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Module 3 Pad and Strip Footings
Construction Technology 2 (Substructure) (300721)
Western Sydney University
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Module 3
Pad and Strip Footings
1. Pad Footings
Pad footings are used for Concentrated Loads and Discontinuous Footing Systems.
The classic example for both, involves portal frame construction. Portal frames are constructed from a steel framework. The framework is quite strong so it can span large distances (up to 50 metres). The superstructure loads are concentrated into a small number of pad footings at the base of each portal (up to 8 metres apart).
Figure 1 Portal frame construction (from steelconstruction)
1 Structural Design of Pad Footings
Pad footings must be designed for two types of action: punching shear and cantilever bending action.
1 Punching Shear
Pad footings carry concentrated loads from columns into the foundation soil. The reinforcement at the base of the column must be sufficiently strong to ensure that the column does not pierce the pad footing.
In turn, the pad footing must be large enough so that it doesn’t punch through foundation soil ( bearing failure ). We have already dealt with bearing failure in Lecture 2. In this lecture, we will investigate the structural action of the pad footing, rather than the foundation soil below.
In punching shear, the column “punches” through the pad footing along the dashed line in Figure 2. If the pad footing was un-reinforced, it would crack along the dashed line.
Figure 2 Punching shear in an unreinforced pad footing
To avoid footing failure, the pad footing must be reinforced at right angles of the cracking. The ideal reinforcement arrangement for punching shear would involve bent reinforcing bars (in orange), as shown in Figure 3.
Figure 3 Reinforcement for punching shear
Bent reinforcing bars are expensive. Generally, pad footing reinforcement is laid straight along the base of the footing. As long as the reinforcing bars intercept the line of cracking shown in Figure 2, they will reinforce the pad footing. However, they must be a larger cross -section, since they do not act in the diagonal direction ( ie. at right angles to the cracking ).
For very severe punching shear from heavy concentrated column loads above, shear reinforcement should be provided at right angles to the cracking pattern (ie. at 45o, angled outward from the footing base). The reinforcement will tie the pad footing together much more
In a lightly loaded outstand, bonding between the reinforcement and concrete near the outer edge can provide sufficient anchorage. Deformed bars can also improve the bonding of the reinforcement with the concrete. However, in most cases, the reinforcement must be carried to the outer edge and “cogged”, as shown in Figure 5.
Figure 5 Reinforcement Cogging (from free-ed)
The reinforcement can also be tied into the footing stem as well, as shown in Figure 6. The starter bars fix the base of a reinforced concrete framework. Just as a steel portal frame can be coupled to a pad footing, a reinforced concrete column can also be coupled. Notice that the starter bars have been bent and angled diagonally to the reinforcement in the base of the pad footing. This ensures that the starter bars have adequate anchorage. The bends act like cogging.
Figure 6 Starter bars
(from tarlings)
1 Effect of foundation type
The behaviour of a pad footing is also complicated by the reaction of the soil. Because clayey soils are able to flow, settlement is concentrated in the centre of the footing. Conversely, the frictional properties mean that sandy soils are able to transmit vertical stresses best where they are confined i n the centre of the footing. Sandy soil will settle most near the edge of the pad footing.
Figure 7 Distribution of vertical displacement: a) clay b) sand s 1 is the settlement directly underneath the column
(from Craig, 1992:p. 175)
1 Construction of a pad footing (portal frame)
Pad footings are used in both houses and industrial buildings. This description applies to industrial buildings. The construction process is more complicated than for housing.
Excavation
Normally, pad footings are excavated by hand. Even for a large number of pad footings, it is difficult for an excavating machine to accurately dig pad footings. A backhoe can speed up the “rough” excavation, but the footing holes must be finished by hand.
Reinforcement
Reinforcement is delivered to site, as bundles of cogged bars (typically). The bars are fabricated together by the concreter, using tie wire and pliers.
The reinforcing mats are placed in the bottom of the pad footing. Before the concrete is poured, the reinforcing mat must be lifted off the base of the footing. The mat is supported by chairs. Reinforcement chairs maintain cover between the steel reinforcement and pad base, so that the reinforcement does not corrode.
