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Re: Railing design

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Mark Trimble wrote:

> Subject: railing design
>
> i have a handrail that has 1/2" X 1/2" verticals at 4" o.c. with a
> nominal flat stock top horizontal.  in applying the code required 200 #
> horizontal load what effective width is allowed.  another words how many
> verticals can contribute?
>
> another question is, what is the codefied deflection criteria?
>

It seems that the responses so far have been somewhat confusing and perhaps
ill-informed.  I will try a fairly comprehensive discussion and would appreciate any
comments/corrections.

First of all, we must define our terms.  What you are describing is a guardrail, not
a handrail.  This is an important distinction in order to read the load table
correctly.

A HANDRAIL is solid member of a particular size that is grasped by the hand, and is
only required on stairways and on some (steep) ramps.  Sometimes it is required on
both sides of a stairway or ramp, and sometimes on one side only.  Stairways with
fewer than four risers (residential only) need not have handrails.  The height of the
handrail above the nosing of the treads must be between 34 and 38 inches, measured
vertically.

A GUARDRAIL is the complete assembly, from the floor deck to the required height,
that prevents a person from stepping off the edge of a floor deck, off the side of a
stairway, into an opening in the floor, etc.  In a stairway,  there is typically
either a wall OR a guardrail on both sides of the stariway.  In other locations (edge
of deck, etc.) a guardrail may be required where there is no wall.  The guardrail, in
most cases, must be 42 inches in height.  However, there are a number of exceptions,
including single-family residences (interior and exterior) and the interior of
multi-family residential occupancies (where the guardrail may 36 inches), and for
theater balcony railings where there are fixed seats (and not at the end of an
aisle), where it may be at 26 inches.  The other exception to the guardrail height
prescription is for guardrails on a stairway (but not at the landings), where the
guardrail may be at the handrail height (and therefore the top rail of the guardrail
may be the handrail).

Now we are done with the architecture, let's do some engineering.

The 1994 or 1997 UBC, Table 16-B, provides all the answers to your first question.

Item 9 in the table lists 3 categories and 3 numbers, and refers to footnotes 8 and
9.  The first two are in the units of plf (per footnote 8) and refer to the required
horizontal load at the top rail of the guardrail.  The third item refers to the
components of the guardrail exclusive of the top rail, which are referred to as
"intermediate rails" in footnote 9 (usually referred as balusters when they are
vertical).  For this load, you must use 25 psf, applied over the whole area, as if it
were solid.  Therefore, your 1/2" by 1/2" verticals (at 4 " oc) must withstand a
loading of 25 x 4/12 = 8.33 plf each.  This load must be carried to its supporting
members, which may include top rails, posts, etc, to the supporting structure.

Footnote 9 also says "Reactions due to this loading need not be combined with those
of footnote 8."  Therefore, if the balusters are directly connected to the top rail,
we must determine which controls, the "components" portion or the "top rail" portion
of Item 9.  Assuming the guardrail is at 42 inches and that the balusters extend down
to the deck, the components portion would be 25 psf x 3.5 ft / 2 = 43.75 plf.  So, if
the guardrail is along an exit facility serving 50 or more, then 50 plf would govern
for the top rail, if not, then the 43.75 plf would govern over the 20 plf you would
otherwise use.  Don't forget, there are other configurations where the intermediate
rails are not connected to the top rail, but rather have members that span
horizontally to the vertical posts, and are thus independent structurally from the
top rail.  In that case, the 20 plf on the top rail would apply.  In either
configuration, the required loading to the posts would be the worst case resulting
from either the "top rail" or the "components".

The 200 pound force (point load) you refer to is for handrails, which is Item 11 in
Table 16-B, including footnote 11.  Of course, if the handrail and the top rail of
the guardrail is the same member (as dicussed above), you must decide which item
controls, depending upon the spacing of the connections of the handrail.  Don't
forget that this 200# force must be applied in any direction to the handrail.

So, in summary, define your terms, apply your loads to each component and determine
the load path to the supporting structure, but don't combine loads that need not be
combined.

As for deflection, I believe that there is no code limit, since Table 16-D refers
only to floor or roof members.  L/120 sounds good to me as rule of thumb.

Good luck

Mark T. Swingle, SE