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Re: CMU[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaoc(--nospam--at)seaoc.org
- Subject: Re: CMU
- From: BCASE1356(--nospam--at)aol.com
- Date: Thu, 31 Jul 1997 00:27:07 -0400 (EDT)
In a message dated 97-07-30 09:57:42 EDT, you write: Questions from Bruce Resnick: << <<The out-of-plane connection we have successfully used to TJI's is a pair of holdowns each side of the TJI web which has been reinforced with a solid web stiffener each side of the web. The web stiffener is usually flush to the outside edge of the top and bottom chords. The stiffener is attached with glue (APA approved glue or glue used by joist manufacturer) and screwed through the TJI web into the stiffener on the opposite side. The screws from both sides provide a clamping force to provide better bonding between the web and web stiffeners.>> Does TrusJoist mind that you are taking your shear force through the glued connection between the web and the top flange of their joist? Does it create any reduction in the vertical load capacity of the joist? Have you used this in LA? Response: It has been a while since I spoke to TrusJoist about this. The original idea came about after some discussions with Standard Structures. Since the glue shear transfer between the joist web and flange is around 1100 pounds/foot and the joist are typically longer than 10 feet in length I don't believe the seismic shear force transfer will be an issue. At an anchor spacing of 8 feet on center you are designing for approximately 2400 pounds (8x300#/ft) to 4000 pounds a connection depending upon wall thickness and height which is less than 11,000 pounds (10'x1100#/ft) of shear transfer capacity. Under the real expected earthquake demand (1.2g to 1.5g) on the wall connection (3.33 x .45w = 1.5g) the design force is 7990 to 13320 pounds which is close to the 11000 pound given in this example. If the load is to large, we have gone to using a parallam. You can also always specity in your general notes that you want the truss manufacturer to verify the joist can handle the seismic load combination as part of your truss design criteria. It is a good point that was made about the joist web splice being a lack of continuity. I would think that since the top and bottom chords are continuous they could transfer the shear force beyond the web splice (3'x 1100#/ft x 2 chords = 6600 pounds). If the web splice occurs under the web stiffeners, then the web stiffeners act as splice members helping to transfer the load (glue and screws) over the web discontinuity. I will investigate this furthur, this may become a limiting factor for the joist axial capacity depending upon if the web splice is a true discontinuity In the case of roof trusses, they are designed for dead and live load, during the seismic event you only have the dead load present reducing the bending demand upon the joist to help offset the axial load demand on the truss. I would think there would be no reduction in joist capacity unless the joist is damaged during the seismic event. I believe we have used this detail on a building in LA. But most building's we have used this detail on have not been that large, if they were that large we typically ended up with a standard panelized roof system using glu-lams and 4x's or Standard Structures Truss Purlins. Question: <<When the joist is parallel to the wall, we use a tie rod threaded through a hole in the TJI webs (reinforced with web stiffeners) with 2x blocking tight against each side of the tie rod . The blocking is nailed to the plywood sheathing using 10d at 4"o.c and attached to the web siffeners with A35's. At the end of the tie rod we have a bearing plate which bears on a web stiffener attached to the web of the TJI and is big enough to overlap the 2x blocking each side of the tie rod on the opposite side of the joist. The wall connection is a threaded anchor bolt and coupler to the tie rod As the wall tries to pull away the tie rod goes into tension, putting bearing on the blocking which is then transferred shear through nailing to the diaphragm.>> Do you worry about shrinkage in the 2x blocking not providing a tight fit over time? Do you worry about loading this connection in compression when the wall pushes on the roof since this is really a "tension only" connection? Would such a load cause the blocks to fall out or separate slightly perhaps creating an impact load in the tension rod when the load reverses, similar to what happens in typical rod X-bracing? Assuming that the tie rod is at the center of the TJI's, do you worry that the eccentricity between the tie rods and the diaphragm will cause the joists to "roll"? Response: Typically we use 2x8 or 2x10 blocking each side of the tie rod, we do not expect any significant axial shrinkage of the blocking since they are only spanning 24 inches usually between the joist, depending upon joist spacing. Obviously, some gaps could occur over time, but we feel these will be rather small and not impact the performance. The design is as a tension brace only, when the wall pushes against the diaphragm, we depend upon the diaphragm to distribute the force to the cross walls. Typically we block the first joist space parallel to the wall to help prevent buckling of the plywood. The added blocking around the tie rods I think would help furthur stiffen the diaphragm against the push force of the CMU wall. Since the end of the tie rod only has a bearing plate against the side of the joist, which is tightened down by a nut on the tie rod, a compression force from the wall pushing on the tie rod would only push the plate away from the joist web (lets say less than 1/64" if any movement at all) resulting in no net compression on the tie rod. I don't think you would experience any elongation of the tie rod similar as occurs in tie rod "x" bracing during a seismic event. Since we are using A35's for the block attachment to the web stiffeners, I don't expect that we would lose any alignment of the blocking. The tie rod elevation is typically 4"-5" below the top of the wall ledger, not at centerline of the TJI. This allows for the use of 2x8 and 2x10 blocking and the tie rod to miss the top chord of the TJI. This eccentricity is small and since we have the blocking over several joist spaces to distribute the loading to the diaphragm by means of nailing, we do not feel that this is causing an eccentricity on the joist that you would have to worry about. Question: Lastly, in this parallel condition, do you use a similar type of tie rod detail completely across the building to satisfy the 24' maximum crosstie spacing requirement, or do you have beams in those locations? Response: Typically we have used a beam for this location. But in many strip malls the TJI joist typically spans from perimeter wall to perimeter wall, so the joist acts as your cross tie. At a spacing of 8 feet on center this keeps the axial loads realitively small. If the member has to act as a diaphragm chord or significant drag element, we use a beam and not a TJI. Hope this helps. Michael Cochran BCASE1356(--nospam--at)aol.com
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