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Shearwalls now allows you to designate walls as Force transfer walls. These walls allow for light gauge steel straps to transfer tensile forces around openings, and blocking to transfer compressive forces, so that the sheathing above and below the opening contributes to shear wall resistance.
Design of the force transfer walls in Shearwalls follows what is known as the Diekmann method, as presented in APA T555, Design for Force Transfer Around Openings (FTAO). The program also conforms to Special Design Provisions for Wind and Seismic (SDPWS) 4.3.4.4 and 4.3.5.2.
In the Diekmann method, shear forces are developed in each wall "pier", defined by the rectangular areas:
A central pier is defined by the top of the highest opening and the bottom of the lowest one.
Force transfer walls are subject to the following limitations:
There must be least one foot of sheathing above and below each opening, to allow shear forces to develop in opening piers. A consequence is that walls with doors cannot be designed as force transfer walls. One solution to this situation is to create a non-shearwall for the door.
As per SDPWS 4.3.5.2 and 4.3.4.4., each central pier (i.e. between openings or between opening and wall end) must conform to the height-to-width limitations given in Table 4.3.4, that is, 3.5:1 for blocked walls and 2:1 for unblocked walls.
As per SDWPS 4.3.5.2.(1), the length of the wall piers (distance between openings or distance between an opening and the wall end), must be 2’ or greater. According to advice we received from FTAO researchers, this limitation has been applied to unblocked walls only; for blocked walls the piers must be 18" or longer.
As per SDWPS 4.3.5.2.(2), a full height segment must be at each end of the wall, that is, a force-transfer wall cannot start at an opening.
Gypsum wallboard or plaster sheathing materials are not compatible with force transfer walls. If such materials exist on a force transfer wall, they are ignored for both design and deflection calculations.
Shearwalls does not consider all the force-transfer details required to ensure that the assumptions of the Diekmann method are satisfied. The designer is responsible for verifying the strength of all elements and connections in the load path using sound engineering judgement. In particular:
SDPWS provides only height-to-width limitations, and in ordinary shear wall segments, width-to-height is rarely an issue. However, for force-transfer walls, narrow piers above openings can have very high width-to-height ratios such that they cannot be relied upon to transfer shear forces. For example, using our 1-foot pier height limitation, a 4-foot opening exceeds the 3.5:1 width-to-height ratio.
In the absence of design code guidance, Shearwalls does not check pier width-to-height and leaves the design of openings to your judgement in this regard.
When sheathing is configured such that panels wrap around the openings in a C- or L-shape with no seam between the corner pier and the opening pier or central pier, shear is transferred through the sheathing rather than via the straps and blocking, and the assumptions of the Diekmann method are not valid. Tests have shown extremely large differences between actual forces and those computed by force-transfer analysis in this case.
Shearwalls does not consider sheathing layout details (other than whether the panels are applied vertically or horizontally). It is the responsibility of the designer to ensure seams exist on the studs at the sides of opening segments or on the force transfer blocking so that force is transferred through blocking and straps rather than sheathing.
A setting called Continuous strap across entire wall has been added to the Design Settings. If checked, it is assumed that the force transfer straps and blocking extend in one straight line across the entire wall, so that all central piers extend from the top of the highest opening on the entire wall to the bottom of the lowest opening on the wall. The opening piers and corner piers are also measured from the top of the highest opening and bottom of the lowest opening, so that some sheathing may be contained within the opening area that does not resist shear force.
If unchecked, it is assumed that strapping and blocking exist at the top and bottom of each opening and extend far enough into the adjacent segments to transfer shear force to the central piers. In this case, the central piers extend from the top of the highest of the two adjacent openings, to the bottom of the lowest of the adjacent openings. The opening piers are measured from the top and bottom of each opening.
The use of non-continuous straps does not provide an advantage in terms of shear forces developed as the force developed in the core of the central pier is the same in either case. Strap/blocking forces may be lower; but more strapping and blocking are required due to overlap. The use of this option is recommended only to avoid large strap forces, or if it is more convenient to construct the wall with blocking at the top and bottom of openings.
A wall type called Force-transfer has been added to the list of wall Types in wall input. Walls of this type that have openings are displayed with square hatching, to distinguish them from perforated walls (diagonal hatching) and segmented walls (solid). A legend item has been added to show the force transfer hatching.
Force transfer walls without openings appear in solid colour.
