A strap footing is a type of foundation in which two footings are strapped together with a strap beam or also called a tie beam. Both Foundation3D and Mat3D can complete a strap footing design for soil supported and pile supported foundations.
In Foundation3D, a strap beam may be added for an exchanger or a horizontal vessel workspace type. The footings supporting saddle supports of an exchanger or a horizontal vessel may be strapped with a beam called "strap beam". When a horizontal exchanger or a horizontal vessel workspace are created or opened, an input page called "Footing Layout" displays. The strap beam may be added on this Footing Layout page. For more details on how to create a strap beam between footings, refer to the section "Strap Footing" on Footing Layout page.
In Mat3D, a strap beam may be added between two footings in a footing group workspace.
Assumptions in a strap beam foundation layout and design:
Strap beams may be added between two footings only - with each footing limited to one strap beam
Strap beams resist the shear and the moment transferred from the applied loads on the footings while the strapped footings resist only axial load
For pile supported strap foundation, only the footings are supported on piles but not the strap beam
For soil supported strap foundation, the strap beam is not subjected to bearing on soil - the two strapped footings resist the bearing. It is therefore recommended that adequate attention to detail be provided under the strap beam so that unintended soil reactions play a role in the design
Two footings that are strapped together can each have only one pier on it
The bottom of strap beam cannot be lower than the lowest of the bottom of the two strapped footings
The top of the strap beam cannot be higher than the lowest of the top of the piers or pedestals on the two footings being strapped together
Regardless of the location of the strap beam, the design of a strap foundation is assumed to be the same. The location of the strap beam is determined by specifying the top and bottom elevation of the strap beam on the Footing Geometry page. If the elevations are so specified such that the strap beam rests entirely above the footing, the calculations to determine the stresses and the reinforcement arrangement in the strap beam and the footings are assumed to stay the same
Strap beam is rigid enough to distribute the shears and moments. The program does not check rigidity of the strap beam. However reference material indicates that the ratio of moment of inertia of the strap beam to that of the footing be greater than 2 in order to assume that the strap beam is rigid enough to transfer moment
Strap footing design is performed only in Analysis mode. The footing and strap dimensions are user entered. A reasonable solution would be one where the strapped footings have somewhat similar if not equal bearing pressures under each footing when subjected to dead loads. This condition would allow for uniform settlement under the footings if any
Internally the program assumes a specific orientation of the strap foundation. The left or main footing is the one that has the least offset and the right or sibling footing is the one that the greatest offset. The design is completed based on this orientation
In the longitudinal direction or strap direction, the entire system is assumed to act as a monolithic unit while in the transverse direction, the footings are assumed to be simple spread footings with no contribution from the strap beam
The piers on the two footings that must be strapped must have the same centerline perpendicular to the strap direction. For eg if the strap direction is X, the Z coordinate of the CL of the two piers on the two footings being strapped must be the same. If the Z offset on one of the piers is changed, the Z coordinate on the other pier will be automatically set to that of the revised offset of the first pier
Analysis:
Strap footing analysis consists of obtaining the reactions under each footing to applied loads on the two footings. These reactions are then used to determine the uniform bearing pressure under each footing. The bearing pressure so obtained is compared to the allowable bearing pressure. The same logic is also applied for factored or ultimate load combinations to determine the factored bearing pressure under each footing. The factored bearing pressure is then used to complete the concrete design of the footings. In this regard, the footings are assumed to resist only the axial load and no shears or moments. The strap beam transfers the moment from one footing to another.
Similarly, a strap foundation with piled footings also assumes that the piles will only carry axial load. The analysis of piled strap foundation is very similar to that of soil supported strap footing. Reactions are obtained under each of the footings and these reactions are assumed to be applied entirely as axial load on piles.
Currently there is no limit on the strap beam width other than it being restrained to the minimum dimension of the piers or footings that it can frame into. Similarly there is no limit on the strap beam thickness other than it must be between the lowest elevations of the piers and the footings within which it is framed.
Tab Layout of Strap Footings:
If a strap foundation is created by joining two adjacent footings with a strap beam, relevant input and output pages show a different arrangement of tabs in the right pane of the window. For eg say the two footings are called F & F1, and if they are strapped together with a strap beam, the tab at the top will display S-F-F1 indicating a strapped footing with a strap beam between footing F & F1. Under this main tab S-F-F1, 3 sub tabs will be displayed F, F1 and S-F-F1. F & F1 display relevant footing related information whereas S-F-F1 displays strap beam information.