprinkler lines or mechanicals hanging from the joists between panel points that aren’t called out on the contract drawings,” says Pote. “Then he comes back to us and asks if it’s okay.” Many times the loads are small (20-50 pounds), so additional reinforcement isn’t necessary. “But we never had a good rule that stated the maximum load,” he says. “I think it helps clear up some of the discrepancy between SJI requirements and actual field practice.”
The manner in which the loads are hung is a key element of the 100-pound rule, says Holtermann. “The rule presumes that the load will be placed concentrically,” he says.
As SJI managing director Ken Charles notes, some trade installers make the mistake of attaching beam clamps to only one bottom chord angle of the joist instead of both. On top of that, he says, “the load isn’t placed concentrically even to that one chord angle, so you’re inducing some torsion and bending of the horizontal leg of the angle.”
His advice: “By far the better way to do it is to make sure the attachment is always concentric and always attached to both chord angles.” (See diagram.)
Steel Joist Do’s and Don’ts
- Recognize how and when to specify total uniform load or factored load and/or net uplift load, and when loads need to be broken out by load category and combinations.
- Know the limits of a concentrated load applied between panel points before an additional web is needed on the joist. The Steel Joist Institute’s new “100-pound rule” clarifies these limits.
- Install bridging early and follow the proper procedures to avoid safety problems, such as a partial building collapse.
- Designate a field representative to inspect the joists during construction and ensure that components are being installed correctly.Ideally, a representative of the team should walk the job site before completion and look for situations where loads aren’t handled correctly.
“A lot of times there’s a disconnect between the design intent and the construction site personnel,” says Charles. “Some sites have representatives in the field to do inspections, but often that’s not the case. Those are the projects that are most likely to have problems down the road when the final punch list is done.”
GET YOUR SPECS STRAIGHT
Joists are designed to handle different types of loads, such as dead load, live load, and wind, snow, and seismic loads. A given joist might experience more than one type of load, so the joist manufacturer needs to know which loads apply and properly combine them according to parameters set by the International Building Code. Most jurisdictions use the 2006 version of the IBC or the 2009 version, which is similar with regard to steel joists.
Brothersen notes that the building code loading combinations are based on the probability of which loads will act simultaneously. “For instance, the probability of a seismic event occurring when the maximum snow load is on the joist is relatively small, so the building code reduces the loads in those cases,” he says. “This is more realistic and doesn’t require manufacturers to design for the maximum load in every category at the same time.”
Pote says manufacturers prefer, when it’s feasible, that loads be provided to them as either a total uniform load or a factored load. In other words, if all components of the joist can be designed for a single downward-acting load case and a single upward-acting load case, the easiest thing for engineers to do is provide the total uniform load and/or net uplift load (dead load minus wind load).
“It’s just easier to communicate,” says Charles. “However, if we have to consider multiple load combinations, we need the breakdown—dead load, live load, snow load, and so forth.”
Holtermann adds that if the structure might be affected by downward-acting winds, the total uniform load should assume the worst-case scenario.
DON’T FORGET THE BRIDGING
Once the joists arrive on site, it’s important that bridging be installed early and correctly, says Brothersen. “If the joists are put up but the bridging isn’t installed, it can become a safety issue.” For example, lack of bridging can result in a partial building collapse when the crew begins stacking bundles of deck material on the joists.
Charles agrees. “The best-laid plans of the engineer and manufacturer can go out the window when the product arrives at the job site and the erector doesn’t follow proper safety regulations and the manufacturer’s advice.” BD+C
--
Editor’s note: To order SJI’s “43rd Edition Catalog of Standard Specifications and Load and Weight Tables for Steel Joists and Joist Girders,” go to: http://steeljoist.org/free-download-form.
Related Stories
| Aug 26, 2013
Chicago Bears kick off season at renovated Halas Hall
An upgraded locker room, expanded weight room, and updated dining room with an outdoor patio greeted the Chicago Bears when they arrived at Halas Hall for practice this month. The improvements are part of a major expansion and renovation of the Bears’ headquarters in Lake Forest, Ill., completed by Mortenson Construction in less than seven months.
| Aug 22, 2013
Energy-efficient glazing technology [AIA Course]
This course discuses the latest technological advances in glazing, which make possible ever more efficient enclosures with ever greater glazed area.
| Aug 22, 2013
Warehouse remake: Conversion project turns derelict freight terminal into modern office space [slideshow]
The goal of the Freight development is to attract businesses to an abandoned industrial zone north of downtown Denver.
| Aug 21, 2013
AIA: Architecture billings on the rise in July
The Architecture Billings Index for July was 52.7, up from a mark of 51.6 in June. This score reflects an increase in demand for design services (any score above 50 indicates an increase in billings).
| Aug 20, 2013
40 Under 40 retrospective: ‘U40s’ take on continuing ed, snake’s blood
Every month we’ll be touching base with past 40 Under 40 honorees to see what’s been happening in their professional and personal lives since winning the award. This month: An accomplished author of test-prep books and an architect who headed to China when the American economy turned sour.
| Aug 20, 2013
Top Data Center Architecture Firms [2013 Giants 300 Report]
Corgan, Gensler, HDR head Building Design+Construction's 2013 ranking of the largest data center architecture and architecture/engineering firms in the U.S.
| Aug 20, 2013
First look: $550 million Billie Jean King National Tennis Center renovation
The United States Tennis Association has announced its plans for a sweeping transformation of the USTABillie Jean King National Tennis Center that will include the construction of two new stadiums, as well as a retractable roof over Arthur Ashe Stadium. The transformation will be implemented in three phases to begin at the conclusion of the 2013 US Open, with the goal of overall completion by the 2018 US Open.
| Aug 15, 2013
Design-build project delivery holds steady at nearly 40% of nonresidential construction: DBIA study
The Design-Build Institute of America (DBIA), the only organization that defines, teaches and promotes best practices in design-build, has released research indicating that design-build project delivery represents nearly 40 percent of total market share in the United States, based on dollar value at the end of 2012.
| Aug 14, 2013
Five projects receive 2013 Educational Facility Design Excellence Award
The American Institute of Architects (AIA) Committee on Architecture for Education (CAE) has selected five educational and cultural facilities for this year’s CAE Educational Facility Design Awards.