The limitations on force-transfer wall geometry are imposed when you change the wall’s geometry in any way, i.e. via lengthening, shortening, changing the height, adding or removing openings, or changing opening dimensions. If this causes segments that are too narrow, piers with disallowed height-to-width ratios, or openings that are too close to the top or bottom of the wall, a warning is issued and the wall configuration is reverted to what it was before the change.
If an existing segmented or perforated wall, or non-shearwall, is changed to a force-transfer wall, the program checks the limitations and if any one is violated, a message is issued, and the change is reverted.
A standard wall called Exterior Force-transfer has been added to the list of standard walls that appear when you first run Shearwalls or select Reset original standard walls from the Default settings.
Walls designated as Force-transfer walls but have not yet had an opening placed in the wall, are in every respect treated by the program as segmented walls.
Shear force distribution to the piers in force-transfer walls is described in APA T555 as a sequence of numerical calculations in a design example of a wall with 2 openings of the same size. The following gives an algebraic formulation of the general procedure used by Shearwalls; it corresponds to the calculations in the T555 example.
The following symbols are used in the shear force distribution equations:
V – Total shear force on wall (lbs)
– Unit shear force in piers above and below openings
– Unit shear force in central piers
– Unit shear force in corner piers above central piers
– Height above openings
– Height above openings
– Height of wall
– Length of wall
– Length of opening
– Length of opening to the right of pier under consideration
– Length of opening to the left of pier under consideration
– Length of central pier under consideration
– Length of central pier to the left of the central pier under consideration
– Length of central pier to the right of the central pier under consideration
– Strap force on the left side of an opening
– Strap force on the right side of an opening
The total shear force in a vertical line on the wall is equal to the hold-down force VH / L (neglecting hold-down offsets), so the unit shear force in the upper and lower piers is that force divided by the length of the combined height of the piers:
The force in the central piers is:
This is the unit diaphragm shear force over the segment with the central pier plus the portion of diaphragm shear force over the openings on either side of the pier transferred to the pier via straps and blocking. This portion is determined by multiplying the unit diaphragm force by the ratio of the adjacent opening lengths to the combined length of the central piers on either side of these openings.
The unit force in the corner pier is the unit force in the central pier minus the strap/blocking forces from each side divided by the length of the pier.
The strap/blocking forces are the total force (not a unit force) above and below the opening apportioned to the left and right side by the proportion of total length of the piers on each side.
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Walls designated as force-transfer without openings are treated as segmented walls.
Although it isn’t explicitly stated in SDPWS 4.3.4.4, we were informed that the intention of the SDPWS is that force-transfer wall central piers that have height-to-width ratios between 2.0 and 3.5 are subject to the Aspect Ratio Factor given in 4.3.4.2 (for deflection-based distribution to segments), and the adjustments in the Exceptions to 4.3.3.4.1 ( for capacity-based distribution).
These are the same factors and adjustments used for segmented wall full-height segments, except that for force-transfer walls they applied to the dimensions of the central piers between two openings or between an opening and the wall end.
The program determines the critical case of pier force / Aspect Ratio Factor for any pier in the wall (including corner and opening piers with a factor of 1.0) and designs the wall for that case.
The contribution of gypsum wallboard and plaster materials is ignored in calculating design strength of force-transfer walls, regardless of what is set in the Ignore contribution of... Design setting.
Force-transfer walls require hold-down devices only at wall ends, so that hold-down forces in Shearwalls are at the ends of force-transfer walls and there is no hold-down at the sides of each opening.
Hold-down devices are input only at the ends of entire walls, as for perforated walls. Hold-down forces are reported in Plan View, Elevation View, and the Hold-down Design table only at the end of walls.
The shear force used for overturning calculations is the shear force applied to the entire wall. Dead and wind uplift loads over the entire length of the wall are distributed to the hold-downs at the end of the wall, so the hold-down force calculations include the dead and wind uplift loads over openings, instead of distributing them to opening supports.
The unit shear forces in the piers that are shown acting horizontally also act vertically along the studs at wall and opening ends. The vertical forces at the wall end add up to the hold-down force, neglecting the hold-down offset.
Beside the openings, tension and compression axial forces are developed in the studs due to the difference in shear force in piers adjacent to the studs. Shearwalls does not calculate or report these forces, but they should be considered when designing the wall.
