US20100018151A1

US20100018151A1 - Portal frame

Info

Publication number: US20100018151A1
Application number: US12/178,078
Authority: US
Inventors: Badri K. J. Hiriyur
Original assignee: Simpson Strong Tie Co Inc
Current assignee: Simpson Strong Tie Co Inc
Priority date: 2008-07-23
Filing date: 2008-07-23
Publication date: 2010-01-28

Abstract

A portal frame is disclosed for use around portals in, for example, a light-framed construction. The portal frame includes a pair of vertically-oriented shearwalls, and a header extending between and connected to the shearwalls. The portal frame may be used around a variety of portals formed in a construction, including for example doors, garage doors and windows. By forming a portal frame using a pair of shearwalls connected to a header, the portal frame in embodiments of the present invention is able to effectively reduce the loads exerted on a foundation by the shearwalls in comparison to a shearwall acting alone (i.e., one that is not part of a portal frame).

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to a portal frame for opposing lateral forces, and in particular to a portal frame including a pair of shearwalls to improve the ability of the portal frame to withstand lateral forces such as those generated in earthquakes, high winds, floods and snow loads.
2. Description of the Related Art
Shearwalls were developed to counteract the potentially devastating effects of natural phenomena such as seismic activity, high winds, floods and snow loads on the structural integrity of light-framed constructions. Prior to shearwalls and lateral bracing systems, lateral forces generated during these natural phenomena often caused the top portion of a wall to move laterally with respect to the bottom portion of the wall, which movement could result in structural failure of the wall and, in some instances, collapse of the building. Shearwalls within wall sections of light-framed constructions provide lateral stability and allow the lateral forces in the wall sections to be transmitted through the shearwalls between the upper portions of the wall and the floor diaphragm or foundation of the building where they are dissipated without structural effect on the wall or building.
One example of a shearwall is disclosed in U.S. patent application Ser. No. 10/734,870, entitled, “Corrugated Shearwall,” which application is assigned to the owner of the present invention, and which application is incorporated herein in its entirety (referred to herein as “the '870 application”). The performance of a shearwall like the one disclosed in '870 application under lateral loads is such that large moment forces may be transferred from the shearwall to the floor diaphragm or foundation on which the shearwall is mounted, at the lower corners of the shearwall. Typically, the load capacity for such shearwalls is limited not by the ability of the wall itself to withstand lateral loads, but rather the ability of the anchorage system and/or the diaphragm or foundation to withstand the large moment forces generated by the shearwall under the lateral loads.
In constructions such as residences and small buildings, portal frames are formed around portals, which may be any opening in a construction such as for example a garage, door or window. A portal frame typically includes vertical studs spaced from each other and affixed to a horizontal top plate, surrounding the portal. A bottom plate may also be included which is typically anchored to the floor diaphragm or foundation. Portal frames are often subjected to larger lateral forces due to the absence of any structural materials within the portal itself. There is therefore a need for a portal frame capable of withstanding large lateral loads, while at the same time prevent large moment forces on the anchorage system and diaphragm, foundation or other base on which the portal frame is mounted.

SUMMARY OF THE INVENTION

Embodiments of the present invention relate to a portal frame for use around portals in, for example, a light-framed construction. The portal frame includes a pair of vertically-oriented shearwalls, and a header extending between and connected to the shearwalls. The portal frame may be used around a variety of portals formed in a construction, including for example doors, garage doors and windows.
In the embodiments, the header may be a steel tube, but may be other materials such as glulam or other engineered lumbers in alternative embodiments. The shearwalls may be identical to each other, and may include a pair of end sections generally defining the width of each shearwall, and at least one corrugation. A sill plate may be affixed at the top of each shearwall which allows distribution of the compressive and tensile forces exerted by the shearwalls on the header. A base plate may be provided at the bottom of each shearwall.
By forming a portal frame using a pair of shearwalls connected to a header, the portal frame in embodiments of the present invention is able to effectively reduce the loads exerted on a foundation by the shearwalls in comparison to a shearwall acting alone (i.e., one that is not part of a portal frame). In one embodiment, the portal frame is essentially pinned to the underlying foundation at the bases of the shearwalls. In such embodiments, very little, if any, moment demand is taken up at the connection point of the shearwalls with the foundation. Thus the demands on the foundation and anchoring mechanisms may be greatly reduced.
In alternative embodiments, it is contemplated that the portal frame may be fixed at its base. In such embodiments, the base plates and anchor mechanisms used may be sturdier to provide constraint against rotation at the base. In such embodiments, the load on the base plates may be substantially similar to the load on the sill plates at the top of the shearwalls.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described with reference to the drawings in which:

