Serviceability Tab

A member must be strong enough to carry the specified loads, of course. Measures of strength include shear and moment capacity.

"Serviceability" refers to a different type of design standard. A joist may be strong enough, but if it sags or vibrates too much it will not be considered "serviceable" by the building occupants. Sag or deflection may cause cracking in finish materials that are attached to the joist, or it may be so noticeable that the occupants do not feel secure. Excessive vibration can also erode confidence in the structural integrity of a floor system, or be found annoying.

Deflection

Choose the deflection limits for live and total loading. The number expresses the deflection limit as a ratio of the span. For example, if the limit is set to 360, the deflection must be less than 1/360 of the span. For a 30' span, the deflection must be less than or equal to 1". See the deflection topic for more information.

Sheathing Attachment

Floor stiffness is affected by the method of sheathing attachment, which in turn affects floor deflection and floor performance. The sheathing attachment selection is not visible for beams or roof members.

FloorChoice� Performance Rating

Floor performance controls are visible only for floor I-joists.

The floor performance analysis is optional. Check the box to find the FloorChoice™ rating once the member is designed.

Sheathing

Thicker sheathing increases floor performance by stiffening the joist and by transferring load to adjacent joists.

This control is also visible for open-web floor trusses. Thicker sheathing reduces deflection due to increased composite stiffness.

Floor Topping

A poured concrete topping decreases performance, as the beneficial increase in stiffness is generally offset by the added mass.

Ceiling

Stiffness and therefore performance may be increased by attaching gypsum sheathing directly to the bottom flange of the joist. Select the appropriate thickness. If the ceiling is suspended, select "None."

Blocking

Adding a row or rows of full-depth blocking may increase floor performance. The potential gain is offset by the difficulty in achieving an effective installation. Good workmanship is required to prevent movement of the blocking and the nail squeaks that go with it. Blocking may be considered as a remedial measure in installed floors. Otherwise RedBuilt™ recommends selecting "None" for blocking.