Formwork
Characteristically, the floor slab of an industrial building is built up to avoid nuisance flooding. The top of the pad footing must be level with the rest of the floor. Hence, the top of the pad footing must be formed up (Figure 8).
The hold-down must be accurately positioned, both in the plane of the portal frame and laterally to the frame. The bolts are placed in a gig, as shown in Figure 8. The gig is nailed off to the formwork.
It is advisable to get a registered surveyor to set out the positions of the hold-down bolts ( Step 1 ) after the site has been leveled for construction and to check the location of the hold-down bolts ( Step 2 ) once the bolt gig has been installed on the formwork.
Portal frame footplates allow a small amount of mis-match with the hold-down bolts. However, if the hold-down bolts are misaligned by more than 5mm, then rectification can be expensive and time-consuming ( flame cutt ing of the footplates, re-alignment of the hold-down bolts, etc. ).
Concreting
Concreting is routine for pad footings on an industrial site.
Concrete pumping is not necessary: - The pad footings are too spread out to justify the installation of concrete pumping pipes - The concrete volumes are much smaller than an entire concrete floor, say
Industrial sites are generally very open. A ready-mix truck can be maneuvered close to each footing and concrete discharged directly into the footing hole.
The concrete should still be compacted to remove air voids. Air voids allow moisture to travel into the footing and corrode the reinforcement. Compaction is achieved by a hand-held vibrator.
The pad footing is screeded by a float to remove excess concrete from the top.
2 Strip Footings
2 Indeterminate Beam Action
Strip footings are designed to carry superstructure loads over the weak points in the soil and distribute the loads in the foundation soil. Hence, the strip beam is subjected to:
Negative curvature (over strong points in the soil) → Reinforcement needed in the top of the beam
Positive curvature (over weak points in the soil)
→ Reinforcement needed in the bottom of the beam
Figure 9 Negative and positive curvature in a strip footing
The strong points represent points of support for the strip footing. Between the strong points, the strip footing will sag so that the bottom of the strip footing is in tension.
The opposite effect occurs over the strong points, where the strip footing “hogs”. Over the strong points, the top of the strip footing is in tension.
The same effect can also occur with underloading. Under a door or window, the load on the strip footing is smaller than the general case, as shown in Figure 10. The strip footing ‘hogs’ at these points and the top of the strip footing is in tension.
Figure 10 Under-loading at openings from Curtins (1994), Structural Foundation Designers Manual p. 155
Figure 11 Articulation of brickwork ( movement of panels has been grossly exaggerated )
Articulation of the superstructure reduces the sensitivity of brittle elements. Small sections of brickwork can move against each other. The movement within the panels is minimized if the panels are allowed to move against each other.
Construction joints must be placed between each panel of brickwork. The spacing of the construction joints depends on the reactivity of the soil.
When the superstructure is made more flexible, less reinforcement is required. The strip footing is allowed to sag and hog to a greater extent.
Seasonal movement of the footing
Ground movement varies from time to time and between different parts of the building.
With clayey soils, the soil expands during wet times and in dry times, the foundation contracts. Soil in colder climates is also subjected to freeze/thaw changes. The building can also be subjected to differential movement. Foundation soil under the inside of the building does not expand or contract at the same rate as the exterior. Internal foundations are subjected to extreme changes in moisture content or temperature.
2 Pier & Beam Action
If surface soil is soft or highly variable, superstructure loads can be carried down to more reliable foundation material. When you cannot ensure that the soil is uniform across the site, then you have to pier down to a constant foundation material. For instance, if you have clay on one part of the site and shale in another, then you must pier down through the clay until you reach the shale. If the building was founded on clay and shale, the differential movement between the clay and shale would cause cracking in the building.
Pier & Beam footings are also used to overcome excessive surface soil movements. The strip beam is disconnected from the soil by an expandable layer (a cardboard former would work well).
Figure 12 Pier & beam footing
If a penetration is placed at the neutral axis, or at least in the middle third of member, it will not affect the structural integrity of the member. A service pipe must be placed in a duct so that any vertical movement of the footing under load or with seasonal effects will not disrupt or rupture the piping.