FIG. 1 is a perspective view of a portal frame according to an embodiment of the present invention;

FIG. 2 is a cross-sectional view through line 2-2 of FIG. 1 showing a shearwall forming part of the portal frame according to the present invention;

FIG. 3 is a perspective view of a portal frame according to an alternative embodiment of the present invention;

FIG. 4 is a front view of a portal frame according to an embodiment of the present invention including an indication of load distribution through the portal;

FIG. 5 is a front view of a portal frame according to an embodiment of the present invention including an indication of an alternative load distribution through the portal.

DETAILED DESCRIPTION

The present invention will now be described with reference to FIGS. 1 through 5, which in embodiments relate to a portal frame including a pair of shearwalls connected to a header. It is understood that the present invention may be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein; rather these embodiments are provided so that this disclosure will be thorough and complete and will fully convey the invention to those skilled in the art. Indeed, the invention is intended to cover alternatives, modifications and equivalents of these embodiments, which are included within the scope and spirit of the invention as defined by the appended claims. Furthermore, in the following detailed description of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be clear to those of ordinary skill in the art that the present invention may be practiced without such specific details.
FIG. 1 is a perspective view of a portal frame 100 according to an embodiment of the present invention. The frame 100 includes a pair of vertically- oriented shearwalls 102, 104, and a header 106 extending between and connected to the shearwalls 102, 104. The portal frame 100 may have a height, width, and depth, each perpendicular to each other and denoted as h, w and d, respectively, in FIG. 1. Each of these dimensions may vary in alternative embodiments, depending in part on the size of the portal around which the portal frame 100 is provided. As one example, the height of the frame may be approximately 8 feet, the width of the frame may be approximately 10 feet, and the depth of the frame may be approximately 2½ inches. It is understood that each of these dimensions may vary in alternative embodiments, proportionately to each other or disproportionately to each other. The portal frame 100 may be used around a variety of portals formed in a construction, including for example doors, garage doors and windows.
The shearwalls 102, 104 may be identical to each other, and the following description applies to both shearwalls 102, 104. Each shearwall may be of the type disclosed in any of the embodiments of the '870 application, previously incorporated by reference, with the exception that, in embodiments, the shearwalls 102, 104 are inverted with respect to the shearwalls of the '870 application. In particular, a sill plate 110, discussed hereinafter, may be affixed at the tops of each shearwall 102, 104, where the sill plate was located at the bottom of the shearwalls of the '870 application. While not optimizing the use of the sill plate 110, it is understood that the shearwalls of the '870 application may be used in their un-inverted position in the portal frame 100 in alternative embodiments.
Referring to FIG. 1 and the cross-sectional view of the shearwall 102, 104 shown in FIG. 2, each shearwall 102, 104 includes a pair of end sections 114 and 116 generally defining the width of each shearwall 102, 104, and at least one corrugation 118. As used herein, a corrugation may be any ridge, groove or angle formed in a shearwall 102, 104 extending in the height direction at least partially between the top and bottom edges of the shearwall. In embodiments of the present invention, each shearwall may have an overall height of 93¼ inches, an overall width of 24 inches, and a depth of 2½ inches. It is understood that each of these dimensions may be varied in alternative embodiments, both proportionately and disproportionately with respect to each other. For example, in one alternative embodiment, the central diaphragm may have an overall width of 18 inches.
In embodiments of the present invention, each shearwall 102, 104 may be formed of 7-gauge sheet steel (0.1875 inches). Other gauges, such as for example 10-gauge sheet steel, and other materials of comparable strength and rigidity may be used in alternative embodiments. One such alternative material may be expanded metal. It is understood that other shearwalls, for example manufactured by Simpson Strong-Tie of Pleasanton, Calif., may be used in place of the shearwall 102, 104 described above.