The procedure for calculating deflection of force transfer walls comes from APA T555, Design for Force Transfer Around Openings (FTAO). This applies only to force transfer walls with openings; those without are treated as segmented walls.
In this method, the deflection due to the shear force in the central pier is calculated on each wall segment between openings, but assuming that the height of all but one of the segments is the height of the central pier plus the top pier only, i.e., the bottom pier is ignored. The one segment that is still considered to be full height is at the far end of the wall from the force direction.
This analysis is done in both force directions using the same wall shear force, then all the deflections from the segments in each direction are averaged to arrive at the deflection of the entire wall; i.e. if there are n segments, 2n segment deflections are averaged.
That is, if E->W wind loads are different than W->E loads, due to for example a monoslope roof, then for the E->W force direction, the E->W force is used for both E->W and W->E directions, then all the deflections are averaged. The same thing is done again for the W->E force direction using W->E loads.
The forces used to calculate the deflection of each segment for the purpose of deflection design are as follows:
The unit shear force v used for deflection calculations is the central pier shear force determined through force-transfer shear force distribution to piers (see ). Therefore, the deflections of the segments within the shear wall that are later averaged are not equalized, even if deflection-based shear distribution to segments is selected in the Design Settings.
The distribution of the force to sheathing on either side of a segment in a two-sided shear wall is determined by equalizing the nail slip plus shear components of the deflection equation.
If deflection-based shear distribution to segments is selected in the Design Settings, the force on the force-transfer shear wall in a shearline is determined by equalizing the deflection of the entire wall with other force-transfer or perforated walls, and with all the segmented wall segments on the line. In other words, the segment-averaging process for a force-transfer wall is incorporated into the iterations that equalize shear wall deflections on the entire line.
The calculation of the shear, bending and nail slip components in the equations for deflection from SDPWS C4.3.2 use the reduced segment height h. The shear and nail slip terms are linear in h; the bending term is proportional to h3.
The hold-down displacement contribution to deflection at each interior segment of the force-transfer wall is calculated as if there was a hold-down device at each opening, even though they are not present in force-transfer walls. The wall anchorage term in the equations for deflection in SDPWS C4.3.2 is calculated as follows:
The shear overturning force is calculated using the reduced moment arm of those segments that are considered to extend from the bottom of the opening to the top of the wall. For full-height end segments, the wall height is used as the moment arm.
Dead loads and wind uplift loads on the same level as the segment are distributed to the segment ends for the purpose of deflection calculations. Dead and wind uplift loads over openings are distributed to the fictitious hold-downs at the sides of the openings, as if it was a segmented wall.
Dead, wind uplift and overturning hold-down forces from floors above are included in the calculation of hold-down deflection if the hold-down forces on the wall above and on the wall below are at the same location. ; that is, these segments behave as segmented walls do in this regard.
The factor h/b in the wall anchorage term uses the reduced segment height h for those segments considered to extend from the bottom of the opening to the top of the wall. For full-height end segments, the wall height is used.
For deflection calculations, the virtual hold-down devices used for the left end of all openings are those selected for the left end of the wall, and those used at the right end of openings are those used at the right end of the wall.
When calculating deflections in the opposite direction, the hold-downs are flipped, so that for one actual force direction, the same hold-down devices are subjected to tension in all the deflections that are averaged to give the resulting wall deflection.
The contribution of gypsum wallboard and plaster materials is ignored in calculating deflections of force-transfer walls, regardless of what is set in the Ignore contribution of.. Design setting.
Collectors are required throughout the full length of the force-transfer wall according to SDPWS 4.3.5.2(4). We received confirmation that this clause refers to the transfer of forces from the diaphragm to the shear wall, not the internal transfer of forces around openings via blocking and straps. A continuous collector is necessary because the shear force distribution to force-transfer piers creates differential forces in the top piers of the wall, so that axial tension and compression forces build up in the top plate of the wall, which acts as a collector dragging or pushing the uniform shear force from the diaphragm above to the piers below.
Shearwalls reports these drag strut forces at the sides of each opening and at the ends of the wall where there is a gap in the shear line. These locations are where the local maxima and minima of tension and compression forces occur, and these forces vary linearly between these points.
These are the same locations drag strut forces appear for segmented walls, but the mechanics for force transfer are quite different.
For a segmented wall, there is no shear resistance in the sheathing above openings, so that the drag strut above the first segment with an opening drags additional force into the segment to the left, and pushes additional force into the segment at the right, typically creating tension in the drag strut at the left side and compression at the right.
