Within the last few years, in an effort to provide for more uniform
design criteria throughout the country and world, building departments
have gone to a more uniform code for the design of buildings, the
Code, IBC. Previously there was the Uniform Building Code (UBC)
which was used in
most the midwest, west and around the world. Building
Code of America, BOCA, was used mostly on the east
The SBC, Southern Building Code, was used mostly in the south.
also had their own codes but they were based on one of those 3 codes
and modified. Still, various cities, counties and states make
modifications to suit their local conditions but they are all now
the International Building Code.
Gravity loads consist of the actual Dead Load weight of
materials. These Dead Loads are always present and must be
accounted for in the design of any structure.
Live Loads, sometimes called service loads, represent the type of
loading a structure will undertake
from use. These take the form of moveable non fixed items that
may include such things as furniture, fixtures and
people, all referred to as transient or service loads. The IBC
defines these Live Loads. For roofs it is generally 20 pounds per
square foot (psf) and residential floor loading of 40 psf. These
Loads are consistent
everywhere. If your building is considered storage, it could be
classified as light storage and need designs for 125 psf.
Wind loads vary throughout the country. Some isolated
areas have special wind loads because of various terrain conditions or
because they are subject to severe conditions such as hurricanes, on
the top of a hill, within a confined area creating a wind tunnel affect
or any number of various other conditions. These special
conditions need to be confirmed with the local building
officials. The IBC defines the wind speed in all areas of the
country at 30 feet above ground. Adjustments are then made based
on height, shape, location within terrain, exposure to elements, and
of the structure.
These wind speeds are then used to calculate the actual wind pressure
at various heights and locations on the structure. These wind
pressures are applied to the
components of the tower to establish the lateral forces on the
tower. The minimum wind speed in an 80 mph area produces a
of about 18 psf, which is based on the formula .00256x wind
speed. The highest wind speeds are in coastal hurricane
areas of about 150 mph. Once the pressures to the structure are
determined these are used to determine the size of
members, size and type of connections, and the width, length and depth
of the foundations.
Under the older codes, it would take about 5-15 min. to determine the
wind loads. With the new codes it takes 6-10 pages of
to go through the design process to determine the loads in all
conditions for the structure. It's much more cumbersome than in
the past and often takes an hour or 2 to determine all the loads.
Seismicity exists everywhere and is investigated for every tower
design. Areas such as the west coast have high seismicity and
more severe earthquakes. Other areas such as the mid west and
east coast have very little to no seismic activity. This is
accounted for in the building code by establishing various seismic zone
factors based site geologic conditions, probability of occurrence and
potential magnitude. Areas such as the west coast
will have higher seismic design factors than the mid west and east
coast and often these will be the governing factors in the tower
design. Other areas, the wind loads will be the governing
factor. Even though you may be in an area of low seismicity,
there is still a potential for ground vibration from earthquakes far
Another thing to be aware of is that even though your site may be in an
area of low seismicity such as the mid west, the largest recorded
earthquake ever was at the New Madrid fault in Missouri. Ground
shaking was felt on both coasts and it went on for days. It
doesn't happen often, fortunately, but there is always a potential for
future activity. Likewise, even though Georgia is pretty much a
very low seismic area, there was a very large earthquake at one time
very near the coast. The codes address all of these situations,
bases on probability, and local site geologic conditions.
Snow loads vary throughout the country. The normal design live
loads for roofs is 20 psf for flat roofs which often reduces to 16-12
psf for sloped
roofs. In areas of low snow, these loads are not a factor because
the normal live loads will be greater than the snow loads. In
areas of high snow, the snow load will be the governing factor.
The IBC defines the basic snow load for various regions throughout the
country but there are areas where it defers to the local building
officials to make a determination of the amount of snow. This
occurs especially in mountainous regions because of high concentration
in some terrain's. Once this ground snow
load is established, this number is used to compute the pressure
exerted by the snow at various heights or slopes of roofs and
pressures are then applied to the tower to establish the forces on the
As I am sure you are aware, soils and geologic conditions vary from
site to site. Every region has a different type of soil.
Some areas may have a lot of sand, clay or rock and various
combinations of all of these. The foundations for these towers is
based on a suitable foundation material that will sustain at least 1000
pounds per square foot pressure. Most soils will meet this and
most will allow for more than this. Typically, unless there are
known problems, the building codes have allowed designers to use this
minimum soil pressure of 1000 psf without any site testing. The
new IBC, however, specifically states that every site shall have a
soils or geologic tests to determine both the vertical load
capabilities and various factors used to determine how it will react
under seismic conditions, no matter how large or small the
building jurisdictions will, however, make an exception to this
requirement based on known information, size or type of structure, or
other reasons known only to them.
