Residential Garage over Wood Framed Floor

[bigmig]

Does anyone have experience with Section 15.1 of the 2005 NDS (Lateral Distribution of A Concentrated Load)? I'm trying to understand some of the limitations of a wood framed garage floor with a 3 to 4" topping slab over a wood system. The NDS describes "at or near the quarter point" as being the worst case shear location. Section 3.4 of the NDS says this location is at d from the support. The garage in mind has two span joists running from a wall to a beam to the adjacent wall. How do I determine the shear load on my joists?

 

[JAE]

Here's my initial take on it:

Section 15.1.1 is solely for the purpose of determining the maximum moment in your garage beams. You take your wheel loads at the center of the span, placed directly over the beam. Table 15.1.1 is used to determine how much of the load is taken by this loaded beam and how much is taken by adjacent beams which are not directly loaded.

So for example if your beams are spaced at 4 feet on center and you have a concrete slab, then the load on the critical beam is S/6.0 = 4/6 = .667. Therefore, your center beam will support 0.667 of the total load and any adjacent beams that are 4 feet away would take the remainder (0.333 x total load).

This would then allow you to design for moment. You place a concentrated load of 0.667(L) at the center of the span and calculate the bending moment. Then design the beam for moment.

Now, to design for shear, you would enter the table 15.1.2 at the 0.667 mark on the left hand column. You would have to interpolate between 0.6 and 0.7.

The first two columns are essentially what you did above - you would have 0.667(L) for the center beam and 0.333(L) for the side beams. Moving to the two columns on the far right, you would get something like 0.75(L)for the center beam and 0.25(L) for the side beams.

So for shear design of the adjacent beams you would use 0.25(L) on the beam and place that load at the 1/4 point of the span to get an end shear.

The center beam shear design would be designed (in my view) for 100% of the load placed anywhere along the length.

What I think they mean by the 1/4 point being the approximate point of maximum shear is this: If you imagine a series of spaced beams (our 4 ft spacing example), then as you move a single concentrated load from the center of the span towards the end, the adjacent beam's shear will initially go up as the load moves closer to the ends.

However, as the load moves past the 1/4 point of the center beam span, the center beam gets pretty stiff (deflection is smaller near its ends) and the ability of the slab to distribute load to the adjacent beams is minimized by that stiffness.

Thus as the load moves between the 1/4 point and the end of the center loaded beam, the shear in the adjacent beams goes to zero and more and more of it remains in the center beam.

So the section 15.1.2 is ONLY about the shear in the adjacent beams.

Section 3.4 still applies for the shear in the center beam as the load moves closer and closer to the end of the span.

I think this whole section (15.1.2) would only be used if the load was pretty static and there were beams that were always "adjacent" to the loaded beam.

For a garage, the wheel load could potentially move all over the place so the design of individual beams would be really based on each beam being a "center", loaded beam.

 

[msquared48]

For the shear load on the floor joists, place the wheel load as close to the end of the beam as the wheel can reasonably reach in the floor structure of the garage. This may or may not be all the way to the end of the span.

For the moment and deflection, place it at the center.

If you have bridging - 8' spacing max - I would put no more than 50% of the wheel load to one joist. If you have reinforcing in the concrete, this should help to span out the wheel load too.

Mike McCann
MMC Engineering