In a further alternative embodiment, a pair of chords (not shown) may be affixed to the end sections 114 and 116 of each shearwall 102, 104 to provide additional structural support to the shearwalls 102, 104. The chords may be formed of wood, such as for example sawn lumber from lumber groups including spruce-pine-fir, Douglas fir-larch, hem-fir and southern pine. The chords may alternatively be formed of engineered lumber, such as glulam and wood composites. Sheathing (not shown) may be affixed over the front and/or back surface of each shearwall 102, 104, and affixed by a variety of affixing mechanisms including screws and glue.
As mentioned above, each shearwall 102, 104 may include a sill plate 110 affixed at the top of the shearwall. The sill plate 110 allows distribution of the compressive and tensile forces exerted by the shearwalls 102, 104 on the header 106. Accordingly, sill plate 110 is provided as a flat plate with a relatively large surface area. The plate 110 has a length which may be greater than the width of each shearwall 102, 104, and a width that is greater than or equal to the depth of each shearwall 102, 104. In one embodiment, the sill plate 110 may be 26 inches long and 4 inches wide. It is understood that the length and/or width of plate 110 may be larger or smaller than these dimensions in alternative embodiments.
Sill plate 110 is also rigid enough to allow even distribution of any localized compressive/tensile forces from the shearwalls 102, 104 on header 106. In one embodiment of the present invention, the sill plate 110 is formed of ½ inch to ⅝ inch thick steel. It is understood that sill plate 110 may have thicknesses other than between ½ inch and ⅝ inches in alternative embodiments. The sill plate 110 may be affixed to each shearwall 102, 104 by affixation methods such as welding, bolting and/or gluing.
A base plate 112 may be provided at the bottom of each shearwall 102, 104. As explained in greater detail below, the portal frame of the present invention is able to effectively reduce the loads exerted on a foundation 120 by shearwalls 102, 104 in comparison to a shearwall acting alone (i.e., one that is not part of a portal frame). Thus, in one embodiment of the present invention explained below with respect to FIG. 4, base plate 112 may be relatively thin, for example, 26 inches long, 4 inches wide and ¼ inch thick. It is understood that the dimensions of the base plate 112 in the embodiment of FIG. 4 may be larger or smaller than that in further embodiments. The embodiment of FIG. 4 is designed as a “pinned base” portal, where very little, if any, moment demand is taken up at the connection point of the shearwalls 102, 104 with the foundation on which the portal frame sits. The shearwalls 102, 104 in this embodiment are essentially assumed to be hinged or pinned at the base, and the shear resistance is taken up in the shearwalls 102, 104 themselves and the joints of the shearwalls with the header 106.
However, in an alternative embodiment shown in FIG. 5 and discussed below, the portal frame 100 is designed to have a “fixed base” condition where the base plates 112 are thicker so as to provide some rotational constraint at the base. This will mean that the columns are resisting moment (rotation) both at the base and at the beam-column connection. In such embodiments, the base plates 112 may have dimensions similar to or even thicker than sill plate 110. As an example, the base plate in the embodiment of FIG. 5 may be 26 inches long, 4 inches wide and ⅝ of an inch thick. It is understood that the base plate 112 in the embodiment of FIG. 5 may be larger or smaller than that in further embodiments. In the embodiments of both FIGS. 4 and 5, the plates 112 may be affixed to each shearwall 102, 104 by affixation mechanisms such as welding, bolting and/or gluing.
FIG. 1 shows the shearwalls 102, 104 mounted on an underlying support surface 120 (a portion of which is shown in FIG. 1). In the embodiment shown, underlying support surface 120 comprises a concrete building foundation, but it is understood that underlying support surface 120 may be any surface on which a conventional shearwall may be located, including for example a floor diaphragm on the building foundation or a floor diaphragm on a top plate of a lower floor. The shearwalls 102, 104 are fastened to the underlying support surface 120 by means of anchors (not shown) protruding up through holes 122 (FIG. 2) formed in the plates 112. Bolts are then fastened over threaded ends of the anchors to anchor the portal frame 100 to the underlying support surface.
The anchors used in the embodiment shown in FIG. 4 may have relatively low load bearing capabilities, while the anchors used in the embodiment shown in FIG. 5 may have higher load bearing capabilities. As examples, a Titen HD screw anchor, model number THD62800H, manufactured by Simpson Strong-Tie of Pleasanton, Calif., may be used in the embodiment of FIG. 4, and a SSWAB screw anchor (e.g. SSWAB1x36HS), also manufactured by Simpson Strong-Tie of Pleasanton, Calif., may be used in the embodiment of FIG. 5.
