Water Towers

Building Code, Design & Geologic Notes

Below are some things to consider or be aware of when planning your tower project.

Planning Issues
All areas of the country have some sort of planning control.  These are administered by local city or county planning departments.  They may have some sort of variation to this name but their function is generally the same.  The Planning Departments are concerned primarily with property use, size, height, and property line setbacks.  This is generally the first place you want to check to see if it is OK to build a tower on your property.  They may have some use restrictions that will either allow or not allow such a use.  They generally also have height requirements that will dictate the maximum size of your tower.  You should verify these before making a decision on the type of tower you want.

Many areas have no height limit.  Many more populated areas of the country have height limits.  The normal is a 35' height limit.  Often there is an exception to this for agricultural or accessory type uses such as water towers.

Other things that they may be concerned with are having access to the tower, such as roads, power if needed, water supply if needed and septic disposal if needed.  Roads and power are often administered by a different department, Public Works Department or similar name.  Water supply and septic disposal are often administered by yet another department, Environmental Health Department or similar name.  If your tower has any of these utilities you should address these issues with these departments before deciding on the type of tower you want.

Building Departments
Building Departments, or similar names, are part of the local government that administers the building code.  Don't confuse them with he Planning Department.  The Building Departments are the ones who review the construction documents, specifications and calculations to determine if the plans meet the building code.  They generally won't do the plan checking until all Planning Department, Public Works and Environmental Health issues have been resolved.  The Building Departments are the ones who issue the permits, do inspections during construction and issue the occupancy permit once the structure is completed.

Building Codes
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 International Building Code, IBC.  Previously there was the Uniform Building Code (UBC) which was used in most the midwest, west and around the world.  Building Officials Code of America, BOCA, was used mostly on the east coast.  The SBC, Southern Building Code, was used mostly in the south.  Various cities 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 based on the International Building Code.

Gravity Loads
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
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 Live 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
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 various parts 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 pressure 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 calculations 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.

Seismic 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 far away. 

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
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 floors.  These pressures are then applied to the tower to establish the forces on the tower.

Geologic Conditions
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 structure.  Most 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 take several weeks and cost thousands of dollars.  You may want to verify with your local building department in advance. 

Ground Freeze
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 the bottom 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.

Fire Code Issues
Code related fire issues may come into play with in a variety of ways.  

Rural Areas:
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:
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 system. 

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.

Energy Issues
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 electric.  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, exposure 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.

Access Issues
Some people wonder why it is I offer the towers without stairs or ladders.  There are a couple of reasons.  First, I cannot guess what purpose or configuration you might want access.  No matter how I try to 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 and then 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:
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 system. 

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 than 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" wide.  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 exterior stairs.

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 considerations. 

Light & Ventilation
Enclosed 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.

Lighting will generally be required for any occupancy at an entry door and possibly at the bottom and/or top of stairs.