For a force-transfer wall, the shear force in the sheathing above the opening is typically greater than the corner pier forces, so that the drag strut pushes additional force into the pier above the first opening from the left, and pulls it into this pier from the right. This typically creates compression in the drag strut at the left side and tension at the right.
The following changes have been made to Elevation View output for force-transfer walls:
The boundaries of force transfer piers are shown as light blue lines.
Shear forces in each pier are shown at the bottom of each pier in the same way that the design shear force is shown at the bottom of the wall for segmented walls. For offset openings and the non-continuous strap setting, the pier force is shown at the bottom of the pier (lowest opening) for consistency, although the full shear force is developed at the bottom of the higher opening.
The program shows only one large arrow at the top of the wall for the total force on the wall, not forces on each segment as it does for segmented walls.
Straps are shown as two thick, closely spaced lines above and below the openings and extending into neighbouring segments. For continuous straps they extend the full length of the wall at the height of the highest opening top and lowest opening bottom. For non continuous straps, the extend the length of the full height segment adjacent to the opening.
The strap/blocking forces are shown at each corner of the opening. The program does not indicate the direction of the force or whether it is in tension or compression; typically for left-to-right shearline forces, the top left strap is tension, top right blocking in compression, bottom left blocking in compression, and bottom right strap in tension.
A legend entry indicates these are strap/blocking forces.
Aspect ratios and factors are shown for central piers only, in the center of the pier.
The vertical dimensions of piers are shown as follows:
Piers above and below openings are dimensioned in the same horizontal location as the opening dimension line.
Central piers are dimensioned if they are a different height than adjacent openings.
Corner piers are not dimensioned as they have the same dimension as an adjacent opening pier.
For continuous strap across the entire wall, only one set of piers is dimensioned; as piers in all segments have the same dimensions.
The program does not shade the areas above and below openings as it does for perforated and segmented walls, as these areas contribute to shear resistance for force-transfer walls.
Items have been added to the Show menu and Options | Display settings to allow you to turn on and off the display of piers, straps and pier dimensions.
The following changes have been made to the Design Results output to accommodate force-transfer walls. These changes apply only to force-transfer walls with openings; walls designated as force-transfer without openings are treated as segmented walls.
In the Shearline, Wall and Opening Dimensions table:
A type FT has been added for force transfer walls.
The aspect ratio for force-transfer walls is the aspect ratio of the central pier; this is explained in the Legend.
The force v in the column headed by v is the force in the central pier.
The column headed by vmax is now vmax/vft and shows the shear force above and below openings and in corner piers. A legend item explains this, and that the aspect ratio factor does not apply to these forces
A line for openings now appears between each segment giving the unit shear force, total shear force, and design ratio of the piers above and below openings (which always have the same unit shear force).
The legend entry for aspect ratio factor now refers to force transfer piers
For other types of walls, this table shows lines for wall segments only. For force-transfer walls, a line for the entire wall has been added to show the averaged deflection of all the segments in both directions.
The shear force shown in this line is the total force on wall divided by wall length. The wall dimensions and wall group are also shown, but all other columns are not used and show a hyphen (-).
The column for segment shear force v shows the force in the central pier.
For end segments, the wall height h and the shear, bending, nail slip, and hold-down components of the deflection equation are the average of those derived from the full wall height and the distance from the bottom of the lowest adjacent opening to the top of wall. This is because the end segments are full height in one direction but not in the other.
The averaging of the bending term, which is non-linear in h, has been determined mathematically and does not correspond to the calculation of that term using the h shown.
For interior segments, the wall height is the distance from the bottom of the lowest adjacent opening to the top of wall, and all deflection components are determined using that height.
The legend has been modified to explain the wall width b, wall height h, shear force v, hold-down devices used, and total deflection for the entire wall and for each segment, referring to APA T555.
The value used for the wall height h to determine total displacement shown for end segments is the average of the full wall height and the distance from the opening bottom to the top of the wall. This is explained in the Legend.
The hold-down device shown is the one used to calculate the displacement at the segment, even though a hold-down device does not exist at that location.
The Drag Strut Forces table has been renamed Collector Forces, and two columns have been added to show the force-transfer strap/blocking forces at the sides of each opening (the same locations that drag strut forces are currently shown), in each force direction.
A legend item has been added for strap/blocking forces.