It is understood that portal frame 100 may be anchored to the underlying support surface by other anchors in alternative embodiments. Moreover, other types of anchoring mechanisms may alternatively be used, such as for example strap anchors, mudsill anchors, retrofit bolts, foundation plate holdowns, straps, ties, nails, screws, framing anchors, ties, plates, straps or a combination thereof.
In the embodiment shown in FIG. 1, the header 106 may be a steel tube, such as for example standard AISC rectangular HSS section (Hollow Structural Steel) with a wall thickness of approximately ⅜ inches. The dimensions of header 106 may vary depending on the moment demand in the header, but in embodiments, the steel tube header may have a length of 10 feet, a height of 3½ inches and a depth of 2½ inches. As indicated, each of these dimensions may vary in alternative embodiments, both proportionately and disproportionately to each other. The header 106 may be affixed to the sill plate 110 of each shearwall 102, 104 via a variety of affixation mechanisms including for example bolts 124 shown in FIG. 1, and/or by welding and gluing. Where bolted, a pipe stiffener may optionally be used within the interior of the tube 106 to add structural support to the header at the bolts as is known in the art.
In embodiments, yield links may be used at the joints between the header 106 and shearwalls 102, 104 to provide a lateral bracing system against the moment forces generated at the joints. Examples of such yield links are disclosed in U.S. patent application Ser. No. 10/847,851, entitled, “Moment Frame Links Wall,” and U.S. patent application Ser. No. 11/959,696, entitled, “Moment Frame Connector.” These applications are assigned to the owner of the present invention and are incorporated by reference herein in their entirety. Yield links may be omitted from embodiments of the present invention.
It is understood that header 106 may be formed of other materials in further embodiments. One such embodiment is shown in FIG. 3, where header 106 is formed of engineered lumber, such as glulam or wood composites. In such embodiments, the header may include recessed bolt holes so that the tops of bolts 124 can be flush with an upper surface of the header. The recessed bolt holes in the header of FIG. 3 may be omitted in further embodiments so that the bolts extend above the glulam header as in FIG. 1.
Referring now to FIG. 4, there is shown a front view of an embodiment of a portal frame 100 together with graphs along the lengths of the shearwalls 102, 104 and header 106. The graphs show an indication of the moment forces exerted per position along the length of the shearwalls 102, 104 and header 106 under a lateral load. As indicated above, the portal frame of the present invention is able to effectively reduce the loads exerted on the foundation 120 by shearwalls 102, 104 in comparison to a shearwall acting alone. Thus, as seen in the embodiment of FIG. 4, there are no moment forces at the base of the shearwalls 102, 104, and the shearwalls are essentially hinged or pinned at their base. As seen in the graphs, the moment forces increase linearly up along the length of the shearwalls 102, 104 (it is conceivable that the shearwalls could be constructed so that the moment forces would increase, but not linearly, up along the length of the shearwalls in further embodiments). Similarly, the moment forces on the header 106 are greatest at its ends (positive at one end, negative at the other), which moment forces decrease to zero at a midsection of header 106.
FIG. 5 shows an alternative embodiment of the portal frame 100 designed for a fixed base condition. In FIG. 5, the shearwalls 102, 104 are affixed to the foundation 120 using stronger base plates 110 and/or anchoring mechanisms. Thus, the portal frame 100 in FIG. 5 is able to provide some rotational constraint at the base. Accordingly, the shearwalls 102, 104 are able to take up a maximum moment force at their base, which force decrease to zero along a length of the shearwalls, and goes to a negative maximum at the top of the shearwalls. The header 106 in FIG. 5 may bear the moment forces in the same manner as in FIG. 4. It is understood that the base may be designed as a combination between FIGS. 4 and 5, so that the base takes up some moment force, but less than the top of the shearwalls.
As discussed in the Background section, shearwalls are able to withstand high lateral loads, but they are typically underutilized in that the load capacity for such shearwalls is limited by the ability of the anchorage system and/or the foundation to withstand the large moment forces generated by the shearwall under the lateral loads. By reducing the forces at the base, the portal frame of the present invention is able to utilize the full lateral bracing capacities of the shearwalls 102 and 104.
The foregoing detailed description of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. The described embodiments were chosen in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.