Roughing in a service penetration is difficult. The best solution is still to place service pipes for sewerage, water and electricity under the footing. A gap of 75mm must be placed between the pipe and the footing.
Figure 14 Pipe penetrations in strip and slab footings (from Cross, 1997:p. 144)
2 Construction of a Domestic Strip Footing
Bulk Excavation
The site must be cleared of vegetation and a level base excavated. On a sloping site, several terraces may have been be excavated for each level of the building.
Set-out
After the bulk excavation of the site, the key corners (including re-entrant corners) are located by a registered surveyor. Minor offsets can be located from the main corners of the building.
Since survey pegs will be disrupted by the subsequent building work, offset pairs of hurdles are installed by the builder to locate the key corners (see Figure 16).
It is also important to establish a datum on the site. The datum establishes a reference height from which all reduced levels (RLs) are measured using a water, dumpy or laser level. The datum is important on a sloping site where there are changes in level. A stable flat surface can be used for the datum.
Piering
For soft or extremely variable foundation soils, piers are installed after the set-out.
Ideally, the piering would happen after the trench excavation, but the trenches impede the movement of machinery across the site.
Pier holes can be set-out: by the surveyor ( expensive but accurate ) or by the builder measuring off the corner pegs of the building at regular int ervals
Pier holes can be excavated using the auger attachment on a backhoe shown on Figure 15.
Very deep piers (ie. slender) can need to be strengthened with steel reinforcement. We will discuss this further in Module 4.
The piers are filled with concrete, to a level 100mm below the base of the strip footing. If the tops of the piers are too high, they can be damaged by subsequent excavation of the strip footings. A damaged pier will not be capable of providing structural support for the strip footing.
Figure 15 Skid steer machine with auger attachment (from wikco)
Trench Excavation
The footing trench is excavated using a backhoe. The bucket size depends on the width of the strip footing. The backhoe operator follows lime lines that are marked out by the builder. It is
Figure 17 Step formwork (from Simpson & Hodgson, 2010:p. 73 )
Steps in the strip footing must be formed up on sloping sites. The formwork must be supported by stakes from the base of the footing. It is important that the strip footing steps are whole-brick courses. The most common complaint bricklayers have, is that the strip footing steps are not whole brick courses. In that case, they are forced to cut bricks longitudinally to provide a level base for the brickwork above. This is an extremely labour-intensive process. The set-out of a building, both horizontally AND vertically, is the responsibility of the builder.
Figure 18 Step leveling (from Simpson & Hodgson, 2010:p. 75)
Water levels work well to set out the vertical height of steps. The water level is used with a staff, that is graduated in brick courses. A water level can also be used to check the general level along the strip footing, during the concrete pour. As we mentioned in the section on Set-out, it is important to establish a datum for the water level (a reservoir with a fixed water level on a solid reference point).
Steel reinforcement
Steel reinforcement cages are fabricated on-site from trench mesh. Characteristically, the trench mesh are 3- or 4-bar lengths. The diameter of the bars ranges from 8mm to 11mm, depending on the foundation soil and articulation.
Figure 19 Strip footing detailing (from AS2870-2011: Table 3)
The top and bottom mesh is spaced apart using stirrups, as shown in Figure 20. The cage is kept in alignment (ie. avoiding collapse) by wire tension braces ( not shown .... ). When the cage has been installed in the trench, it is supported off the bottom of the trench using chairs.
Figure 20 Reinforcement cage (from designsmyth.files.wordpress)
References
Cowan, Henry J.; Gunaratnam, David & Wilson, Forrest (1995), Structural Systems , Department of Architectural and Design Science, University of Sydney.
Craig, R. (1992), Soil Mechanics , Fifth edition, Chapman & Hall, London.
Cross, George (1997), Basics & Footings , Domestic House Construction, Construction Technology series, Volume One, TAFE publications, Collingwood, Victoria.
Portal frames steelconstruction accessed 14 May, 2013
Created 2004 Modified 2013
Module 3 Pad and Strip Footings
Course: Construction Technology 2 (Substructure) (300721)
University: Western Sydney University
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