Frequently Asked Questions
Why can't I use the same field modification from a previous job for the same joist type and same dimensions in the current job?
Every joist is designed and manufactured according to the material in inventory at that particular time. Therefore, the material size may differ. A joist will always be designed to carry the minimum of the published catalog load, but the maximum load capacity may be different.
How do I pick a joist size if I do not know where all of the point loads will be located?
You can use a constant shear joist, otherwise known as a KCS joists. First, determine what your maximum shear and moment will be on the joist with all gravity loads added. Second, determine what depth of joist you need. Finally, look in the Quincy Joist Company Catalog on page 41 and choose a joist that meets both the maximum shear and maximum moment requirements.
What if I need to add additional loads to a constant shear (KCS) joist?
If additional loads need to be added to a constant shear joist then the maximum shear and moment using all of the gravity loads needs to be recalculated and compared to the present sized joist's shear and moment capacities in the Quincy Joist Company Catalog on page 41. If the shear or the moment capacities are exceeded then the joist needs to be resized.
Can I add mechanical units to a joist after it has been erected?
If new loads were to be added to joists already in the field, then Quincy Joist Company would need to be notified of the change prior to the addition of the loads and review the design of the joists in question to see if additional reinforcement is required. Please contact your project manager or salesperson if such an incident should occur.
What is dead load camber?
Dead load camber is a misnomer. In actuality, a joist supplier is being asked to camber the joist and/or joist girder for the amount of theoretical dead load deflection based on the project load criteria. If no other special requirements on camber are provided at the time of fabrication, the joists and girders will be fabricated with the SJI recommended camber. The approximate cambers for different lengths are given in the Quincy Joist Company Catalog on pages 17, 55 and 85.
Why do I still have to place additional struts in a constant shear joist when there is a point load applied that does not hit at a panel point?
While a constant shear joist is indeed designed for the worst-case moment and shear, the top chord is not designed to support a point load in bending. For that reason an additional vertical or strut must be field applied when the final point load location is determined and it is not applied at a panel point.
Is x - bridging always required between a wall and the first joist?
No, x - bridging should be used typically for joist-to-joist bridging. If "x" - bridging is used between the first joist and a wall the bridging will actually prevent the roof from properly deflecting and may damage the wall.
Can joists be designed completely cantilevered, i.e. supported on one end only?
No, joists must be supported at two points. If this is not possible due to architectural design, then by presenting all of the requirements to our Engineering department, one of our engineers may be able to provide you with alternate solutions to obtain the same results.
What joist sizes do you keep in stock?
Each and every joist is manufactured to specific design criteria based on each job once the information is received. It would not be practical to try and anticipate what joists to build and just store away, therefore all joists are built once ordered.
Can joists and girders be designed with zero deflection?
No, joists and girders cannot be designed with zero deflection. If very little deflection were required, we would recommend contacting our Engineering Department to coordinate what the desired result is and discussing different options, whether it be increasing the material sizes or providing a larger camber.
What do the SJI designations for joist girders stand for?
A typical girder would be designated by the depth, the number of spaces between the joists and the load at each joist location. For example, a 48G10N9.5K would be 48 inches deep, have 10 joist spaces and be loaded 9.5 kips (9500 lbs.) at each joist location.
What is net uplift and how is it calculated?
Net uplift is equal to the gross uplift minus the sustained dead load and is furnished by the engineer of record on the structural drawings, typically in pounds per a square foot. When designing joists we would need the uplift given to us in pounds per a linear foot. To convert pounds per a square foot to pounds per a linear foot, just multiply by the joist spacing or the tributary area.