design brief -ramky

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1 DESIGN CRITERIA FOR FOUNDATION OF SOLAR PANEL MOUNTING STRUCTURE

Introdution:

Generally, driven pile method was the original preferred method for installing solar modules, the

repurpose of landfills and contaminated superfund sites as home to solar farms has the use of CAST IN

SITE CONCRETE BEAM + SLAB RAFT FOOTING stand as the apt foundation approach for the landfill sites.

Landfills and contaminated superfund sites are ideal candidates for solar farms because they are

considered already disturbed lands and thereby relieve the pressure to develop on undisturbed or

uncontaminated lands.

However, many of these sites do not permit or allow ground penetration for obvious reasons. Once you

get past the three feet of top cap soil, you reach the contaminated soil below. This is one area where

cast in site concrete raft footings win out over driven piles because they provide a non penetrating

solution. Other areas where cast on site concrete raft footings are finding success, is in solar installations

going in over bedrock where penetration is difficult if not impossible, solar installations with high water

tables, and installations with adverse soil conditions such as corrosive soils or soils with poor passive

earth‐pressure characteristics.

In addition to not penetrating the ground, cast in site beam +slab raft footings offer a variety of other

benefits such as:

• No soil Penetration.

• Minimal site excavation/preparation needed.

• Accommodates most site locations and conditions.

• Design performance is based on solar asset weight which is calculated precisely at 650kg for a carrying

capacity of 22 solar panels per steel structure. (on the contrary driven piles rely on assumed passive soil

pressures and other assumptions)

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DESIGN CONSIDERATION

The advantage of cast in site beam + slab raft concrete footings over pile+ post mount /pre cast ballast

foundation system and the 3 stability issues that are taken into design consideration are:

• Overturning (up – lift)

• Sliding

• Over‐shading

OVERTURNING (UP‐LIFT)

Excessive wind load forces can cause over turning moments (what is commonly referred to as uplift).

Figure 1 –Overturning on pre cast Ballasted Footing System

Figure 2 –Overturning on Post Mount System

FIGURE 1, results from a failure of the precast ballasted footing to successfully counter excessive wind

loads. (This means the footing was the wrong weight and or size for the application.)

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Overturning on a pile driven or post mount system, FIGURE 2, results from a failure of the mounting

hardware, racking system, or post/soil interaction due to excessive wind loads.

SLIDING

Sliding is a potential failure that has been overcome with the concept of beam + slab raft footing

combination.

Although not as catastrophic as overturning, sliding can occur due to the wind induced forces on the

solar array system. The wind not only tries to push the Solar Array system horizontally, but it also

creates a lift which reduces the actual dead load of the entire system, see FIGURE ‐3. This reduced dead

load coupled with the horizontal wind force is what creates the possibility of a solar module and its

foundation sliding horizontally. Taking into consideration the type of Sub grade material on which the

beam + slab raft footings will be installed is the primary factor to provide resistance to sliding.

The ideal type of Sub grade is a well drained granular course material such as sand.

Figure 3 – Sliding of pre cast ballasted Footings

OVER‐SHADING

Another common mistake is to install rows of solar modules too close together which results in

“over‐shading”. In driven pile or post mount applications this can result from not fully understanding

the affect that a change in height can have on the way shadows fall on the solar system as a whole. With

precast ballasted footings this can occur as a result of altering the thickness of the ballasted footing in an

attempt to add weight when additional weight is required due to regions of higher wind speeds. Thicker

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ballasted footings could increase the overall height of the solar electrical system which could require an

increase in distance between rows to eliminate possible adjacent‐row over‐shading,see Figures 4&5.

Figure 4–Over‐shading on Ballasted Footing System

Figure 5–Over‐shading on Post Mount System

This can avoided by uniform thickness of cast In site concrete beam + slab raft footing .

OTHER FACTORS

One of the biggest misconceptions within the solar industry in regards to cast in site concrete footings

is that the footing size and the unit cost are directly related to the energy output or

watts of the solar electrical system. This could not be farther from the truth. The footing designs have

nothing to do with the power output or price per watt, and everything to do with the following:

• Bearing Pressure.

• Tilt angle and tracking characteristics of the solar power system.

• Local design wind speeds where the solar power system is to be installed.

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• Support and racking configuration.

• Overall solar module system size and weight.