Claims

1. A portal frame, comprising:

a pair of shearwalls, each shearwall including:

a top edge and a bottom edge generally defining a height of the shearwall,

first and second ends, extending between the top and bottom edges, generally defining a width of the shearwall, and

a corrugated section extending at least partially between said top edge and said bottom edge in between said first and second ends, said corrugated section forming at least one corrugation; and

a header extending between and mounted to top edges of the pair of shearwalls, the pair of shearwalls and header together defining a frame for a portal of a construction.

2. A portal frame as recited in claim 1, further comprising a sill plate affixed at the top edge of each shearwall and having a footprint at least as large as the footprint of each shearwall.

3. A portal frame as recited in claim 1, further comprising a bottom plate affixed at the bottom edge of each shearwall and having a footprint at least as large as the footprint of each shearwall, the bottom plates affixing the shearwalls to a surface on which the portal frame is supported.

4. A portal frame as recited in claim 3, wherein a lateral load on the portal frame results in negligible forces between the bottom plates and support surface.

5. A portal frame as recited in claim 1, further comprising a sill plate at the top edge of each shearwall and a bottom plate at the bottom edge of each shearwall, the header affixed to the sill plates and the bottom plates affixed to a surface on which the portal frame is supported, wherein a lateral load on the portal results in substantially equal forces between the sill plates and header on the one hand, and between the bottom plates and support surface on the other hand.

6. A portal frame as recited in claim 1, wherein the header is formed of tube steel.

7. A portal frame as recited in claim 1, further comprising bolts for bolting the header to the shearwalls.

8. A portal frame as recited in claim 1, wherein the header is formed of engineered lumber.

9. A portal frame as recited in claim 8, wherein the engineered lumber is glulam.

10. A portal frame as recited in claim 8, further comprising bolts for bolting the header to the shearwalls, the bolts fitting within recessed holes in the engineered lumber.

11. A portal frame as recited in claim 1, further comprising yield links at the joints between the header and shearwalls.

12. A portal frame as recited in claim 1, further comprising first and second chords affixed one each to said first and second ends of each shearwall.

13. A portal frame as recited in claim 1, wherein said shearwalls are formed of 7-gauge steel.

14. A portal frame, comprising:

a pair of shearwalls, each shearwall including:

a top edge and a bottom edge generally defining a height of the shearwall,

a corrugated section extending at least partially between said top edge and said bottom edge in between said first and second ends, said corrugated section forming at least one corrugation;

sill plates affixed at the top edges of the shearwalls;

a header extending between and mounted on top of the sill plates, the pair of shearwalls and header together defining a frame for a portal of a construction; and

base plates affixed at bottom edges of the shearwalls, the base plates affixing the portal frame to a surface on which the portal frame is supported,

wherein a lateral load on the portal frame results in negligible forces between the bottom plates and support surface.

15. A portal frame as recited in claim 14, wherein the header is formed of tube steel.

16. A portal frame as recited in claim 14, wherein the header is formed of engineered lumber.

17. A portal frame as recited in claim 16, wherein the engineered lumber is glulam.

18. A portal frame as recited in claim 16, further comprising bolts for bolting the header to the shearwalls, the bolts fitting within recessed holes in the engineered lumber.

19. A portal frame as recited in claim 14, further comprising yield links at the joints between the header and shearwalls.

20. A portal frame, comprising:

a pair of shearwalls, each shearwall including:

a top edge and a bottom edge generally defining a height of the shearwall,

sill plates affixed at the top edges of the shearwalls;

one of a first and second pair of base plates affixed at bottom edges of the shearwalls, the base plates affixing the portal frame to a surface on which the portal frame is supported, the first pair of base plates capable of bearing less load than the second pair of base plates,

wherein a lateral load on the portal frame results in negligible forces between the bottom plates and support surface when the first pair of base plates are affixed to the bottom edges of the shearwalls, and

wherein a lateral load on the portal results in substantially equal forces between the sill plates and header on the one hand, and between the bottom plates and support surface on the other hand, when the second pair of base plates are affixed to the bottom edges of the shearwalls.

21. A portal frame as recited in claim 20, further comprising a first pair of anchors for anchoring the portal frame to the support surface when the first pair of base plates are affixed to the bottom edges of the shearwalls.

22. A portal frame as recited in claim 21, further comprising a second pair of anchors for anchoring the portal frame to the support surface when the second pair of base plates are affixed to the bottom edges of the shearwalls, the second pair of anchors capable of supporting a larger load than the first pair of anchors.

23. A portal frame as recited in claim 20, wherein the header is formed of tube steel.

24. A portal frame as recited in claim 20, wherein the header is formed of glulam.

25. A portal frame as recited in claim 20, further comprising yield links at the joints between the header and shearwalls.