The point of this is that it may be possible your local building
department will not accept the design of the tower and footings unless
a geologic report is done. I design all the towers based on
minimal soil values but if
your local building department requires more
geologic information, you may be required to obtain a report which can
several weeks and cost thousands of dollars.
You may want
verify with your local building department in advance.
Some areas of the country undergo severe cold and freezing. In
these areas, the ground freezes. In doing so, the ground expands
and can move the foundations. In such areas it is required that
of the footings extend beyond a point were the ground will
freeze. If this were not done, there would be considerable
movement of the footing resulting in damage to the structure. If
you are in an area of ground freeze, it may be required to lower the
footings and or install expansion materials around the footings.
Areas of ground freeze need to have the bottom of the
footings anywhere between 2-5 foot deep depending on how much ground
freeze occurs. Verify what it is in
your area if you are in an area of ground freeze.
Code related fire issues may come into play with in a variety of
In rural areas where there is a high potential for wild and forest
fires, it is often a requirement that the plans be processed through
the local or state Department of Forestry, or similar named
department. Their concerns are generally that there is access to
the structure such that fire fighting equipment can get to it.
They often require a space where they can turn vehicles around and may
require a water source they can hook up to. They also generally
have some requirement about removing vegetation for a certain distance
around the structure.
In areas of high fire threat, such as areas of California, there are
now fairly strict requirements not only on site conditions but also on
building materials and systems. The intent is to minimize the
potential for fires by imposing restrictions on construction and
materials for eaves, windows, doors, openings in roofing or siding
etc. You can find more specific regulations for these issues from
documentation from the State Fire Marshall. The state of
California has issued a document that more specifically defines these
regulations. The Wildland Urban Interface (WUI) Products
requirements can be seen at: http://osfm.fire.ca.gov/strucfireengineer/pdf/bml/wuiproducts.pdf
Open towers often come under the classification of agricultural or
accessory buildings. As such, their requirements are not a
stringent as enclosed occupied structures.
Enclosed Non-Residential Towers:
These type of towers are often part of a water storage or producing
system. They may consists of attics, pump rooms, storage rooms or
similar uses. The requirements for these is not too much
different than for Open Towers but they will require access, maybe some
lighting and possibly more fire resistance.
A Lookout Tower is often an open framed tower with an enclosed room on
top. The room on top will have to meet some of these requirements.
For non-residential use, the tower will most likely be classified as a
"U" occupancy. If it is storing somewhat more hazardous
materials, it may be classified as an "S" occupancy. The towers
are built as a Construction Type "V", either A or B. A
construction type V-A is allowed 9,000 sf, 2 stories and 50 ft. height
max. A construction type V-B is allowed 5,500 sf, 1 story and 40
ft. height max. There are allowable area increases if you
are set back far from from the property lines. You can get a 1
story increase by having a fire sprinkler system. You can also
get higher limits on heights and area with a fire sprinkler
In all the years I've been doing towers, which is many, I've never seen
a situation where they have required a fire sprinkler system.
I've also never had a situation where they have denied the construction
based on these requirements. It doesn't mean you won't have to
address these issues as most building departments are pretty lenient on
agricultural type buildings. It is something you should be aware
of and something you should check with your local building department
before ordering plans. The plans I provide assume this will not
be an issue. Any problems of this type will be solely the
responsibility of the buyer of the plans.
Another consideration is for higher towers to use a different
classification, Type IV. This is referred to has "Heavy
Timber". These requirements require main posts to be at least
8x's, 6x10 min. for floor joists and beams, and 4x8 min. for roof
rafters. It also requires a minimum of 2x decking on roofs and 3x
decking on floors. All this makes for a good fire resistant
building. What it allows you is to increase the height to 65 ft.,
4 stories, and 18,000 sf. The same increases apply for fire
sprinkler systems and property line set backs.
I've designed towers taller than this as heavy timber and not had a
problem with the building departments. There always is a chance
you may have to address these issues, again I suggest checking with
your local building department before ordering the plans.
Enclosed Residential Towers:
For residential use, there is a whole different set of
requirements. Besides the requirements noted above for
non-residential use, the tower is now classified as an "R-3" type of
occupancy. For this type of occupancy you are limited to 3
stories for either Type V-A or Type V-B construction. For Type IV
you are limited to 4 stories. The areas are not limited and
increases are allowed for set backs and fire sprinkler systems as noted
above. Under many jurisdictions though, they require a fire
sprinkler system for residences 3 stories and taller. This is
something you should verify with your local building department prior
to ordering plans. You will need a water source for this.