• Local design codes and project requirements.

• Soil characteristic relative to friction, sliding, consolidation, slope stability, etc.

Bearing Pressures:

Once the footing has been designed (sized) to prevent overturning and sliding failure, the soil bearing

pressures should be checked to ensure that they are in compliance with a soil’s engineer’s report that

may specify maximum bearing pressures.

Tilt Angle:

To maximize the output of the solar power system, especially in PV Solar Array applications, the

Optimal tilt angle is typically specified for non‐tracking systems, and remains fixed.

Structurally, higher tilt angles result in an increased wind load on the solar module which would require

a larger ballasted footing. In addition, higher tilt angles may require an increase in distance between

rows to eliminate adjacent‐row over‐shading, see FIGURE ‐4&5. A lower tilt angle is often desired to

minimize the wind forces and reduce the footing size.

A lower tilt angle also results in reduced adjacent‐row over‐shading potential which allows

the module rows to be spaced closer together. A cost comparison could be done to compare the tilt

angle versus the adjacent‐row over‐shading versus the ballasted footing size. In addition, lower tilt

angles allow for more compact utilization of the land available by minimizing the unusable area that is in

shade thus offering an opportunity for more solar modules.

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Wind Speed:

The most significant impact on the cast in site concrete footing design is due to the local design wind

speed. The local design wind speed can be found from the International Building Code, IBC, or from

the local building department where the solar power system is to be installed. It is imperative to get

the correct design wind speed as it has a direct impact on the footing dimensions, and thereby the

economics of the project. The force on the solar power system from the wind is directly related to the

wind speed squared ,”V” Square.

STRUCTURAL STANDARDS AND CODES:‐

The sub‐structure for Solar Power Plant panel are designed as reinforced concrete structure beam+raft

foundation subjected to vertical loads, self weight, dead load, live load and horizontal loads such as wind

loads. The various Indian standards followed for evaluating loads, structural analysis and design are

given below.

SL.No Code Description

1

2

3

IS: 875 (Part 1) – 1987

IS: 875 (Part 2) – 1987

IS: 875 (Part 3) – 1987

Code of Practice for Design Loads (Other then

Earthquake) for Building and Structures – Unit Weight of

Building Materials and Stored Materials


Earthquake) for Building and Structures – Imposed

Loads


Earthquake) for Building and Structures – Wind Loads


Earthquake) for Building and Structures – Special Loads

and Load Combinations.

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4

5

6

7

8

9

IS: 875 (Part 5) – 1987

IS: 1893 – 2002 (Part I)

IS: 4326‐1993

IS:13920 – 1993

IS: 456 – 2000

IS: 1786 – 1985

Criteria for Earthquake Resistance Design of structures.

Earthquake Resistance Design and Construction of

Building – Code of Practice.

Ductile Detailing of Reinforced Concrete Structures

Subjected to Seismic forces – Code of Practice.

Code of Practice for Plain and Reinforced Concrete.

Specification for High Strength Deformed Steel Bars and

Wires for concrete reinforcement.

10

IS: 432 (Part 1) – 1982

Specification for Mild Steel and Medium Tensile Steel

Bars and Hard Drawn Steel Wire for Concrete

Reinforcement – Mild Steel and Medium Tensile Steel

Bars

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11

12

13

14

15

16

17

18

IS: 432 (Part 2) – 1982

IS: 3370 (Part 1) – 1965

IS: 3370 (Part 2) – 1965

IS: 3370 (Part 4) – 1965

IS: 2062‐1999

IS:1904 – 1986

IS:800‐2007

IS:801

Specification for Mild Steel and Medium Tensile Steel

Bars and Hard Drawn Steel Wire for Concrete

Reinforcement – Hard Drawn Steel Wire.

Code of Practice for Concrete Structure for the storage

of liquids‐ General Requirements.


of liquids‐ Reinforced Concrete Structures.


of liquids‐ Design Tables.

Steel for General Structural purpose – Specification.

Code of practice for Design and Construction of

foundation in soils General Requirements.

General Construction in Steel — Code of practice

CODE OF PRACTICE FOR USE OF COLD‐FORMED LIGHT

GAUGE STEEL STRUCTURAL MEMBERS IN GENERAL

BUILDING CONSTRUCTION

design brief -ramky

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