BWally
Structural
- Sep 29, 2006
- 21
I have a “square tube column to H-beam” connection that consists of an H-beam resting on top of a square tube column. The beam ends at the column. There will be a base plate welded to the top of the tube and bolted to the bottom flange of the beam. There are no beam moments or beam axial forces of any significance to deal with here—only the shear at the end of the beam becoming a compressive axial force in the column. (The bolts will be arranged in such a way as to basically prevent this connection from having any moment capacity.)
How do I check the concentrated force capacity of this connection?
I’ve thought of a couple of logical methods of checking this, but logical doesn’t always mean valid:
1. Check the beam using the AISC ASD Chapter K concentrated force criteria that we all use for concentrated forces on beams. Use the square tube width as the bearing width. If the web is too weak, size a stiffener on each side of the web using the Chapter K guidelines. For the assumption that bearing width = tube width, the base plate has to be able to make this happen: it has to be able to get the load from all 4 walls of the tube into a length of beam equal to the tube width without becoming overstressed itself. How do I determine the required thickness of the base plate to make this happen?
2. Check the beam using the AISC ASD Chapter K concentrated force criteria, but use the square tube wall thickness as the bearing width for half the load, assuming only the 2 walls of the square tube perpendicular to the beam axis are active for the load transfer at the connection (ie, the tube becomes an H-beam, with only its “flanges” transferring the load; its other 2 walls are inactive). If the web is too weak, place a pair of stiffeners above each of the tube’s “flanges”. The base plate thickness is added to the thickness of the beam bottom flange to get “tf = base plate thickness + beam bottom flange thickness” for use in the Chapter K equations. To check the base plate for getting the load into the tube walls, I would use the AISC LRFD Spec for Steel HSS (yr 2000), which gives some equations for the situation of an axially loaded square tube with a base plate. There is a bearing width involved in checking the local stress in the square tube wall, for which I would use the beam web thickness. There is also a “t1” term, for which I would use “t1 = actual base plate thickness + beam bottom flange thickness”.
How do I check the concentrated force capacity of this connection?
I’ve thought of a couple of logical methods of checking this, but logical doesn’t always mean valid:
1. Check the beam using the AISC ASD Chapter K concentrated force criteria that we all use for concentrated forces on beams. Use the square tube width as the bearing width. If the web is too weak, size a stiffener on each side of the web using the Chapter K guidelines. For the assumption that bearing width = tube width, the base plate has to be able to make this happen: it has to be able to get the load from all 4 walls of the tube into a length of beam equal to the tube width without becoming overstressed itself. How do I determine the required thickness of the base plate to make this happen?
2. Check the beam using the AISC ASD Chapter K concentrated force criteria, but use the square tube wall thickness as the bearing width for half the load, assuming only the 2 walls of the square tube perpendicular to the beam axis are active for the load transfer at the connection (ie, the tube becomes an H-beam, with only its “flanges” transferring the load; its other 2 walls are inactive). If the web is too weak, place a pair of stiffeners above each of the tube’s “flanges”. The base plate thickness is added to the thickness of the beam bottom flange to get “tf = base plate thickness + beam bottom flange thickness” for use in the Chapter K equations. To check the base plate for getting the load into the tube walls, I would use the AISC LRFD Spec for Steel HSS (yr 2000), which gives some equations for the situation of an axially loaded square tube with a base plate. There is a bearing width involved in checking the local stress in the square tube wall, for which I would use the beam web thickness. There is also a “t1” term, for which I would use “t1 = actual base plate thickness + beam bottom flange thickness”.