There are a number of other fire related requirements that will also
come into play. This may include, but not be limited to, such
things as, fire rated gyp board under stairs, double pane windows,
smoke detectors, fire rate doors, windows need to be large enough to
escape, electrical wiring etc.
If you have an enclosed occupied tower you will be required to heat
it. This is a building code requirement that will require you to
have some sort of heater whether it be solar, fuel fired or
Most jurisdictions these days have some sort of energy requirement that
must be met for heated buildings. This may take the form of
requiring such things as a certain amount of insulation, efficiency of
the heating device, sealing of penetrations of the building envelope,
window and door infiltration etc. Site location, orientation,
and a variety of other issues are used in making energy use
determinations and conformance. It usually requires a separate
set of calculation documents by an architect, mechanical engineer or
energy consultant. These issues are not something I do as part of
the tower plans I provide. I can do these separately or you can
have other local design professionals address these issues if needed.
Some people like to use wood fireplaces for heating but here in
California it is very difficult to get them approved based on the T-24
energy requirements. If you are considering an active solar
system of some sort, you will need to have a battery storage area and
will need a solar designer to address these issues.
If you have a bathroom or kitchen, you will also need hot water.
This will require a hot water heating system that also must meet the
requirements of energy use.
Some people wonder why it is I offer the towers without stairs or
ladders. There are a couple of reasons. First, I cannot
purpose or configuration you might want access. No matter how I
standardize the access, everybody seems to have their own idea of how
they want to get up and down the tower so I offer them without access
work with the client to provide access if desired. More often
than not it is more difficult to design the access than the tower.
Another reason is because many people are concerned about the
liability and vandalism. Often the towers are in isolated
non monitored areas where it is possible for kids, vandals or others to
access the tower. As such they do not want stairs making it more
difficult for non authorized access. In these cases they often
use portable ladders.
Open towers considered as agricultural or accessory buildings generally
don't have access requirements. This means that you can build a
tower with no ladder or stairs. You can access various heights of
the tower by a temporary ladder or other means. For convenience
though, you may want to have a permanent or fixed stair or ladder
Since the towers are generally tall compared to the horizontal space,
it is difficult to get a conventional stair system to fit within the
confines of the tower. A typical conventional stair takes about
13-14 feet of run plus landings. The towers generally have less
this and even less as you go up a tapered tower. Since this type
of tower does not require access, you can build something that is in
between a conventional stair and a ladder, i.e., something that is more
steep. This becomes more like a ships ladder and can still
provide relatively easy access.
Another approach is to have the stairs on the outside of the
tower. This allows for more conventional stairs and can wind
around the tower to get access at various levels without consuming all
the interior space of the towers. On tall towers though, the
stairs can become a dominant feature and distract from the appearance
of the tower.
Enclosed Non-Residential Towers:
Again, since these are considered agricultural or accessory buildings
they generally don't have to have access requirements and can be
addressed as described above for Open Towers:
Enclosed Residential Towers:
If the tower is used for residential purposes, it needs access to each
level by a conventional stair system. A conventional stair system
requires risers to be no more than 7" high and treads at least 11"
In addition, it needs to be at least 3' wide, have landings at least
the width of the stairs and it must have handrails. All of this
will take up considerable room within the tower. In a tapered
tower it becomes even more difficult as the space narrows. It is
almost impossible to get a conventional stair to fit within the upper
levels of the towers. For this reason I generally suggest
Enclosed Commercial Towers:
If the use of the tower is for public or commercial uses, in addition
to the requirements noted above for residential towers, it must also
meet the requirements for Accessibility. This means it needs to
meet ADA (Americans Disability Act) requirements. Often landings
need to be larger, the stair runs need to be enclosed, there needs to
be visual designations of change of surfaces and a variety of other
occupied towers, whether residential or commercial, require light and
ventilation. The code generally specifies natural light and
ventilation as 10% of the floor area at each level or room for light
and one half of this must be operable for ventilation. In
addition, for residential use, each sleeping area must have an open
able window within 44" of the floor of a minimum size such that a
person can escape during a fire.
If these requirements are not met naturally, they must be addressed
mechanically by a heating and air conditioning system and electrical
Lighting will generally be required for any occupancy at an entry door
and possibly at the bottom and/or top of stairs.